About Sleep's Role in Memory

At least one cited source doesn’t support this claim or points to a different paper, and none confirmed it — see the verdicts below.

§ About sleep's role in memory > Direct evidence for a causal role of slow oscillations on sleepdependent memory consolidation is provided by studies experimentally inducing slow oscillations by transcranial direct current stimulation (tDCS; FIGURE 5A). In humans, tDCS that oscillated at a very low frequency (0.003 Hz; 30-s on/30-s off) and was applied to the prefrontal cortex during early noctur-nal non-REM sleep, increased endogenous SWA, and produced a significant improvement in the overnight retention of word pair memories (785).Effects on word pair memories were even more consistent with tDCS oscillating at 0.75 Hz, i.e., a frequency mimicking the endogenous slow oscillations (FIGURE 5B, Ref. 783).This type of stimulation applied during early non-REM sleep specifically enhanced Ͻ1 Hz slow oscillations and additionally increased frontal slow spindle activity (10)(11)(12).tDCS at the same frequency (0.75 Hz) during late REM-rich sleep was ineffective in enhancing both endogenous slow oscillations and overnight retention of word pairs.Further controls ensured that the effects depended on the frequency of the oscillating stimulation and on the brain state (FIGURE 5C): tDCS at the 5 Hz theta frequency during early non-REM sleep had an immediate suppressing rather than enhancing effect on endogenous slow oscillation and frontal slow spindle activity, and impaired overnight retention of word pairs (784).Applying the 0.75 Hz slow oscillatory stimulation during wakefulness induced a widespread increase in theta (4 -8 Hz), rather than slow oscillation activity, and this increase in theta activity was associated with a significant improvement in the encoding of declarative memories, rather than affecting retention of these memories (FIGURE 5D, Ref. 643).These results agree with findings that reveal increased cortical excitability following oscillatory electrical stimulation in the wake state (83,484) and provided initial hints at a causal role of EEG theta activity for the encoding of new memories in humans (646, 647, 687).In fact, combination changes observed after tDCS at different frequencies seem to indicate that the cortical networks oscillating at the theta frequency during encoding of hippocampus-dependent memories are functionally linked to the networks that oscillate at the slow oscillation frequency during subsequent SWS to consolidate these memories.

It may state two separate claims as one.

At least one cited source doesn’t support this claim or points to a different paper, and none confirmed it — see the verdicts below.

§ About sleep's role in memory > 2. Consolidation of procedural memory In contrast to the strong beneficial effects of non-REM sleep on the consolidation of declarative memory in children, studies examining procedural memories consistently demonstrated that in children, unlike in adults, posttraining sleep does not produce a robust improvement of skill.This finding is surprising considering that the acquisition of skills like learning to walk and to speak, to write, and to ride a bicycle represent major challenges of childhood.Also, the neuroanatomic structures underlying skill learning mature quite early during development, i.e., within the first 3 years of life (180, 468).In fact, the majority of studies on sleepdependent procedural memory formation in children indicated an impairing rather than improving effect of sleep on skill (385, 947, 949, 1318), remarkably similar to findings in juvenile song birds where overnight sleep likewise deteriorated performance on the tutored song (280).In the respective studies, the sleep-dependent overnight gains in skill were measured with reference to performance before sleep rather than with reference to performance changes across a corresponding wake interval, which excludes that the absent gain of skill across sleep in children is due to a relatively greater capability in children to stabilize memories during wakefulness (318).Interestingly, sleep restored daytime deficits in procedural memory (serial reaction time task) in children with attention-deficit hyperactivity disorder, while again no overnight improvements were observed in healthy children (949). One factor explaining the missing overnight gain in skill in children might be their fairly low initial skill levels.In adults, the level of skill performance at learning has repeatedly been demonstrated to modulate sleep-dependent memory consolidation, whereby benefits from sleep appeared to be most robust with intermediate performance levels (16,297,679,1151).In children, skill performance is generally much slower, less accurate, and less automated than in adults, particularly in the initial stage of training (318, 385, 1185), which gives rise to the question of whether sleep enhances skill memories also in children if presleep performance is improved to a level comparable with that in adults.Indeed, children (aged 4 -6 years) who received high amounts of training in a finger-tapping task did develop a significant gain in tapping skill across sleep, whereas no such gain was observed in low performing children who had received the standard amount of training (1320).In the adults of this study, only the group with minimum training showed a consistent benefit from the nap.In combination, the findings in children and adults suggest that across age groups, the likelihood that sleep produces a distinct gain in skill performance is greatest when presleep performance is at an intermediate level.Owing to their generally quite low performance of skills, children reach levels of strength at which sleep-dependent benefits directly translate into benefits in behavioral speed and accuracy only after extensive levels of training.

It may state two separate claims as one.

Every source the paper cites for this claim was checked and supports it.

§ About sleep's role in memory > A. Sleep

Probably not a substantial claim. The paper’s conclusions may not depend on it. It may state two separate claims as one.

The cited sources support only part of this claim — read each source’s verdict below.

§ About sleep's role in memory > 3. The standard two-stage memory system Why does the consolidation of memory have to take place during sleep?The hypotheses that sleep serves memory consolidation is conceptually rooted in the standard two-stage memory system which is currently the most influential model of human memory, and has been developed as a solution to several key problems arising from simple associative network models of memory (175,780,800).The foremost of these problems is that although simple association networks are in fact able to store information very rapidly, as is the case in the declarative memory system , the uptake of new conflicting information has a strong tendency to erase the older memories, thus inducing so-called "catastrophic interference" (1005).The critical question is how the neuronal network can learn new patterns without simultaneously forgetting older memories, an issue that has also been referred to as the "stability-plasticity dilemma" (e.g., see Ref. 3).In addition, unstructured recurrent networks have been demonstrated to face essential capacity constraints (723).The two-stage memory formation mechanism first proposed by Marr (780) offers a solution to these problems.It assumes that memories are initially encoded into a fast learning store (i.e., the hippocampus in the declarative memory system) and then gradually transferred to a slow learning store for long-term storage (i.e., the neocortex).The fast learning store ensures quick and efficient encoding of memories, even in one attempt (one-trial learning).Yet, these representations are unstable and vulnerable to (retroactive) interference by newly encoded information.Over time, the information is gradually integrated in the slowly learning long-term store without overwriting older, more remote memories.It is assumed that by the repeated reactivation of the new memories during off-line periods like sleep, the slowly learning long-term store is trained and the new memories are gradually strengthened and adapted to preexisting long-term memories.The transformation new memory representations undergo in this system consolidation process comprises also the extraction of invariants and the development of prototypes and schemas, as the core of the newly learned information is reactivated more frequently than divergent details (734,800,1239).For the declarative memory system, the two-stage model has received strong support from lesion studies, indicating that lesions of the hippocampus abolish the ability to acquire new declarative memory and simultaneously produce a temporally graded retrograde amnesia where older memories remain intact (231,418).The time interval for a memory to reach a state of hippocampus-independent retrieval can vary from one day to several months or years, depending on the acquired information and the schemas preexisting in long-term memory (1210,1308).The standard twostage model of memory has been also successfully applied to nondeclarative kinds of memory, like procedural memory (668), suggesting that the offline reactivation of recent memories and their redistribution from a fast encoding temporary to a slowly learning permanent store could be a general feature of long-term memory formation.

It may state two separate claims as one.

• AssumptionThe authors state this as an assumption: Memories are initially encoded into a fast learning store (the hippocampus in the declarative memory system) and then gradually transferred to a slow learning store for long-term storage (the neocortex).
• AssumptionThe authors state this as an assumption: By repeated reactivation of new memories during off-line periods like sleep, the slowly learning long-term store is trained and the new memories are gradually strengthened and adapted to preexisting long-term memories.

The cited sources support only part of this claim — read each source’s verdict below.

§ About sleep's role in memory > A. Sleep Acts by Passively Protecting Memory From Retroactive Interference

It may state two separate claims as one.

The cited sources support only part of this claim — read each source’s verdict below.

§ About sleep's role in memory > C. Sleep and the Erasure of Information: Accounts of Emotional Memory The idea that sleep might be involved in the erasure or filtering of information has been put forward by several authors (e.g., Refs. 210,236,362,369,854).In particular in 1983, Crick and Mitchison (236) proposed, based on a neurocomputational model of associative learning, that dreaming during REM sleep helps to forget "parasitic modes" of activity, thus ensuring an efficient mode of operation of the brain during waking.Such parasitic modes of activity particularly occurred after stimulation overload and included "fantasy" (i.e., the net produces far-fetched and bizarre associations), "obsession" (i.e., iterates the same response, irrespective of input) and "hallucination"like responses (i.e., responds to inappropriate input signals).As a solution to this problem, the authors proposed a "reverse learning" mechanism during REM sleep-dreaming that dampens synaptic weights to reduce the probability of these parasitic activity modes and thereby also enhances the efficacy and storage capacity of the network.Thus, according to this account, dreaming reduces unwanted and bizarre forms of representations in memory, which enhances new learning the next day as well as retrieval of memories acquired before sleep (237).In simulation studies, repeated unlearning procedures indeed improved the learning capability of the network and retrieval of recently learned patterns, but concurrently weakened more remote memories (564, 1237). Although computational scientists agree in that a mechanism is necessary that limits the strength of synaptic weights in artificial neural networks, empirical evidence for the proposed function of REM sleep, specifically with regard to the removal of "unwanted modes" of activation, is so far lacking.Several authors have reported an influence of REM sleep on emotional reactivity or mood (490, 491, 1243), but a specific influence on "obsessive" or "hallucination"-like behaviors has not yet been tested.In contrast, after awakening from REM sleep, the brain appears to remain in a "hyperassociative" mode, in which weak semantic primes produced distinctly stronger priming effects than during consolidated wakefulness or after awakenings from non-REM sleep (1154).Similarly, REM sleep awakenings also resulted in a 32% increase in a complex anagram solving task (1288), and conversely, the need for creative thinking increased subsequent REM sleep (730).Furthermore, priming before naps filled with REM sleep had a much stronger impact on later creative answers compared with naps without REM or quiet resting, indicating that REM sleep improves creative problem solving instead of reducing creativity (163).In volunteers asked to voluntarily suppress "unwanted memories," sleep and particularly REM sleep appeared to counteract this suppression as reflected by an improved retrieval for previously suppressed items, rather than to enhance forgetting of these unwanted memories (381).Finally, repetitive nightmares, which are highly prevalent in patients suffering from posttraumatic stress disorders (PTSD), do not lead to forgetting of the traumatic event, but are rather associated with increased severity of the disorder and considered a risk factor in the development of PTSD (75,1240,1331).Overall, these findings speak against the view posed by Crick and Mitchison (236). Of note, in other computational models the mechanisms limiting the strength of synaptic weights in the neural network have been linked to non-REM rather than REM sleep (210).A recent version of this idea is the "synaptic homeostasis hypothesis" (1203, 1204) which will be discussed in section IVB and assumes that a global downscaling in the strength of synaptic connections takes place during SWS to prevent saturation and to reduce place and energy demands, thereby preparing the network for the encoding of new information during succeeding wakefulness (565, 566). If the emotional tone of a memory is considered an "unwanted activation," then the "sleep to forget sleep to remember" (SFSR) hypothesis recently proposed by Walker and van der Helm (1282, 1284) bears some similarities with the ideas about REM sleep by Crick and Mitchison.The SFSR hypothesis assumes that REM sleep after an (aversive) emotional experience strengthens the content of the respective representations in memory, but simultaneously reduces the emotional tone associated with this memory, i.e., reduces the emotional response when the memory is retrieved.The process is not restricted to one night after encoding, but would continue during multiple nights.In depressed patients showing enhanced REM sleep, according to the SFSR hypothesis, this enhancement would bias strengthening of memories towards increased storage of negative contents, while suppression of REM sleep through antidepressants counteracts this bias.The impairment of mood associated with this REM-related process might indicate that the attenuation of emotional tone by REM sleep is not functional in these patients.Similarly, in traumatized patients, increased nightmare frequency would point to a failure to attenuate memory-associated emotions during REM sleep. Consistent with the theory, several studies have shown that emotional memories are particularly strengthened across sleep (560,574,732,861,895,898,1274,1276,1278,1294), in particular when containing high amounts of REM sleep (179,478,479,1274).The enhancing effect of postencoding sleep on emotional memories was detectable even after several years (1276).Duration and latency of REM sleep significantly correlated with the later recall emotional memories (861).In the context of fear learning, sleep promoted the generalization of extinction learning (878), increased intersession habituation to emotional stimuli (879), and in both humans and rodents, the deprivation of REM sleep impaired fear extinction (421, 1084, 1133).Further support for the SFSR hypothesis was provided by functional magnetic resonance imaging studies indicating that the strengthening of negative emotional memories by sleep is accompanied by a reduced amygdala activation, i.e., a diminished emotional response during retrieval (1147, 1231). However, there are also contradicting findings.For example, two studies have shown that memory recall of negative pictures is less impaired by sleep deprivation after encoding compared with neutral pictures, suggesting a reduced dependency of negative memories on sleep-dependent consolidation processes (46,1147).With respect to emotional reactivity, one study showed that REM sleep-rich sleep amplified subjectively experienced aversion to previously viewed emotional pictures (1273).Conversely, selective REM sleep deprivation after picture viewing reduced arousal ratings to negative pictures presented again on the next morning (704).Others found that emotional reactivity decreased across wakefulness, but was preserved during sleep, with the preserving effect on emotional reactivity being specifically linked to REM sleep (60,481).In addition, amygdala responses to correctly recognized emotional objects (with reference to neutral objects) increased rather than decreased after sleep, compared with an assessment after a wake retention interval (896).The increased amygdala response was accompanied by a stronger connectivity among limbic regions after sleep in this study.Sleep after fear conditioning in humans increases the conditioned response and the associated amygdala activity, with this enhancement being positively associated with postlearning REM sleep (Menz, Rihm, Born, Kalisch, Pape, Marshall, and Büchel, unpublished observation).Sleep likewise facilitated the generalization of implicit fear responses (678).In animals, REM sleep has been consistently associated with a strengthening of conditioned fear memories (see sect.IIB; for reviews, see, e.g., Refs. 538, 807, 1092Refs. 538, 807, , 1094)).Furthermore, a recent study points towards an involvement of adrenergic activity during SWS instead of REM sleep for the consolidation of emotional information (482).In conclusion, it is still an open question whether the consolidation of emotional memories actually differs in quality from that of neutral declarative memories, or whether it is the same consolidation process that is simply enhanced or accelerated by the emotional arousal that is attached to the representation at encoding.

It may state two separate claims as one.

The sources with a verdict support this claim; the rest weren’t checked yet, have no full text we could read, or their verdicts aren’t rated yet.

§ About sleep's role in memory > C. Sleep and the Erasure of Information: Accounts of Emotional Memory The idea that sleep might be involved in the erasure or filtering of information has been put forward by several authors (e.g., Refs. 210,236,362,369,854).In particular in 1983, Crick and Mitchison (236) proposed, based on a neurocomputational model of associative learning, that dreaming during REM sleep helps to forget "parasitic modes" of activity, thus ensuring an efficient mode of operation of the brain during waking.Such parasitic modes of activity particularly occurred after stimulation overload and included "fantasy" (i.e., the net produces far-fetched and bizarre associations), "obsession" (i.e., iterates the same response, irrespective of input) and "hallucination"like responses (i.e., responds to inappropriate input signals).As a solution to this problem, the authors proposed a "reverse learning" mechanism during REM sleep-dreaming that dampens synaptic weights to reduce the probability of these parasitic activity modes and thereby also enhances the efficacy and storage capacity of the network.Thus, according to this account, dreaming reduces unwanted and bizarre forms of representations in memory, which enhances new learning the next day as well as retrieval of memories acquired before sleep (237).In simulation studies, repeated unlearning procedures indeed improved the learning capability of the network and retrieval of recently learned patterns, but concurrently weakened more remote memories (564, 1237). Although computational scientists agree in that a mechanism is necessary that limits the strength of synaptic weights in artificial neural networks, empirical evidence for the proposed function of REM sleep, specifically with regard to the removal of "unwanted modes" of activation, is so far lacking.Several authors have reported an influence of REM sleep on emotional reactivity or mood (490, 491, 1243), but a specific influence on "obsessive" or "hallucination"-like behaviors has not yet been tested.In contrast, after awakening from REM sleep, the brain appears to remain in a "hyperassociative" mode, in which weak semantic primes produced distinctly stronger priming effects than during consolidated wakefulness or after awakenings from non-REM sleep (1154).Similarly, REM sleep awakenings also resulted in a 32% increase in a complex anagram solving task (1288), and conversely, the need for creative thinking increased subsequent REM sleep (730).Furthermore, priming before naps filled with REM sleep had a much stronger impact on later creative answers compared with naps without REM or quiet resting, indicating that REM sleep improves creative problem solving instead of reducing creativity (163).In volunteers asked to voluntarily suppress "unwanted memories," sleep and particularly REM sleep appeared to counteract this suppression as reflected by an improved retrieval for previously suppressed items, rather than to enhance forgetting of these unwanted memories (381).Finally, repetitive nightmares, which are highly prevalent in patients suffering from posttraumatic stress disorders (PTSD), do not lead to forgetting of the traumatic event, but are rather associated with increased severity of the disorder and considered a risk factor in the development of PTSD (75,1240,1331).Overall, these findings speak against the view posed by Crick and Mitchison (236). Of note, in other computational models the mechanisms limiting the strength of synaptic weights in the neural network have been linked to non-REM rather than REM sleep (210).A recent version of this idea is the "synaptic homeostasis hypothesis" (1203, 1204) which will be discussed in section IVB and assumes that a global downscaling in the strength of synaptic connections takes place during SWS to prevent saturation and to reduce place and energy demands, thereby preparing the network for the encoding of new information during succeeding wakefulness (565, 566). If the emotional tone of a memory is considered an "unwanted activation," then the "sleep to forget sleep to remember" (SFSR) hypothesis recently proposed by Walker and van der Helm (1282, 1284) bears some similarities with the ideas about REM sleep by Crick and Mitchison.The SFSR hypothesis assumes that REM sleep after an (aversive) emotional experience strengthens the content of the respective representations in memory, but simultaneously reduces the emotional tone associated with this memory, i.e., reduces the emotional response when the memory is retrieved.The process is not restricted to one night after encoding, but would continue during multiple nights.In depressed patients showing enhanced REM sleep, according to the SFSR hypothesis, this enhancement would bias strengthening of memories towards increased storage of negative contents, while suppression of REM sleep through antidepressants counteracts this bias.The impairment of mood associated with this REM-related process might indicate that the attenuation of emotional tone by REM sleep is not functional in these patients.Similarly, in traumatized patients, increased nightmare frequency would point to a failure to attenuate memory-associated emotions during REM sleep. Consistent with the theory, several studies have shown that emotional memories are particularly strengthened across sleep (560,574,732,861,895,898,1274,1276,1278,1294), in particular when containing high amounts of REM sleep (179,478,479,1274).The enhancing effect of postencoding sleep on emotional memories was detectable even after several years (1276).Duration and latency of REM sleep significantly correlated with the later recall emotional memories (861).In the context of fear learning, sleep promoted the generalization of extinction learning (878), increased intersession habituation to emotional stimuli (879), and in both humans and rodents, the deprivation of REM sleep impaired fear extinction (421, 1084, 1133).Further support for the SFSR hypothesis was provided by functional magnetic resonance imaging studies indicating that the strengthening of negative emotional memories by sleep is accompanied by a reduced amygdala activation, i.e., a diminished emotional response during retrieval (1147, 1231). However, there are also contradicting findings.For example, two studies have shown that memory recall of negative pictures is less impaired by sleep deprivation after encoding compared with neutral pictures, suggesting a reduced dependency of negative memories on sleep-dependent consolidation processes (46,1147).With respect to emotional reactivity, one study showed that REM sleep-rich sleep amplified subjectively experienced aversion to previously viewed emotional pictures (1273).Conversely, selective REM sleep deprivation after picture viewing reduced arousal ratings to negative pictures presented again on the next morning (704).Others found that emotional reactivity decreased across wakefulness, but was preserved during sleep, with the preserving effect on emotional reactivity being specifically linked to REM sleep (60,481).In addition, amygdala responses to correctly recognized emotional objects (with reference to neutral objects) increased rather than decreased after sleep, compared with an assessment after a wake retention interval (896).The increased amygdala response was accompanied by a stronger connectivity among limbic regions after sleep in this study.Sleep after fear conditioning in humans increases the conditioned response and the associated amygdala activity, with this enhancement being positively associated with postlearning REM sleep (Menz, Rihm, Born, Kalisch, Pape, Marshall, and Büchel, unpublished observation).Sleep likewise facilitated the generalization of implicit fear responses (678).In animals, REM sleep has been consistently associated with a strengthening of conditioned fear memories (see sect.IIB; for reviews, see, e.g., Refs. 538, 807, 1092Refs. 538, 807, , 1094)).Furthermore, a recent study points towards an involvement of adrenergic activity during SWS instead of REM sleep for the consolidation of emotional information (482).In conclusion, it is still an open question whether the consolidation of emotional memories actually differs in quality from that of neutral declarative memories, or whether it is the same consolidation process that is simply enhanced or accelerated by the emotional arousal that is attached to the representation at encoding.

It may state two separate claims as one.

The cited sources support only part of this claim — read each source’s verdict below.

§ About sleep's role in memory > B. REM Sleep and Memory Consolidation The hypothesis has been around for some time that REM sleep contributes to memory consolidation, stimulated in particular by the wakelike EEG activity during this sleep stage together with frequent reports of vivid dreams after awakening from REM sleep.Very consistent evidence for a role of REM sleep for memory was provided by studies in animals (for comprehensive reviews, see, e.g., Refs. 101, 388, 807, 901, 936, 943, 1092Refs. 101, 388, 807, 901, 936, 943, , 1095Refs. 101, 388, 807, 901, 936, 943, , 1098Refs. 101, 388, 807, 901, 936, 943, , 1260)).With the use of a variety of tasks including classic, aversive, and appetitive conditioning procedures, a large number of studies consistently revealed increases in REM sleep after learning in rats, mice, and cats (100, 284, 389, 540, 708, 711-713, 747, 944, 1102, 1104, 1110, 1111).Rats living in enriched compared with impoverished environments likewise exhibited enhanced REM sleep, although increases in non-REM sleep were also observed (278,493,494,669,823,1086,1168).Increasing REM pharmacologically by administration of carbachol into the pontine reticular formation and of corticotrophin-like intermediate lobe peptide (CLIP) as well as a REM sleep rebound after prior REM deprivation, all improved memory for a Y-maze discrimination task when applied after learning of the task (1316).Deprivation of REM sleep (mostly without simultaneous sleep recording) appeared to primarily impair memory formation on complex tasks, like two-way shuttle box avoid-ance and complex mazes, which encompass a change in the animals regular repertoire (69, 100, 312, 516, 525, 539, 644, 710, 713, 714, 787, 900, 903-906, 992, 1021, 1072, 1111, 1113, 1238, 1352, 1353).In contrast, long-term memory for simpler tasks, like one-way active avoidance and simple mazes, were less consistently affected (15, 249, 386, 390, 495, 558, 611, 644, 821, 872, 902, 907-909, 1072, 1091, 1334). REM sleep increases were mostly observed in the first hours after learning, partly reflecting the fact that recording was limited to these hours.With prolonged recording sessions, elevated periods of REM sleep occurred up to 4 -6 days after learning, sometimes following a cyclic pattern (1111,1116).Here, REM sleep increases were typically most prominent during specific time periods and dependent on the task.In the Morris water maze task, it started more than 2 h after learning and persisted for 22 h (1099).In several other avoidance tasks, REM sleep increases were less persistent, emerged later (i.e., 9 -12 h post-learning), and sometimes reemerged 17-20 h post-learning (1094).In appetitive learning tasks, REM sleep increases started after 4 h and persisted for 12 h (1104).The increases in REM sleep during the specific time periods predicted later memory recall and reliably separated between learners and nonlearners (1095, 1110).Learning in these studies induced distinct and prolonged waves of REM increases possibly involved in memory formation.Based on these findings, Smith proposed the concept of "paradoxical sleep windows" (PSW) mediating memory formation (1092, 1097).Indeed, selective deprivation of REM sleep during, but not outside of, identified PSWs impaired memory (719,1101,1105,1106,1113,1115).Inhibition of protein synthesis by anisomycin also impaired memory only when intraperitoneally injected during a PSW 9 h after learning a shuttle avoidance task (1108).Similar results were obtained when blocking muscarinic cholinergic receptors during a PSW by scopolamine (720, 1108), pointing to a crucial involvement of protein synthesis as well as cholinergic activation in PSW-associated memory processes.Interestingly, blocking of NMDA receptors was most effective in impairing memory consolidation when administered after a PSW, suggesting that PSW-associated memory processing induces subsequent NMDA-dependent plasticity (1094, 1099). Quite a number of studies in this context have been criticized as they employed the "flower-pot" method to deprive the animal from REM sleep.In this procedure, the rat rests on a small platform (i.e., the flower pot) surrounded by water and, owing to complete muscle atonia, falls into the water whenever REM sleep starts (119, 387, 569).Awakenings induced in this way are highly stressful and may per se impair later memory performance (953).However, impairing effects of REM sleep deprivation on memory retention have been also demonstrated with less stressful procedures like gentle head lifting (263), mild touching (596), or after pharmacological REM suppression (720, 721, 1098, 1108).Ponto-geniculo-occipital (PGO) waves which occur associated with REMs in rats and cats have been proposed as a mechanism promoting plastic processes underlying memory formation during REM sleep (259, 260) (see sect.

Probably not a substantial claim. The paper’s conclusions may not depend on it.

The sources with a verdict support this claim; the rest weren’t checked yet, have no full text we could read, or their verdicts aren’t rated yet.

§ About sleep's role in memory > C. Sleep and the Erasure of Information: Accounts of Emotional Memory The idea that sleep might be involved in the erasure or filtering of information has been put forward by several authors (e.g., Refs. 210,236,362,369,854).In particular in 1983, Crick and Mitchison (236) proposed, based on a neurocomputational model of associative learning, that dreaming during REM sleep helps to forget "parasitic modes" of activity, thus ensuring an efficient mode of operation of the brain during waking.Such parasitic modes of activity particularly occurred after stimulation overload and included "fantasy" (i.e., the net produces far-fetched and bizarre associations), "obsession" (i.e., iterates the same response, irrespective of input) and "hallucination"like responses (i.e., responds to inappropriate input signals).As a solution to this problem, the authors proposed a "reverse learning" mechanism during REM sleep-dreaming that dampens synaptic weights to reduce the probability of these parasitic activity modes and thereby also enhances the efficacy and storage capacity of the network.Thus, according to this account, dreaming reduces unwanted and bizarre forms of representations in memory, which enhances new learning the next day as well as retrieval of memories acquired before sleep (237).In simulation studies, repeated unlearning procedures indeed improved the learning capability of the network and retrieval of recently learned patterns, but concurrently weakened more remote memories (564, 1237). Although computational scientists agree in that a mechanism is necessary that limits the strength of synaptic weights in artificial neural networks, empirical evidence for the proposed function of REM sleep, specifically with regard to the removal of "unwanted modes" of activation, is so far lacking.Several authors have reported an influence of REM sleep on emotional reactivity or mood (490, 491, 1243), but a specific influence on "obsessive" or "hallucination"-like behaviors has not yet been tested.In contrast, after awakening from REM sleep, the brain appears to remain in a "hyperassociative" mode, in which weak semantic primes produced distinctly stronger priming effects than during consolidated wakefulness or after awakenings from non-REM sleep (1154).Similarly, REM sleep awakenings also resulted in a 32% increase in a complex anagram solving task (1288), and conversely, the need for creative thinking increased subsequent REM sleep (730).Furthermore, priming before naps filled with REM sleep had a much stronger impact on later creative answers compared with naps without REM or quiet resting, indicating that REM sleep improves creative problem solving instead of reducing creativity (163).In volunteers asked to voluntarily suppress "unwanted memories," sleep and particularly REM sleep appeared to counteract this suppression as reflected by an improved retrieval for previously suppressed items, rather than to enhance forgetting of these unwanted memories (381).Finally, repetitive nightmares, which are highly prevalent in patients suffering from posttraumatic stress disorders (PTSD), do not lead to forgetting of the traumatic event, but are rather associated with increased severity of the disorder and considered a risk factor in the development of PTSD (75,1240,1331).Overall, these findings speak against the view posed by Crick and Mitchison (236). Of note, in other computational models the mechanisms limiting the strength of synaptic weights in the neural network have been linked to non-REM rather than REM sleep (210).A recent version of this idea is the "synaptic homeostasis hypothesis" (1203, 1204) which will be discussed in section IVB and assumes that a global downscaling in the strength of synaptic connections takes place during SWS to prevent saturation and to reduce place and energy demands, thereby preparing the network for the encoding of new information during succeeding wakefulness (565, 566). If the emotional tone of a memory is considered an "unwanted activation," then the "sleep to forget sleep to remember" (SFSR) hypothesis recently proposed by Walker and van der Helm (1282, 1284) bears some similarities with the ideas about REM sleep by Crick and Mitchison.The SFSR hypothesis assumes that REM sleep after an (aversive) emotional experience strengthens the content of the respective representations in memory, but simultaneously reduces the emotional tone associated with this memory, i.e., reduces the emotional response when the memory is retrieved.The process is not restricted to one night after encoding, but would continue during multiple nights.In depressed patients showing enhanced REM sleep, according to the SFSR hypothesis, this enhancement would bias strengthening of memories towards increased storage of negative contents, while suppression of REM sleep through antidepressants counteracts this bias.The impairment of mood associated with this REM-related process might indicate that the attenuation of emotional tone by REM sleep is not functional in these patients.Similarly, in traumatized patients, increased nightmare frequency would point to a failure to attenuate memory-associated emotions during REM sleep. Consistent with the theory, several studies have shown that emotional memories are particularly strengthened across sleep (560,574,732,861,895,898,1274,1276,1278,1294), in particular when containing high amounts of REM sleep (179,478,479,1274).The enhancing effect of postencoding sleep on emotional memories was detectable even after several years (1276).Duration and latency of REM sleep significantly correlated with the later recall emotional memories (861).In the context of fear learning, sleep promoted the generalization of extinction learning (878), increased intersession habituation to emotional stimuli (879), and in both humans and rodents, the deprivation of REM sleep impaired fear extinction (421, 1084, 1133).Further support for the SFSR hypothesis was provided by functional magnetic resonance imaging studies indicating that the strengthening of negative emotional memories by sleep is accompanied by a reduced amygdala activation, i.e., a diminished emotional response during retrieval (1147, 1231). However, there are also contradicting findings.For example, two studies have shown that memory recall of negative pictures is less impaired by sleep deprivation after encoding compared with neutral pictures, suggesting a reduced dependency of negative memories on sleep-dependent consolidation processes (46,1147).With respect to emotional reactivity, one study showed that REM sleep-rich sleep amplified subjectively experienced aversion to previously viewed emotional pictures (1273).Conversely, selective REM sleep deprivation after picture viewing reduced arousal ratings to negative pictures presented again on the next morning (704).Others found that emotional reactivity decreased across wakefulness, but was preserved during sleep, with the preserving effect on emotional reactivity being specifically linked to REM sleep (60,481).In addition, amygdala responses to correctly recognized emotional objects (with reference to neutral objects) increased rather than decreased after sleep, compared with an assessment after a wake retention interval (896).The increased amygdala response was accompanied by a stronger connectivity among limbic regions after sleep in this study.Sleep after fear conditioning in humans increases the conditioned response and the associated amygdala activity, with this enhancement being positively associated with postlearning REM sleep (Menz, Rihm, Born, Kalisch, Pape, Marshall, and Büchel, unpublished observation).Sleep likewise facilitated the generalization of implicit fear responses (678).In animals, REM sleep has been consistently associated with a strengthening of conditioned fear memories (see sect.IIB; for reviews, see, e.g., Refs. 538, 807, 1092Refs. 538, 807, , 1094)).Furthermore, a recent study points towards an involvement of adrenergic activity during SWS instead of REM sleep for the consolidation of emotional information (482).In conclusion, it is still an open question whether the consolidation of emotional memories actually differs in quality from that of neutral declarative memories, or whether it is the same consolidation process that is simply enhanced or accelerated by the emotional arousal that is attached to the representation at encoding.

Probably not a substantial claim. The paper’s conclusions may not depend on it. It may state two separate claims as one.

The sources with a verdict support this claim; the rest weren’t checked yet, have no full text we could read, or their verdicts aren’t rated yet.

§ About sleep's role in memory > D. The Dual Process Hypothesis The dual processes hypothesis assumes that different sleep stages serve the consolidation of different types of memories (428, 765, 967, 1096).Specifically, it has been assumed that declarative memory profits from SWS, whereas the consolidation of nondeclarative memory is supported by REM sleep.The hypothesis received support mainly from studies in humans, particularly from those employing the "nighthalf paradigm."This paradigm, originally developed by Ekstrand and co-workers (341, 408, 1340), basically compares retention performance across retention intervals that cover either the early or late half of nocturnal sleep.Whereas in the early sleep condition, participants learn (to criterion) the memory tasks in the evening and then sleep for 3-4 h before a later recall test, in the late sleep condition, participants first sleep for ϳ3 h (to satisfy the need of SWS) and then are subjected to the learning phase, followed by the late night retention sleep.Due to the circadian rhythm, early nocturnal sleep contains most of SWS, whereas late nocturnal sleep is dominated by REM sleep.Time in stage 1 or 2 sleep usually does not differ between early and late sleep retention conditions.The approach thus allows for comparing the effects of sleep rich in SWS versus REM sleep, elegantly avoiding possible confounding effects resulting from stressful repeated awakenings accompanying standard procedures of selective sleep deprivation.To control possible confounds of the circadian rhythm, the effects of early and late sleep retention periods are typically compared additionally with the effects of wake retention periods that cover the same early and late phases of the night.These two wake control conditions are also necessary to ensure that depth of encoding during the learning phase is comparable between the conditions, because prior sleep is known to influence encoding capabilities (328,342,486,515,548,1190,1232,1345), and learning is preceded by prior sleep only in the late sleep condition, but not in the early sleep condition. Yarush et al. (1340) were the first to report a beneficial effect of SWS-rich early sleep on declarative memory (word pairs), compared with retention performance across a REM sleep-rich late sleep or across corresponding wake intervals, and these findings were replicated in a later study of the same group (408).A similar benefit for declarative (pairedassociates) memories was revealed when controlling for circadian influences by placing SWS-rich and REM sleep-rich sleep periods at the same circadian phase, between 3 and 7 a.m.(63).While these studies quite compellingly showed that declarative memory for neutral materials is enhanced by SWS, emotional declarative memories appear to additionally benefit from REM-rich late sleep (1273, 1274).Building on these early studies, Plihal and Born (930, 931) not only demonstrated a benefit for declarative memories (word pairs, spatial information) from early SWS-rich sleep, but also demonstrated that late REM sleep-rich retention sleep selectively improved procedural and im-plicit memories (mirror tracing skill, word-stem priming), compared with corresponding wake-retention intervals.Later studies replicated beneficial effects of late REM sleep-rich sleep on implicit memories (e.g., faces, masked stimuli) (1248, 1277), and altogether these findings fit well with the notion of an involvement of REM sleep in procedural and implicit memory processes as revealed by standard REM sleep deprivation procedures (described in sect.IIB). Improving the effects of early SWS-rich sleep on declarative memory appeared to be overall more consistent with free (or cued) recall measures than with recognition measures of memory (265, 326, 667, 966; see Ref. 297, for a review), possibly reflecting that unlike recollection, familiarity based recognition taps to a greater extent implicit components of these memories (1343,1344).Two studies found enhanced recollection (of words) after early, SWS-rich sleep, compared with a late REM-rich sleep or wake intervals, while familiarity-based recognition measures remained unaffected (265,326).In a third study measuring recognition right after free recall, this effect was not replicated, although SWS-rich sleep still enhanced free recall of the declarative materials (966). Overall, studies using the night-half paradigm have provided substantial evidence for the dual processes hypothesis, such as hippocampus-dependent declarative memories preferentially profiting from SWS, whereas nondeclarative aspects of memory, such as procedural, implicit, and emotional, additionally profiting from REM sleep (121, 428).The hypothesis has been challenged by findings showing that procedural tasks like visuomotor adaptation and visual texture discrimination also benefit from SWS (10,433,581,582).However, training such skills does not proceed entirely detached from declarative memory mechanisms, especially at an initial stage of training (e.g., Refs. 198, 533, 1013Refs. 198, 533, , 1014Refs. 198, 533, , 1042)).Such declarative components might have mediated the strong benefits for skills from SWS in those studies (910). A major weakness of the night-half paradigm is that it ignores possible contributions of stage 2 sleep to memory.Although the amounts of stage 2 sleep were comparable in the early and late sleep conditions of the studies reported above, sleep in this sleep stage may have substantially differed between the two phases, for example, with regard to spindle density (440, 441, 550), heart rate, or levels of neuromodulators like catecholamines and cortisol (95,124,131,313,554,960).There is consistent evidence for an involvement of stage 2 sleep and sleep spindles in motor learning (139,396,822,862,921), and memory for a simple motor task is impaired after selective stage 2 sleep deprivation (1112, 1117).Training on motor tasks increased the time spent in stage 2 and the density of fast (but not slow) spindles (399, 837, 920, 921, 1173, 1174), and sleep stage 2 duration and fast spindle density also predicted sleep-dependent improvements in a finger tapping task (59,961,1285,1286).Based on these findings, Smith and coworkers (846, 1114) proposed that especially simple motor tasks require stage 2 sleep, whereas complex motor tasks may require REM sleep.However, sleep stage 2 spindle activity and spindle counts were found to similarly correlate with the overnight retention of verbal and visuospatial memories, suggesting that spindles and stage 2 sleep are also involved in declarative memory formation (213,214,1016).

It may state two separate claims as one.

The sources with a verdict support this claim; the rest weren’t checked yet, have no full text we could read, or their verdicts aren’t rated yet.

§ About sleep's role in memory > C. Sleep and the Erasure of Information: Accounts of Emotional Memory The idea that sleep might be involved in the erasure or filtering of information has been put forward by several authors (e.g., Refs. 210,236,362,369,854).In particular in 1983, Crick and Mitchison (236) proposed, based on a neurocomputational model of associative learning, that dreaming during REM sleep helps to forget "parasitic modes" of activity, thus ensuring an efficient mode of operation of the brain during waking.Such parasitic modes of activity particularly occurred after stimulation overload and included "fantasy" (i.e., the net produces far-fetched and bizarre associations), "obsession" (i.e., iterates the same response, irrespective of input) and "hallucination"like responses (i.e., responds to inappropriate input signals).As a solution to this problem, the authors proposed a "reverse learning" mechanism during REM sleep-dreaming that dampens synaptic weights to reduce the probability of these parasitic activity modes and thereby also enhances the efficacy and storage capacity of the network.Thus, according to this account, dreaming reduces unwanted and bizarre forms of representations in memory, which enhances new learning the next day as well as retrieval of memories acquired before sleep (237).In simulation studies, repeated unlearning procedures indeed improved the learning capability of the network and retrieval of recently learned patterns, but concurrently weakened more remote memories (564, 1237). Although computational scientists agree in that a mechanism is necessary that limits the strength of synaptic weights in artificial neural networks, empirical evidence for the proposed function of REM sleep, specifically with regard to the removal of "unwanted modes" of activation, is so far lacking.Several authors have reported an influence of REM sleep on emotional reactivity or mood (490, 491, 1243), but a specific influence on "obsessive" or "hallucination"-like behaviors has not yet been tested.In contrast, after awakening from REM sleep, the brain appears to remain in a "hyperassociative" mode, in which weak semantic primes produced distinctly stronger priming effects than during consolidated wakefulness or after awakenings from non-REM sleep (1154).Similarly, REM sleep awakenings also resulted in a 32% increase in a complex anagram solving task (1288), and conversely, the need for creative thinking increased subsequent REM sleep (730).Furthermore, priming before naps filled with REM sleep had a much stronger impact on later creative answers compared with naps without REM or quiet resting, indicating that REM sleep improves creative problem solving instead of reducing creativity (163).In volunteers asked to voluntarily suppress "unwanted memories," sleep and particularly REM sleep appeared to counteract this suppression as reflected by an improved retrieval for previously suppressed items, rather than to enhance forgetting of these unwanted memories (381).Finally, repetitive nightmares, which are highly prevalent in patients suffering from posttraumatic stress disorders (PTSD), do not lead to forgetting of the traumatic event, but are rather associated with increased severity of the disorder and considered a risk factor in the development of PTSD (75,1240,1331).Overall, these findings speak against the view posed by Crick and Mitchison (236). Of note, in other computational models the mechanisms limiting the strength of synaptic weights in the neural network have been linked to non-REM rather than REM sleep (210).A recent version of this idea is the "synaptic homeostasis hypothesis" (1203, 1204) which will be discussed in section IVB and assumes that a global downscaling in the strength of synaptic connections takes place during SWS to prevent saturation and to reduce place and energy demands, thereby preparing the network for the encoding of new information during succeeding wakefulness (565, 566). If the emotional tone of a memory is considered an "unwanted activation," then the "sleep to forget sleep to remember" (SFSR) hypothesis recently proposed by Walker and van der Helm (1282, 1284) bears some similarities with the ideas about REM sleep by Crick and Mitchison.The SFSR hypothesis assumes that REM sleep after an (aversive) emotional experience strengthens the content of the respective representations in memory, but simultaneously reduces the emotional tone associated with this memory, i.e., reduces the emotional response when the memory is retrieved.The process is not restricted to one night after encoding, but would continue during multiple nights.In depressed patients showing enhanced REM sleep, according to the SFSR hypothesis, this enhancement would bias strengthening of memories towards increased storage of negative contents, while suppression of REM sleep through antidepressants counteracts this bias.The impairment of mood associated with this REM-related process might indicate that the attenuation of emotional tone by REM sleep is not functional in these patients.Similarly, in traumatized patients, increased nightmare frequency would point to a failure to attenuate memory-associated emotions during REM sleep. Consistent with the theory, several studies have shown that emotional memories are particularly strengthened across sleep (560,574,732,861,895,898,1274,1276,1278,1294), in particular when containing high amounts of REM sleep (179,478,479,1274).The enhancing effect of postencoding sleep on emotional memories was detectable even after several years (1276).Duration and latency of REM sleep significantly correlated with the later recall emotional memories (861).In the context of fear learning, sleep promoted the generalization of extinction learning (878), increased intersession habituation to emotional stimuli (879), and in both humans and rodents, the deprivation of REM sleep impaired fear extinction (421, 1084, 1133).Further support for the SFSR hypothesis was provided by functional magnetic resonance imaging studies indicating that the strengthening of negative emotional memories by sleep is accompanied by a reduced amygdala activation, i.e., a diminished emotional response during retrieval (1147, 1231). However, there are also contradicting findings.For example, two studies have shown that memory recall of negative pictures is less impaired by sleep deprivation after encoding compared with neutral pictures, suggesting a reduced dependency of negative memories on sleep-dependent consolidation processes (46,1147).With respect to emotional reactivity, one study showed that REM sleep-rich sleep amplified subjectively experienced aversion to previously viewed emotional pictures (1273).Conversely, selective REM sleep deprivation after picture viewing reduced arousal ratings to negative pictures presented again on the next morning (704).Others found that emotional reactivity decreased across wakefulness, but was preserved during sleep, with the preserving effect on emotional reactivity being specifically linked to REM sleep (60,481).In addition, amygdala responses to correctly recognized emotional objects (with reference to neutral objects) increased rather than decreased after sleep, compared with an assessment after a wake retention interval (896).The increased amygdala response was accompanied by a stronger connectivity among limbic regions after sleep in this study.Sleep after fear conditioning in humans increases the conditioned response and the associated amygdala activity, with this enhancement being positively associated with postlearning REM sleep (Menz, Rihm, Born, Kalisch, Pape, Marshall, and Büchel, unpublished observation).Sleep likewise facilitated the generalization of implicit fear responses (678).In animals, REM sleep has been consistently associated with a strengthening of conditioned fear memories (see sect.IIB; for reviews, see, e.g., Refs. 538, 807, 1092Refs. 538, 807, , 1094)).Furthermore, a recent study points towards an involvement of adrenergic activity during SWS instead of REM sleep for the consolidation of emotional information (482).In conclusion, it is still an open question whether the consolidation of emotional memories actually differs in quality from that of neutral declarative memories, or whether it is the same consolidation process that is simply enhanced or accelerated by the emotional arousal that is attached to the representation at encoding.

Probably not a substantial claim. The paper’s conclusions may not depend on it.

The cited sources support only part of this claim — read each source’s verdict below.

§ About sleep's role in memory > D. The Dual Process Hypothesis The dual processes hypothesis assumes that different sleep stages serve the consolidation of different types of memories (428, 765, 967, 1096).Specifically, it has been assumed that declarative memory profits from SWS, whereas the consolidation of nondeclarative memory is supported by REM sleep.The hypothesis received support mainly from studies in humans, particularly from those employing the "nighthalf paradigm."This paradigm, originally developed by Ekstrand and co-workers (341, 408, 1340), basically compares retention performance across retention intervals that cover either the early or late half of nocturnal sleep.Whereas in the early sleep condition, participants learn (to criterion) the memory tasks in the evening and then sleep for 3-4 h before a later recall test, in the late sleep condition, participants first sleep for ϳ3 h (to satisfy the need of SWS) and then are subjected to the learning phase, followed by the late night retention sleep.Due to the circadian rhythm, early nocturnal sleep contains most of SWS, whereas late nocturnal sleep is dominated by REM sleep.Time in stage 1 or 2 sleep usually does not differ between early and late sleep retention conditions.The approach thus allows for comparing the effects of sleep rich in SWS versus REM sleep, elegantly avoiding possible confounding effects resulting from stressful repeated awakenings accompanying standard procedures of selective sleep deprivation.To control possible confounds of the circadian rhythm, the effects of early and late sleep retention periods are typically compared additionally with the effects of wake retention periods that cover the same early and late phases of the night.These two wake control conditions are also necessary to ensure that depth of encoding during the learning phase is comparable between the conditions, because prior sleep is known to influence encoding capabilities (328,342,486,515,548,1190,1232,1345), and learning is preceded by prior sleep only in the late sleep condition, but not in the early sleep condition. Yarush et al. (1340) were the first to report a beneficial effect of SWS-rich early sleep on declarative memory (word pairs), compared with retention performance across a REM sleep-rich late sleep or across corresponding wake intervals, and these findings were replicated in a later study of the same group (408).A similar benefit for declarative (pairedassociates) memories was revealed when controlling for circadian influences by placing SWS-rich and REM sleep-rich sleep periods at the same circadian phase, between 3 and 7 a.m.(63).While these studies quite compellingly showed that declarative memory for neutral materials is enhanced by SWS, emotional declarative memories appear to additionally benefit from REM-rich late sleep (1273, 1274).Building on these early studies, Plihal and Born (930, 931) not only demonstrated a benefit for declarative memories (word pairs, spatial information) from early SWS-rich sleep, but also demonstrated that late REM sleep-rich retention sleep selectively improved procedural and im-plicit memories (mirror tracing skill, word-stem priming), compared with corresponding wake-retention intervals.Later studies replicated beneficial effects of late REM sleep-rich sleep on implicit memories (e.g., faces, masked stimuli) (1248, 1277), and altogether these findings fit well with the notion of an involvement of REM sleep in procedural and implicit memory processes as revealed by standard REM sleep deprivation procedures (described in sect.IIB). Improving the effects of early SWS-rich sleep on declarative memory appeared to be overall more consistent with free (or cued) recall measures than with recognition measures of memory (265, 326, 667, 966; see Ref. 297, for a review), possibly reflecting that unlike recollection, familiarity based recognition taps to a greater extent implicit components of these memories (1343,1344).Two studies found enhanced recollection (of words) after early, SWS-rich sleep, compared with a late REM-rich sleep or wake intervals, while familiarity-based recognition measures remained unaffected (265,326).In a third study measuring recognition right after free recall, this effect was not replicated, although SWS-rich sleep still enhanced free recall of the declarative materials (966). Overall, studies using the night-half paradigm have provided substantial evidence for the dual processes hypothesis, such as hippocampus-dependent declarative memories preferentially profiting from SWS, whereas nondeclarative aspects of memory, such as procedural, implicit, and emotional, additionally profiting from REM sleep (121, 428).The hypothesis has been challenged by findings showing that procedural tasks like visuomotor adaptation and visual texture discrimination also benefit from SWS (10,433,581,582).However, training such skills does not proceed entirely detached from declarative memory mechanisms, especially at an initial stage of training (e.g., Refs. 198, 533, 1013Refs. 198, 533, , 1014Refs. 198, 533, , 1042)).Such declarative components might have mediated the strong benefits for skills from SWS in those studies (910). A major weakness of the night-half paradigm is that it ignores possible contributions of stage 2 sleep to memory.Although the amounts of stage 2 sleep were comparable in the early and late sleep conditions of the studies reported above, sleep in this sleep stage may have substantially differed between the two phases, for example, with regard to spindle density (440, 441, 550), heart rate, or levels of neuromodulators like catecholamines and cortisol (95,124,131,313,554,960).There is consistent evidence for an involvement of stage 2 sleep and sleep spindles in motor learning (139,396,822,862,921), and memory for a simple motor task is impaired after selective stage 2 sleep deprivation (1112, 1117).Training on motor tasks increased the time spent in stage 2 and the density of fast (but not slow) spindles (399, 837, 920, 921, 1173, 1174), and sleep stage 2 duration and fast spindle density also predicted sleep-dependent improvements in a finger tapping task (59,961,1285,1286).Based on these findings, Smith and coworkers (846, 1114) proposed that especially simple motor tasks require stage 2 sleep, whereas complex motor tasks may require REM sleep.However, sleep stage 2 spindle activity and spindle counts were found to similarly correlate with the overnight retention of verbal and visuospatial memories, suggesting that spindles and stage 2 sleep are also involved in declarative memory formation (213,214,1016).

It may state two separate claims as one.

The cited sources support only part of this claim — read each source’s verdict below.

§ About sleep's role in memory > D. The Dual Process Hypothesis The dual processes hypothesis assumes that different sleep stages serve the consolidation of different types of memories (428, 765, 967, 1096).Specifically, it has been assumed that declarative memory profits from SWS, whereas the consolidation of nondeclarative memory is supported by REM sleep.The hypothesis received support mainly from studies in humans, particularly from those employing the "nighthalf paradigm."This paradigm, originally developed by Ekstrand and co-workers (341, 408, 1340), basically compares retention performance across retention intervals that cover either the early or late half of nocturnal sleep.Whereas in the early sleep condition, participants learn (to criterion) the memory tasks in the evening and then sleep for 3-4 h before a later recall test, in the late sleep condition, participants first sleep for ϳ3 h (to satisfy the need of SWS) and then are subjected to the learning phase, followed by the late night retention sleep.Due to the circadian rhythm, early nocturnal sleep contains most of SWS, whereas late nocturnal sleep is dominated by REM sleep.Time in stage 1 or 2 sleep usually does not differ between early and late sleep retention conditions.The approach thus allows for comparing the effects of sleep rich in SWS versus REM sleep, elegantly avoiding possible confounding effects resulting from stressful repeated awakenings accompanying standard procedures of selective sleep deprivation.To control possible confounds of the circadian rhythm, the effects of early and late sleep retention periods are typically compared additionally with the effects of wake retention periods that cover the same early and late phases of the night.These two wake control conditions are also necessary to ensure that depth of encoding during the learning phase is comparable between the conditions, because prior sleep is known to influence encoding capabilities (328,342,486,515,548,1190,1232,1345), and learning is preceded by prior sleep only in the late sleep condition, but not in the early sleep condition. Yarush et al. (1340) were the first to report a beneficial effect of SWS-rich early sleep on declarative memory (word pairs), compared with retention performance across a REM sleep-rich late sleep or across corresponding wake intervals, and these findings were replicated in a later study of the same group (408).A similar benefit for declarative (pairedassociates) memories was revealed when controlling for circadian influences by placing SWS-rich and REM sleep-rich sleep periods at the same circadian phase, between 3 and 7 a.m.(63).While these studies quite compellingly showed that declarative memory for neutral materials is enhanced by SWS, emotional declarative memories appear to additionally benefit from REM-rich late sleep (1273, 1274).Building on these early studies, Plihal and Born (930, 931) not only demonstrated a benefit for declarative memories (word pairs, spatial information) from early SWS-rich sleep, but also demonstrated that late REM sleep-rich retention sleep selectively improved procedural and im-plicit memories (mirror tracing skill, word-stem priming), compared with corresponding wake-retention intervals.Later studies replicated beneficial effects of late REM sleep-rich sleep on implicit memories (e.g., faces, masked stimuli) (1248, 1277), and altogether these findings fit well with the notion of an involvement of REM sleep in procedural and implicit memory processes as revealed by standard REM sleep deprivation procedures (described in sect.IIB). Improving the effects of early SWS-rich sleep on declarative memory appeared to be overall more consistent with free (or cued) recall measures than with recognition measures of memory (265, 326, 667, 966; see Ref. 297, for a review), possibly reflecting that unlike recollection, familiarity based recognition taps to a greater extent implicit components of these memories (1343,1344).Two studies found enhanced recollection (of words) after early, SWS-rich sleep, compared with a late REM-rich sleep or wake intervals, while familiarity-based recognition measures remained unaffected (265,326).In a third study measuring recognition right after free recall, this effect was not replicated, although SWS-rich sleep still enhanced free recall of the declarative materials (966). Overall, studies using the night-half paradigm have provided substantial evidence for the dual processes hypothesis, such as hippocampus-dependent declarative memories preferentially profiting from SWS, whereas nondeclarative aspects of memory, such as procedural, implicit, and emotional, additionally profiting from REM sleep (121, 428).The hypothesis has been challenged by findings showing that procedural tasks like visuomotor adaptation and visual texture discrimination also benefit from SWS (10,433,581,582).However, training such skills does not proceed entirely detached from declarative memory mechanisms, especially at an initial stage of training (e.g., Refs. 198, 533, 1013Refs. 198, 533, , 1014Refs. 198, 533, , 1042)).Such declarative components might have mediated the strong benefits for skills from SWS in those studies (910). A major weakness of the night-half paradigm is that it ignores possible contributions of stage 2 sleep to memory.Although the amounts of stage 2 sleep were comparable in the early and late sleep conditions of the studies reported above, sleep in this sleep stage may have substantially differed between the two phases, for example, with regard to spindle density (440, 441, 550), heart rate, or levels of neuromodulators like catecholamines and cortisol (95,124,131,313,554,960).There is consistent evidence for an involvement of stage 2 sleep and sleep spindles in motor learning (139,396,822,862,921), and memory for a simple motor task is impaired after selective stage 2 sleep deprivation (1112, 1117).Training on motor tasks increased the time spent in stage 2 and the density of fast (but not slow) spindles (399, 837, 920, 921, 1173, 1174), and sleep stage 2 duration and fast spindle density also predicted sleep-dependent improvements in a finger tapping task (59,961,1285,1286).Based on these findings, Smith and coworkers (846, 1114) proposed that especially simple motor tasks require stage 2 sleep, whereas complex motor tasks may require REM sleep.However, sleep stage 2 spindle activity and spindle counts were found to similarly correlate with the overnight retention of verbal and visuospatial memories, suggesting that spindles and stage 2 sleep are also involved in declarative memory formation (213,214,1016).

It may state two separate claims as one.

The cited sources that could be checked couldn’t confirm or dispute this claim.

§ About sleep's role in memory > D. The Dual Process Hypothesis The dual processes hypothesis assumes that different sleep stages serve the consolidation of different types of memories (428, 765, 967, 1096).Specifically, it has been assumed that declarative memory profits from SWS, whereas the consolidation of nondeclarative memory is supported by REM sleep.The hypothesis received support mainly from studies in humans, particularly from those employing the "nighthalf paradigm."This paradigm, originally developed by Ekstrand and co-workers (341, 408, 1340), basically compares retention performance across retention intervals that cover either the early or late half of nocturnal sleep.Whereas in the early sleep condition, participants learn (to criterion) the memory tasks in the evening and then sleep for 3-4 h before a later recall test, in the late sleep condition, participants first sleep for ϳ3 h (to satisfy the need of SWS) and then are subjected to the learning phase, followed by the late night retention sleep.Due to the circadian rhythm, early nocturnal sleep contains most of SWS, whereas late nocturnal sleep is dominated by REM sleep.Time in stage 1 or 2 sleep usually does not differ between early and late sleep retention conditions.The approach thus allows for comparing the effects of sleep rich in SWS versus REM sleep, elegantly avoiding possible confounding effects resulting from stressful repeated awakenings accompanying standard procedures of selective sleep deprivation.To control possible confounds of the circadian rhythm, the effects of early and late sleep retention periods are typically compared additionally with the effects of wake retention periods that cover the same early and late phases of the night.These two wake control conditions are also necessary to ensure that depth of encoding during the learning phase is comparable between the conditions, because prior sleep is known to influence encoding capabilities (328,342,486,515,548,1190,1232,1345), and learning is preceded by prior sleep only in the late sleep condition, but not in the early sleep condition. Yarush et al. (1340) were the first to report a beneficial effect of SWS-rich early sleep on declarative memory (word pairs), compared with retention performance across a REM sleep-rich late sleep or across corresponding wake intervals, and these findings were replicated in a later study of the same group (408).A similar benefit for declarative (pairedassociates) memories was revealed when controlling for circadian influences by placing SWS-rich and REM sleep-rich sleep periods at the same circadian phase, between 3 and 7 a.m.(63).While these studies quite compellingly showed that declarative memory for neutral materials is enhanced by SWS, emotional declarative memories appear to additionally benefit from REM-rich late sleep (1273, 1274).Building on these early studies, Plihal and Born (930, 931) not only demonstrated a benefit for declarative memories (word pairs, spatial information) from early SWS-rich sleep, but also demonstrated that late REM sleep-rich retention sleep selectively improved procedural and im-plicit memories (mirror tracing skill, word-stem priming), compared with corresponding wake-retention intervals.Later studies replicated beneficial effects of late REM sleep-rich sleep on implicit memories (e.g., faces, masked stimuli) (1248, 1277), and altogether these findings fit well with the notion of an involvement of REM sleep in procedural and implicit memory processes as revealed by standard REM sleep deprivation procedures (described in sect.IIB). Improving the effects of early SWS-rich sleep on declarative memory appeared to be overall more consistent with free (or cued) recall measures than with recognition measures of memory (265, 326, 667, 966; see Ref. 297, for a review), possibly reflecting that unlike recollection, familiarity based recognition taps to a greater extent implicit components of these memories (1343,1344).Two studies found enhanced recollection (of words) after early, SWS-rich sleep, compared with a late REM-rich sleep or wake intervals, while familiarity-based recognition measures remained unaffected (265,326).In a third study measuring recognition right after free recall, this effect was not replicated, although SWS-rich sleep still enhanced free recall of the declarative materials (966). Overall, studies using the night-half paradigm have provided substantial evidence for the dual processes hypothesis, such as hippocampus-dependent declarative memories preferentially profiting from SWS, whereas nondeclarative aspects of memory, such as procedural, implicit, and emotional, additionally profiting from REM sleep (121, 428).The hypothesis has been challenged by findings showing that procedural tasks like visuomotor adaptation and visual texture discrimination also benefit from SWS (10,433,581,582).However, training such skills does not proceed entirely detached from declarative memory mechanisms, especially at an initial stage of training (e.g., Refs. 198, 533, 1013Refs. 198, 533, , 1014Refs. 198, 533, , 1042)).Such declarative components might have mediated the strong benefits for skills from SWS in those studies (910). A major weakness of the night-half paradigm is that it ignores possible contributions of stage 2 sleep to memory.Although the amounts of stage 2 sleep were comparable in the early and late sleep conditions of the studies reported above, sleep in this sleep stage may have substantially differed between the two phases, for example, with regard to spindle density (440, 441, 550), heart rate, or levels of neuromodulators like catecholamines and cortisol (95,124,131,313,554,960).There is consistent evidence for an involvement of stage 2 sleep and sleep spindles in motor learning (139,396,822,862,921), and memory for a simple motor task is impaired after selective stage 2 sleep deprivation (1112, 1117).Training on motor tasks increased the time spent in stage 2 and the density of fast (but not slow) spindles (399, 837, 920, 921, 1173, 1174), and sleep stage 2 duration and fast spindle density also predicted sleep-dependent improvements in a finger tapping task (59,961,1285,1286).Based on these findings, Smith and coworkers (846, 1114) proposed that especially simple motor tasks require stage 2 sleep, whereas complex motor tasks may require REM sleep.However, sleep stage 2 spindle activity and spindle counts were found to similarly correlate with the overnight retention of verbal and visuospatial memories, suggesting that spindles and stage 2 sleep are also involved in declarative memory formation (213,214,1016).

Probably not a substantial claim. The paper’s conclusions may not depend on it.

Every source the paper cites for this claim was checked and supports it.

§ About sleep's role in memory > 1. Reorganization of procedural and declarative memories during sleep With regard to procedural memory, findings that sleep after training of perceptual and motor skills like visual texture discrimination and finger sequence tapping can produce significant improvement (i.e., a gain in skill at a later retesting) have pointed towards an active reprocessing of skill representations occurring during sleep that sharpens the respective representations (47,57,147,274,275,291,322,383,384,433,582,622,631,664,665,679,762,770,808,810,863,930,1003,1153,1157,1286,1287,1290,1318).However, sleep-dependent gains may not equally occur for all types of skills (e.g., Refs. 221, 1002Refs. 221, , 1015)).Moreover, recent studies have shown that such gains in skill that are typically measured with reference to the performance level at the end of training, can occur also within a few hours after training in the absence of sleep, and may thus partly reflect a recovery process that is independent of sleep (364,573,(1002)(1003)(1004).Furthermore, some studies reported that no sleep-dependent gains in procedural motor tasks occur when circadian and homeostatic influences (e.g., time of day, time since sleep) are controlled (164, 991, 995).Also, a mere gain, for example, of speed in finger sequence tapping could be explained solely on the basis of synaptic consolidation processes that strengthen connections formed during training without reorganizing the memory representation.However, convincing behavioral clues for a reorganization of skill representations during sleep come from investigations of sequence-finger tapping skills that show that sleep favors the emergence of an effector independent representation, i.e., sleep benefited pressing the sequence of target keys independent of whether the sequence was tapped with the right or left hand, whereas the sequenced tapping movements per se appeared to benefit also from a wake retention interval (218, 1330).Addi-tionally, sleep enhanced sequence-finger tapping performance when learning occurred by observation (1234) or motor imagery (274), a further hint towards qualitative changes in the skill representations induced by sleep. In connection with declarative memory processes, evidence for an active consolidation process during sleep that leads to a qualitative reorganization of memory representations has been provided by studies showing that sleep preferentially supports memory for the "gist" in the learned material, thereby supporting processes of abstraction, inference, and insight.Thus, in adults and children, sleep promoted the integration of newly learned spoken words into existing knowledge networks as measured by a lexical competition task (333, 334, 530; but see Ref. 735), with this effect being associated with increased spindle activity (1175).Sleep likewise promoted grammar-related abstraction processes in language learning tasks in infants (469, 589).When participants learned a hierarchy of pairwise presented elements (e.g., A Ͼ B and D Ͼ E), sleep after learning improved the ability to infer the correct relation between the most distant elements (A Ͼ E) (345).Comparable results were obtained after a nap using relations between different faces paired with the same object, whereby the amount of SWS correlated with the accuracy of relational memory (705,706).Sleep also increased the production of false memories in the Deese-Roediger-McDermott (DRM) paradigm (1007), in which in the learning phase participants listened to lists of semantically related words (e.g., nurse, patient, hospital, sick, medicine, etc.), whereas the semantic "topic" of the list is not presented (the "critical lure," e.g., doctor).Compared with wake retention conditions, after sleep subjects showed a greater tendency to "falsely" recall the critical lure, in line with the notion that sleep promotes processes of abstraction and the extraction of the gist information from the list (294, 897).However, the effect of sleep in the DRM paradigm was less consistent when recall of the critical lure was tested with a recognition procedure rather than by free recall (255, 296, 373; see Ref. 1159 for an overview). Using the Number Reduction Task (1189), Wagner et al. (1275) showed that sleep facilitates the gain of insight, i.e., explicit knowledge of a hidden structure that was embedded in strings of digits which the subjects had to process before sleep.Subsequent studies of the same group specified that the gain of insight depended on the occurrence of spindle activity during early SWS-rich nocturnal sleep (1346 -1350).Very similar results were obtained with an approach combining the classical Serial Reaction Time Task (SRTT) with a so-called "generation task."On the SRTT, the subject is trained to press as fast and as accurately as possible different keys corresponding to the changing positions of a cue.Unknown to the subject, the changes in the cue position follow a repeating sequence.Typically, during training subjects acquire an implicit knowledge of this sequence as indicated by faster responses to cue positions that follow the sequence compared with responses to random positions.Training does not lead to an immediate formation of explicit sequence knowledge, as the subjects typically cannot deliberately reproduce the SRTT sequence, when explicitly asked to do so right after training (in a generation task).Yet, significant explicit sequence knowledge developed when SRTT training was followed by nocturnal sleep (324,382).Generation task performance in the wake control group in these studies remained at chance level.The sleepinduced extraction of explicit sequence knowledge from an implicitly trained SRTT was particularly pronounced in children, where this extraction process was correlated with enhanced EEG slow oscillation activity during posttraining sleep (1321).Similarly, in adults performing on a statistical learning task, sleep promoted the abstraction of probabilistic regularities in tone sequences, with this effect being associated with increased SWS during postlearning sleep (337).Complementary evidence for a neuronal reorganization of memory representations by sleep was provided by studies imaging brain activation during learning and retrieval.In experiments by Takashima et al. (1170), participants napped for 90 min after studying pictures.Recognition of the pictures was tested 1, 30, and 90 days later.Activation of the hippocampus gradually decreased over time, whereas cortical activation in ventromedial prefrontal areas increased.The duration of SWS during the nap predicted the reduction in hippocampal activity and was also associated with better recognition performance on day 1.A second study of this group confirmed that one night of sleep decreases hippocampal activation and increases activation in neocortex areas (1169).In another study (427), the sleepinduced improvement in word pair memories, at a retrieval test 2 days later, was accompanied by an increased functional connectivity between the hippocampus and the medial prefrontal cortex, although contrary to Takashima et al.'s findings, hippocampal activation per se was enhanced at recall in the sleep group.At a follow-up test 6 mo later, the sleep group exhibited increased cortical activation compared with the subjects who stayed awake on the night after word pair learning.Sleep following learning spatial memories of a virtual town produced increased activation in the striatum at a recall test 3 days later, when compared with a group deprived of sleep after learning (873).Activation in the right striatum positively predicted way finding in the virtual town only in the sleep group, and also functional connectivity between the striatum and hippocampus was modulated by sleep after learning.Similarly, the sleep-enhancing effect of statistical learning (337) was accompanied by a shift in brain activation from the medial temporal lobe to the striatum (336).A sleep-induced reorganization of neuronal representations was also revealed for emotional memories (pictures) after 1 night (896) or 3 days after learning (1147).In the later study, the reorganization led to increased hippocampal and cortical activation as well as increased connectivity between the medial frontal cortical and hippocampal areas, compared with a control group, which stayed awake on the night after learning.Some of the effects of sleep on brain activation during recall of emotional memories were still observed 6 mo later (1148).Finally, sleep-dependent changes in brain activity indicative for a reorganization of memory representations have also been reported for procedural tasks like sequences finger tapping and visual texture discrimination (384, 1292, 1293).

It may state two separate claims as one.

Every source the paper cites for this claim was checked and supports it.

§ About sleep's role in memory > 1. Reorganization of procedural and declarative memories during sleep With regard to procedural memory, findings that sleep after training of perceptual and motor skills like visual texture discrimination and finger sequence tapping can produce significant improvement (i.e., a gain in skill at a later retesting) have pointed towards an active reprocessing of skill representations occurring during sleep that sharpens the respective representations (47,57,147,274,275,291,322,383,384,433,582,622,631,664,665,679,762,770,808,810,863,930,1003,1153,1157,1286,1287,1290,1318).However, sleep-dependent gains may not equally occur for all types of skills (e.g., Refs. 221, 1002Refs. 221, , 1015)).Moreover, recent studies have shown that such gains in skill that are typically measured with reference to the performance level at the end of training, can occur also within a few hours after training in the absence of sleep, and may thus partly reflect a recovery process that is independent of sleep (364,573,(1002)(1003)(1004).Furthermore, some studies reported that no sleep-dependent gains in procedural motor tasks occur when circadian and homeostatic influences (e.g., time of day, time since sleep) are controlled (164, 991, 995).Also, a mere gain, for example, of speed in finger sequence tapping could be explained solely on the basis of synaptic consolidation processes that strengthen connections formed during training without reorganizing the memory representation.However, convincing behavioral clues for a reorganization of skill representations during sleep come from investigations of sequence-finger tapping skills that show that sleep favors the emergence of an effector independent representation, i.e., sleep benefited pressing the sequence of target keys independent of whether the sequence was tapped with the right or left hand, whereas the sequenced tapping movements per se appeared to benefit also from a wake retention interval (218, 1330).Addi-tionally, sleep enhanced sequence-finger tapping performance when learning occurred by observation (1234) or motor imagery (274), a further hint towards qualitative changes in the skill representations induced by sleep. In connection with declarative memory processes, evidence for an active consolidation process during sleep that leads to a qualitative reorganization of memory representations has been provided by studies showing that sleep preferentially supports memory for the "gist" in the learned material, thereby supporting processes of abstraction, inference, and insight.Thus, in adults and children, sleep promoted the integration of newly learned spoken words into existing knowledge networks as measured by a lexical competition task (333, 334, 530; but see Ref. 735), with this effect being associated with increased spindle activity (1175).Sleep likewise promoted grammar-related abstraction processes in language learning tasks in infants (469, 589).When participants learned a hierarchy of pairwise presented elements (e.g., A Ͼ B and D Ͼ E), sleep after learning improved the ability to infer the correct relation between the most distant elements (A Ͼ E) (345).Comparable results were obtained after a nap using relations between different faces paired with the same object, whereby the amount of SWS correlated with the accuracy of relational memory (705,706).Sleep also increased the production of false memories in the Deese-Roediger-McDermott (DRM) paradigm (1007), in which in the learning phase participants listened to lists of semantically related words (e.g., nurse, patient, hospital, sick, medicine, etc.), whereas the semantic "topic" of the list is not presented (the "critical lure," e.g., doctor).Compared with wake retention conditions, after sleep subjects showed a greater tendency to "falsely" recall the critical lure, in line with the notion that sleep promotes processes of abstraction and the extraction of the gist information from the list (294, 897).However, the effect of sleep in the DRM paradigm was less consistent when recall of the critical lure was tested with a recognition procedure rather than by free recall (255, 296, 373; see Ref. 1159 for an overview). Using the Number Reduction Task (1189), Wagner et al. (1275) showed that sleep facilitates the gain of insight, i.e., explicit knowledge of a hidden structure that was embedded in strings of digits which the subjects had to process before sleep.Subsequent studies of the same group specified that the gain of insight depended on the occurrence of spindle activity during early SWS-rich nocturnal sleep (1346 -1350).Very similar results were obtained with an approach combining the classical Serial Reaction Time Task (SRTT) with a so-called "generation task."On the SRTT, the subject is trained to press as fast and as accurately as possible different keys corresponding to the changing positions of a cue.Unknown to the subject, the changes in the cue position follow a repeating sequence.Typically, during training subjects acquire an implicit knowledge of this sequence as indicated by faster responses to cue positions that follow the sequence compared with responses to random positions.Training does not lead to an immediate formation of explicit sequence knowledge, as the subjects typically cannot deliberately reproduce the SRTT sequence, when explicitly asked to do so right after training (in a generation task).Yet, significant explicit sequence knowledge developed when SRTT training was followed by nocturnal sleep (324,382).Generation task performance in the wake control group in these studies remained at chance level.The sleepinduced extraction of explicit sequence knowledge from an implicitly trained SRTT was particularly pronounced in children, where this extraction process was correlated with enhanced EEG slow oscillation activity during posttraining sleep (1321).Similarly, in adults performing on a statistical learning task, sleep promoted the abstraction of probabilistic regularities in tone sequences, with this effect being associated with increased SWS during postlearning sleep (337).Complementary evidence for a neuronal reorganization of memory representations by sleep was provided by studies imaging brain activation during learning and retrieval.In experiments by Takashima et al. (1170), participants napped for 90 min after studying pictures.Recognition of the pictures was tested 1, 30, and 90 days later.Activation of the hippocampus gradually decreased over time, whereas cortical activation in ventromedial prefrontal areas increased.The duration of SWS during the nap predicted the reduction in hippocampal activity and was also associated with better recognition performance on day 1.A second study of this group confirmed that one night of sleep decreases hippocampal activation and increases activation in neocortex areas (1169).In another study (427), the sleepinduced improvement in word pair memories, at a retrieval test 2 days later, was accompanied by an increased functional connectivity between the hippocampus and the medial prefrontal cortex, although contrary to Takashima et al.'s findings, hippocampal activation per se was enhanced at recall in the sleep group.At a follow-up test 6 mo later, the sleep group exhibited increased cortical activation compared with the subjects who stayed awake on the night after word pair learning.Sleep following learning spatial memories of a virtual town produced increased activation in the striatum at a recall test 3 days later, when compared with a group deprived of sleep after learning (873).Activation in the right striatum positively predicted way finding in the virtual town only in the sleep group, and also functional connectivity between the striatum and hippocampus was modulated by sleep after learning.Similarly, the sleep-enhancing effect of statistical learning (337) was accompanied by a shift in brain activation from the medial temporal lobe to the striatum (336).A sleep-induced reorganization of neuronal representations was also revealed for emotional memories (pictures) after 1 night (896) or 3 days after learning (1147).In the later study, the reorganization led to increased hippocampal and cortical activation as well as increased connectivity between the medial frontal cortical and hippocampal areas, compared with a control group, which stayed awake on the night after learning.Some of the effects of sleep on brain activation during recall of emotional memories were still observed 6 mo later (1148).Finally, sleep-dependent changes in brain activity indicative for a reorganization of memory representations have also been reported for procedural tasks like sequences finger tapping and visual texture discrimination (384, 1292, 1293).

It may state two separate claims as one.

Every source the paper cites for this claim was checked and supports it.

§ About sleep's role in memory > 1. Reorganization of procedural and declarative memories during sleep With regard to procedural memory, findings that sleep after training of perceptual and motor skills like visual texture discrimination and finger sequence tapping can produce significant improvement (i.e., a gain in skill at a later retesting) have pointed towards an active reprocessing of skill representations occurring during sleep that sharpens the respective representations (47,57,147,274,275,291,322,383,384,433,582,622,631,664,665,679,762,770,808,810,863,930,1003,1153,1157,1286,1287,1290,1318).However, sleep-dependent gains may not equally occur for all types of skills (e.g., Refs. 221, 1002Refs. 221, , 1015)).Moreover, recent studies have shown that such gains in skill that are typically measured with reference to the performance level at the end of training, can occur also within a few hours after training in the absence of sleep, and may thus partly reflect a recovery process that is independent of sleep (364,573,(1002)(1003)(1004).Furthermore, some studies reported that no sleep-dependent gains in procedural motor tasks occur when circadian and homeostatic influences (e.g., time of day, time since sleep) are controlled (164, 991, 995).Also, a mere gain, for example, of speed in finger sequence tapping could be explained solely on the basis of synaptic consolidation processes that strengthen connections formed during training without reorganizing the memory representation.However, convincing behavioral clues for a reorganization of skill representations during sleep come from investigations of sequence-finger tapping skills that show that sleep favors the emergence of an effector independent representation, i.e., sleep benefited pressing the sequence of target keys independent of whether the sequence was tapped with the right or left hand, whereas the sequenced tapping movements per se appeared to benefit also from a wake retention interval (218, 1330).Addi-tionally, sleep enhanced sequence-finger tapping performance when learning occurred by observation (1234) or motor imagery (274), a further hint towards qualitative changes in the skill representations induced by sleep. In connection with declarative memory processes, evidence for an active consolidation process during sleep that leads to a qualitative reorganization of memory representations has been provided by studies showing that sleep preferentially supports memory for the "gist" in the learned material, thereby supporting processes of abstraction, inference, and insight.Thus, in adults and children, sleep promoted the integration of newly learned spoken words into existing knowledge networks as measured by a lexical competition task (333, 334, 530; but see Ref. 735), with this effect being associated with increased spindle activity (1175).Sleep likewise promoted grammar-related abstraction processes in language learning tasks in infants (469, 589).When participants learned a hierarchy of pairwise presented elements (e.g., A Ͼ B and D Ͼ E), sleep after learning improved the ability to infer the correct relation between the most distant elements (A Ͼ E) (345).Comparable results were obtained after a nap using relations between different faces paired with the same object, whereby the amount of SWS correlated with the accuracy of relational memory (705,706).Sleep also increased the production of false memories in the Deese-Roediger-McDermott (DRM) paradigm (1007), in which in the learning phase participants listened to lists of semantically related words (e.g., nurse, patient, hospital, sick, medicine, etc.), whereas the semantic "topic" of the list is not presented (the "critical lure," e.g., doctor).Compared with wake retention conditions, after sleep subjects showed a greater tendency to "falsely" recall the critical lure, in line with the notion that sleep promotes processes of abstraction and the extraction of the gist information from the list (294, 897).However, the effect of sleep in the DRM paradigm was less consistent when recall of the critical lure was tested with a recognition procedure rather than by free recall (255, 296, 373; see Ref. 1159 for an overview). Using the Number Reduction Task (1189), Wagner et al. (1275) showed that sleep facilitates the gain of insight, i.e., explicit knowledge of a hidden structure that was embedded in strings of digits which the subjects had to process before sleep.Subsequent studies of the same group specified that the gain of insight depended on the occurrence of spindle activity during early SWS-rich nocturnal sleep (1346 -1350).Very similar results were obtained with an approach combining the classical Serial Reaction Time Task (SRTT) with a so-called "generation task."On the SRTT, the subject is trained to press as fast and as accurately as possible different keys corresponding to the changing positions of a cue.Unknown to the subject, the changes in the cue position follow a repeating sequence.Typically, during training subjects acquire an implicit knowledge of this sequence as indicated by faster responses to cue positions that follow the sequence compared with responses to random positions.Training does not lead to an immediate formation of explicit sequence knowledge, as the subjects typically cannot deliberately reproduce the SRTT sequence, when explicitly asked to do so right after training (in a generation task).Yet, significant explicit sequence knowledge developed when SRTT training was followed by nocturnal sleep (324,382).Generation task performance in the wake control group in these studies remained at chance level.The sleepinduced extraction of explicit sequence knowledge from an implicitly trained SRTT was particularly pronounced in children, where this extraction process was correlated with enhanced EEG slow oscillation activity during posttraining sleep (1321).Similarly, in adults performing on a statistical learning task, sleep promoted the abstraction of probabilistic regularities in tone sequences, with this effect being associated with increased SWS during postlearning sleep (337).Complementary evidence for a neuronal reorganization of memory representations by sleep was provided by studies imaging brain activation during learning and retrieval.In experiments by Takashima et al. (1170), participants napped for 90 min after studying pictures.Recognition of the pictures was tested 1, 30, and 90 days later.Activation of the hippocampus gradually decreased over time, whereas cortical activation in ventromedial prefrontal areas increased.The duration of SWS during the nap predicted the reduction in hippocampal activity and was also associated with better recognition performance on day 1.A second study of this group confirmed that one night of sleep decreases hippocampal activation and increases activation in neocortex areas (1169).In another study (427), the sleepinduced improvement in word pair memories, at a retrieval test 2 days later, was accompanied by an increased functional connectivity between the hippocampus and the medial prefrontal cortex, although contrary to Takashima et al.'s findings, hippocampal activation per se was enhanced at recall in the sleep group.At a follow-up test 6 mo later, the sleep group exhibited increased cortical activation compared with the subjects who stayed awake on the night after word pair learning.Sleep following learning spatial memories of a virtual town produced increased activation in the striatum at a recall test 3 days later, when compared with a group deprived of sleep after learning (873).Activation in the right striatum positively predicted way finding in the virtual town only in the sleep group, and also functional connectivity between the striatum and hippocampus was modulated by sleep after learning.Similarly, the sleep-enhancing effect of statistical learning (337) was accompanied by a shift in brain activation from the medial temporal lobe to the striatum (336).A sleep-induced reorganization of neuronal representations was also revealed for emotional memories (pictures) after 1 night (896) or 3 days after learning (1147).In the later study, the reorganization led to increased hippocampal and cortical activation as well as increased connectivity between the medial frontal cortical and hippocampal areas, compared with a control group, which stayed awake on the night after learning.Some of the effects of sleep on brain activation during recall of emotional memories were still observed 6 mo later (1148).Finally, sleep-dependent changes in brain activity indicative for a reorganization of memory representations have also been reported for procedural tasks like sequences finger tapping and visual texture discrimination (384, 1292, 1293).

It may state two separate claims as one.

Every source the paper cites for this claim was checked and supports it.

§ About sleep's role in memory > 1. Reorganization of procedural and declarative memories during sleep With regard to procedural memory, findings that sleep after training of perceptual and motor skills like visual texture discrimination and finger sequence tapping can produce significant improvement (i.e., a gain in skill at a later retesting) have pointed towards an active reprocessing of skill representations occurring during sleep that sharpens the respective representations (47,57,147,274,275,291,322,383,384,433,582,622,631,664,665,679,762,770,808,810,863,930,1003,1153,1157,1286,1287,1290,1318).However, sleep-dependent gains may not equally occur for all types of skills (e.g., Refs. 221, 1002Refs. 221, , 1015)).Moreover, recent studies have shown that such gains in skill that are typically measured with reference to the performance level at the end of training, can occur also within a few hours after training in the absence of sleep, and may thus partly reflect a recovery process that is independent of sleep (364,573,(1002)(1003)(1004).Furthermore, some studies reported that no sleep-dependent gains in procedural motor tasks occur when circadian and homeostatic influences (e.g., time of day, time since sleep) are controlled (164, 991, 995).Also, a mere gain, for example, of speed in finger sequence tapping could be explained solely on the basis of synaptic consolidation processes that strengthen connections formed during training without reorganizing the memory representation.However, convincing behavioral clues for a reorganization of skill representations during sleep come from investigations of sequence-finger tapping skills that show that sleep favors the emergence of an effector independent representation, i.e., sleep benefited pressing the sequence of target keys independent of whether the sequence was tapped with the right or left hand, whereas the sequenced tapping movements per se appeared to benefit also from a wake retention interval (218, 1330).Addi-tionally, sleep enhanced sequence-finger tapping performance when learning occurred by observation (1234) or motor imagery (274), a further hint towards qualitative changes in the skill representations induced by sleep. In connection with declarative memory processes, evidence for an active consolidation process during sleep that leads to a qualitative reorganization of memory representations has been provided by studies showing that sleep preferentially supports memory for the "gist" in the learned material, thereby supporting processes of abstraction, inference, and insight.Thus, in adults and children, sleep promoted the integration of newly learned spoken words into existing knowledge networks as measured by a lexical competition task (333, 334, 530; but see Ref. 735), with this effect being associated with increased spindle activity (1175).Sleep likewise promoted grammar-related abstraction processes in language learning tasks in infants (469, 589).When participants learned a hierarchy of pairwise presented elements (e.g., A Ͼ B and D Ͼ E), sleep after learning improved the ability to infer the correct relation between the most distant elements (A Ͼ E) (345).Comparable results were obtained after a nap using relations between different faces paired with the same object, whereby the amount of SWS correlated with the accuracy of relational memory (705,706).Sleep also increased the production of false memories in the Deese-Roediger-McDermott (DRM) paradigm (1007), in which in the learning phase participants listened to lists of semantically related words (e.g., nurse, patient, hospital, sick, medicine, etc.), whereas the semantic "topic" of the list is not presented (the "critical lure," e.g., doctor).Compared with wake retention conditions, after sleep subjects showed a greater tendency to "falsely" recall the critical lure, in line with the notion that sleep promotes processes of abstraction and the extraction of the gist information from the list (294, 897).However, the effect of sleep in the DRM paradigm was less consistent when recall of the critical lure was tested with a recognition procedure rather than by free recall (255, 296, 373; see Ref. 1159 for an overview). Using the Number Reduction Task (1189), Wagner et al. (1275) showed that sleep facilitates the gain of insight, i.e., explicit knowledge of a hidden structure that was embedded in strings of digits which the subjects had to process before sleep.Subsequent studies of the same group specified that the gain of insight depended on the occurrence of spindle activity during early SWS-rich nocturnal sleep (1346 -1350).Very similar results were obtained with an approach combining the classical Serial Reaction Time Task (SRTT) with a so-called "generation task."On the SRTT, the subject is trained to press as fast and as accurately as possible different keys corresponding to the changing positions of a cue.Unknown to the subject, the changes in the cue position follow a repeating sequence.Typically, during training subjects acquire an implicit knowledge of this sequence as indicated by faster responses to cue positions that follow the sequence compared with responses to random positions.Training does not lead to an immediate formation of explicit sequence knowledge, as the subjects typically cannot deliberately reproduce the SRTT sequence, when explicitly asked to do so right after training (in a generation task).Yet, significant explicit sequence knowledge developed when SRTT training was followed by nocturnal sleep (324,382).Generation task performance in the wake control group in these studies remained at chance level.The sleepinduced extraction of explicit sequence knowledge from an implicitly trained SRTT was particularly pronounced in children, where this extraction process was correlated with enhanced EEG slow oscillation activity during posttraining sleep (1321).Similarly, in adults performing on a statistical learning task, sleep promoted the abstraction of probabilistic regularities in tone sequences, with this effect being associated with increased SWS during postlearning sleep (337).Complementary evidence for a neuronal reorganization of memory representations by sleep was provided by studies imaging brain activation during learning and retrieval.In experiments by Takashima et al. (1170), participants napped for 90 min after studying pictures.Recognition of the pictures was tested 1, 30, and 90 days later.Activation of the hippocampus gradually decreased over time, whereas cortical activation in ventromedial prefrontal areas increased.The duration of SWS during the nap predicted the reduction in hippocampal activity and was also associated with better recognition performance on day 1.A second study of this group confirmed that one night of sleep decreases hippocampal activation and increases activation in neocortex areas (1169).In another study (427), the sleepinduced improvement in word pair memories, at a retrieval test 2 days later, was accompanied by an increased functional connectivity between the hippocampus and the medial prefrontal cortex, although contrary to Takashima et al.'s findings, hippocampal activation per se was enhanced at recall in the sleep group.At a follow-up test 6 mo later, the sleep group exhibited increased cortical activation compared with the subjects who stayed awake on the night after word pair learning.Sleep following learning spatial memories of a virtual town produced increased activation in the striatum at a recall test 3 days later, when compared with a group deprived of sleep after learning (873).Activation in the right striatum positively predicted way finding in the virtual town only in the sleep group, and also functional connectivity between the striatum and hippocampus was modulated by sleep after learning.Similarly, the sleep-enhancing effect of statistical learning (337) was accompanied by a shift in brain activation from the medial temporal lobe to the striatum (336).A sleep-induced reorganization of neuronal representations was also revealed for emotional memories (pictures) after 1 night (896) or 3 days after learning (1147).In the later study, the reorganization led to increased hippocampal and cortical activation as well as increased connectivity between the medial frontal cortical and hippocampal areas, compared with a control group, which stayed awake on the night after learning.Some of the effects of sleep on brain activation during recall of emotional memories were still observed 6 mo later (1148).Finally, sleep-dependent changes in brain activity indicative for a reorganization of memory representations have also been reported for procedural tasks like sequences finger tapping and visual texture discrimination (384, 1292, 1293).

It may state two separate claims as one.

The cited sources support only part of this claim — read each source’s verdict below.

§ About sleep's role in memory > 1. Reorganization of procedural and declarative memories during sleep With regard to procedural memory, findings that sleep after training of perceptual and motor skills like visual texture discrimination and finger sequence tapping can produce significant improvement (i.e., a gain in skill at a later retesting) have pointed towards an active reprocessing of skill representations occurring during sleep that sharpens the respective representations (47,57,147,274,275,291,322,383,384,433,582,622,631,664,665,679,762,770,808,810,863,930,1003,1153,1157,1286,1287,1290,1318).However, sleep-dependent gains may not equally occur for all types of skills (e.g., Refs. 221, 1002Refs. 221, , 1015)).Moreover, recent studies have shown that such gains in skill that are typically measured with reference to the performance level at the end of training, can occur also within a few hours after training in the absence of sleep, and may thus partly reflect a recovery process that is independent of sleep (364,573,(1002)(1003)(1004).Furthermore, some studies reported that no sleep-dependent gains in procedural motor tasks occur when circadian and homeostatic influences (e.g., time of day, time since sleep) are controlled (164, 991, 995).Also, a mere gain, for example, of speed in finger sequence tapping could be explained solely on the basis of synaptic consolidation processes that strengthen connections formed during training without reorganizing the memory representation.However, convincing behavioral clues for a reorganization of skill representations during sleep come from investigations of sequence-finger tapping skills that show that sleep favors the emergence of an effector independent representation, i.e., sleep benefited pressing the sequence of target keys independent of whether the sequence was tapped with the right or left hand, whereas the sequenced tapping movements per se appeared to benefit also from a wake retention interval (218, 1330).Addi-tionally, sleep enhanced sequence-finger tapping performance when learning occurred by observation (1234) or motor imagery (274), a further hint towards qualitative changes in the skill representations induced by sleep. In connection with declarative memory processes, evidence for an active consolidation process during sleep that leads to a qualitative reorganization of memory representations has been provided by studies showing that sleep preferentially supports memory for the "gist" in the learned material, thereby supporting processes of abstraction, inference, and insight.Thus, in adults and children, sleep promoted the integration of newly learned spoken words into existing knowledge networks as measured by a lexical competition task (333, 334, 530; but see Ref. 735), with this effect being associated with increased spindle activity (1175).Sleep likewise promoted grammar-related abstraction processes in language learning tasks in infants (469, 589).When participants learned a hierarchy of pairwise presented elements (e.g., A Ͼ B and D Ͼ E), sleep after learning improved the ability to infer the correct relation between the most distant elements (A Ͼ E) (345).Comparable results were obtained after a nap using relations between different faces paired with the same object, whereby the amount of SWS correlated with the accuracy of relational memory (705,706).Sleep also increased the production of false memories in the Deese-Roediger-McDermott (DRM) paradigm (1007), in which in the learning phase participants listened to lists of semantically related words (e.g., nurse, patient, hospital, sick, medicine, etc.), whereas the semantic "topic" of the list is not presented (the "critical lure," e.g., doctor).Compared with wake retention conditions, after sleep subjects showed a greater tendency to "falsely" recall the critical lure, in line with the notion that sleep promotes processes of abstraction and the extraction of the gist information from the list (294, 897).However, the effect of sleep in the DRM paradigm was less consistent when recall of the critical lure was tested with a recognition procedure rather than by free recall (255, 296, 373; see Ref. 1159 for an overview). Using the Number Reduction Task (1189), Wagner et al. (1275) showed that sleep facilitates the gain of insight, i.e., explicit knowledge of a hidden structure that was embedded in strings of digits which the subjects had to process before sleep.Subsequent studies of the same group specified that the gain of insight depended on the occurrence of spindle activity during early SWS-rich nocturnal sleep (1346 -1350).Very similar results were obtained with an approach combining the classical Serial Reaction Time Task (SRTT) with a so-called "generation task."On the SRTT, the subject is trained to press as fast and as accurately as possible different keys corresponding to the changing positions of a cue.Unknown to the subject, the changes in the cue position follow a repeating sequence.Typically, during training subjects acquire an implicit knowledge of this sequence as indicated by faster responses to cue positions that follow the sequence compared with responses to random positions.Training does not lead to an immediate formation of explicit sequence knowledge, as the subjects typically cannot deliberately reproduce the SRTT sequence, when explicitly asked to do so right after training (in a generation task).Yet, significant explicit sequence knowledge developed when SRTT training was followed by nocturnal sleep (324,382).Generation task performance in the wake control group in these studies remained at chance level.The sleepinduced extraction of explicit sequence knowledge from an implicitly trained SRTT was particularly pronounced in children, where this extraction process was correlated with enhanced EEG slow oscillation activity during posttraining sleep (1321).Similarly, in adults performing on a statistical learning task, sleep promoted the abstraction of probabilistic regularities in tone sequences, with this effect being associated with increased SWS during postlearning sleep (337).Complementary evidence for a neuronal reorganization of memory representations by sleep was provided by studies imaging brain activation during learning and retrieval.In experiments by Takashima et al. (1170), participants napped for 90 min after studying pictures.Recognition of the pictures was tested 1, 30, and 90 days later.Activation of the hippocampus gradually decreased over time, whereas cortical activation in ventromedial prefrontal areas increased.The duration of SWS during the nap predicted the reduction in hippocampal activity and was also associated with better recognition performance on day 1.A second study of this group confirmed that one night of sleep decreases hippocampal activation and increases activation in neocortex areas (1169).In another study (427), the sleepinduced improvement in word pair memories, at a retrieval test 2 days later, was accompanied by an increased functional connectivity between the hippocampus and the medial prefrontal cortex, although contrary to Takashima et al.'s findings, hippocampal activation per se was enhanced at recall in the sleep group.At a follow-up test 6 mo later, the sleep group exhibited increased cortical activation compared with the subjects who stayed awake on the night after word pair learning.Sleep following learning spatial memories of a virtual town produced increased activation in the striatum at a recall test 3 days later, when compared with a group deprived of sleep after learning (873).Activation in the right striatum positively predicted way finding in the virtual town only in the sleep group, and also functional connectivity between the striatum and hippocampus was modulated by sleep after learning.Similarly, the sleep-enhancing effect of statistical learning (337) was accompanied by a shift in brain activation from the medial temporal lobe to the striatum (336).A sleep-induced reorganization of neuronal representations was also revealed for emotional memories (pictures) after 1 night (896) or 3 days after learning (1147).In the later study, the reorganization led to increased hippocampal and cortical activation as well as increased connectivity between the medial frontal cortical and hippocampal areas, compared with a control group, which stayed awake on the night after learning.Some of the effects of sleep on brain activation during recall of emotional memories were still observed 6 mo later (1148).Finally, sleep-dependent changes in brain activity indicative for a reorganization of memory representations have also been reported for procedural tasks like sequences finger tapping and visual texture discrimination (384, 1292, 1293).

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§ About sleep's role in memory > 2. Selectivity of memory consolidation during sleep

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§ About sleep's role in memory > 2. Selectivity of memory consolidation during sleep

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§ About sleep's role in memory > 2. Selectivity of memory consolidation during sleep

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§ About sleep's role in memory > 1. Reactivation of hippocampal place cells during sleep

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§ About sleep's role in memory > 1. Reactivation of hippocampal place cells during sleep

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§ About sleep's role in memory > 1. Animal studies

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§ About sleep's role in memory > 3. Comparing cueing of memories during wakefulness and sleep

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§ About sleep's role in memory > 2. Human studies

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§ About sleep's role in memory > 3. Memory reactivations and dreaming It is unclear whether neuronal signs of memory reactivation during sleep are in any way linked to the recall of dreams after awakening from sleep.Highly vivid and emotional dreams are typically reported after awakenings from REM sleep, whereas more thoughtlike dream reports can be obtained after awakenings from non-REM sleep (199,554).Subjectively, reported dreams often cover an extended time period involving a sequence of events, whereas signs of memory reactivation are usually restricted to brief intervals in the range of several 100 ms.However, time perception during dreams might be compressed, as has been reported for replay of hippocampal neuron assemblies during SWS.Nevertheless, although semantic features from past experience are often included, only a very small portion of dream reports (1-2%) incorporate genuinely episodic memories experienced during presleep waking (405, 1050).Also, experimental cueing of memories during sleep did not produce specific dream reports of task-related themes (959, 1018; Rihm et al., unpublished observation).On the other hand, several experiments showed that specific waking behaviors can influence subsequent dream content (1152).When participants played an emotionally engaging ski computer game, 30% of dreams reported upon awakenings (from non-REM sleep stage 1 and 2) shortly after sleep onset contained elements related to the computer game (1304).The same group also reported a link between dream mentation during non-REM sleep and memory consolidation after a nap (1303), although the effect was based on only four subjects who actually reported task-related mentation. Overall, there is so far no convincing evidence for a direct link between the reactivation of newly encoded memory representations during sleep as evidenced by the recording of neuronal activity and reported dreams.Although it cannot be excluded that aside from consolidating memory, these reactivations occasionally trigger certain fragments of memory that then are incorporated into dreams.

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§ About sleep's role in memory > 3. Comparing cueing of memories during wakefulness and sleep

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§ About sleep's role in memory > 2. Human studies

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§ About sleep's role in memory > 1. Animal studies

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§ About sleep's role in memory > 2. Human studies

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§ About sleep's role in memory > 1. Animal studies

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§ About sleep's role in memory > 1. Animal studies

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§ About sleep's role in memory > 1. Generation, propagation, and homeostatic regulation

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§ About sleep's role in memory > 1. Generation, propagation, and homeostatic regulation

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§ About sleep's role in memory > 1. Generation, propagation, and homeostatic regulation

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§ About sleep's role in memory > 1. Generation, propagation, and homeostatic regulation

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§ About sleep's role in memory > 1. Generation, propagation, and homeostatic regulation

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§ About sleep's role in memory > 1. Generation, propagation, and homeostatic regulation

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§ About sleep's role in memory > 1. Generation, propagation, and homeostatic regulation

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§ About sleep's role in memory > 1. Generation, propagation, and homeostatic regulation

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§ About sleep's role in memory > 1. Generation, propagation, and homeostatic regulation

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§ About sleep's role in memory > 3. Comparing cueing of memories during wakefulness and sleep

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§ About sleep's role in memory > Direct evidence for a causal role of slow oscillations on sleepdependent memory consolidation is provided by studies experimentally inducing slow oscillations by transcranial direct current stimulation (tDCS; FIGURE 5A). In humans, tDCS that oscillated at a very low frequency (0.003 Hz; 30-s on/30-s off) and was applied to the prefrontal cortex during early noctur-nal non-REM sleep, increased endogenous SWA, and produced a significant improvement in the overnight retention of word pair memories (785).Effects on word pair memories were even more consistent with tDCS oscillating at 0.75 Hz, i.e., a frequency mimicking the endogenous slow oscillations (FIGURE 5B, Ref. 783).This type of stimulation applied during early non-REM sleep specifically enhanced Ͻ1 Hz slow oscillations and additionally increased frontal slow spindle activity (10)(11)(12).tDCS at the same frequency (0.75 Hz) during late REM-rich sleep was ineffective in enhancing both endogenous slow oscillations and overnight retention of word pairs.Further controls ensured that the effects depended on the frequency of the oscillating stimulation and on the brain state (FIGURE 5C): tDCS at the 5 Hz theta frequency during early non-REM sleep had an immediate suppressing rather than enhancing effect on endogenous slow oscillation and frontal slow spindle activity, and impaired overnight retention of word pairs (784).Applying the 0.75 Hz slow oscillatory stimulation during wakefulness induced a widespread increase in theta (4 -8 Hz), rather than slow oscillation activity, and this increase in theta activity was associated with a significant improvement in the encoding of declarative memories, rather than affecting retention of these memories (FIGURE 5D, Ref. 643).These results agree with findings that reveal increased cortical excitability following oscillatory electrical stimulation in the wake state (83,484) and provided initial hints at a causal role of EEG theta activity for the encoding of new memories in humans (646, 647, 687).In fact, combination changes observed after tDCS at different frequencies seem to indicate that the cortical networks oscillating at the theta frequency during encoding of hippocampus-dependent memories are functionally linked to the networks that oscillate at the slow oscillation frequency during subsequent SWS to consolidate these memories.

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§ About sleep's role in memory > 2. The relation between SWA and memory benefits

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§ About sleep's role in memory > Direct evidence for a causal role of slow oscillations on sleepdependent memory consolidation is provided by studies experimentally inducing slow oscillations by transcranial direct current stimulation (tDCS; FIGURE 5A). In humans, tDCS that oscillated at a very low frequency (0.003 Hz; 30-s on/30-s off) and was applied to the prefrontal cortex during early noctur-nal non-REM sleep, increased endogenous SWA, and produced a significant improvement in the overnight retention of word pair memories (785).Effects on word pair memories were even more consistent with tDCS oscillating at 0.75 Hz, i.e., a frequency mimicking the endogenous slow oscillations (FIGURE 5B, Ref. 783).This type of stimulation applied during early non-REM sleep specifically enhanced Ͻ1 Hz slow oscillations and additionally increased frontal slow spindle activity (10)(11)(12).tDCS at the same frequency (0.75 Hz) during late REM-rich sleep was ineffective in enhancing both endogenous slow oscillations and overnight retention of word pairs.Further controls ensured that the effects depended on the frequency of the oscillating stimulation and on the brain state (FIGURE 5C): tDCS at the 5 Hz theta frequency during early non-REM sleep had an immediate suppressing rather than enhancing effect on endogenous slow oscillation and frontal slow spindle activity, and impaired overnight retention of word pairs (784).Applying the 0.75 Hz slow oscillatory stimulation during wakefulness induced a widespread increase in theta (4 -8 Hz), rather than slow oscillation activity, and this increase in theta activity was associated with a significant improvement in the encoding of declarative memories, rather than affecting retention of these memories (FIGURE 5D, Ref. 643).These results agree with findings that reveal increased cortical excitability following oscillatory electrical stimulation in the wake state (83,484) and provided initial hints at a causal role of EEG theta activity for the encoding of new memories in humans (646, 647, 687).In fact, combination changes observed after tDCS at different frequencies seem to indicate that the cortical networks oscillating at the theta frequency during encoding of hippocampus-dependent memories are functionally linked to the networks that oscillate at the slow oscillation frequency during subsequent SWS to consolidate these memories.

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§ About sleep's role in memory > Direct evidence for a causal role of slow oscillations on sleepdependent memory consolidation is provided by studies experimentally inducing slow oscillations by transcranial direct current stimulation (tDCS; FIGURE 5A). In humans, tDCS that oscillated at a very low frequency (0.003 Hz; 30-s on/30-s off) and was applied to the prefrontal cortex during early noctur-nal non-REM sleep, increased endogenous SWA, and produced a significant improvement in the overnight retention of word pair memories (785).Effects on word pair memories were even more consistent with tDCS oscillating at 0.75 Hz, i.e., a frequency mimicking the endogenous slow oscillations (FIGURE 5B, Ref. 783).This type of stimulation applied during early non-REM sleep specifically enhanced Ͻ1 Hz slow oscillations and additionally increased frontal slow spindle activity (10)(11)(12).tDCS at the same frequency (0.75 Hz) during late REM-rich sleep was ineffective in enhancing both endogenous slow oscillations and overnight retention of word pairs.Further controls ensured that the effects depended on the frequency of the oscillating stimulation and on the brain state (FIGURE 5C): tDCS at the 5 Hz theta frequency during early non-REM sleep had an immediate suppressing rather than enhancing effect on endogenous slow oscillation and frontal slow spindle activity, and impaired overnight retention of word pairs (784).Applying the 0.75 Hz slow oscillatory stimulation during wakefulness induced a widespread increase in theta (4 -8 Hz), rather than slow oscillation activity, and this increase in theta activity was associated with a significant improvement in the encoding of declarative memories, rather than affecting retention of these memories (FIGURE 5D, Ref. 643).These results agree with findings that reveal increased cortical excitability following oscillatory electrical stimulation in the wake state (83,484) and provided initial hints at a causal role of EEG theta activity for the encoding of new memories in humans (646, 647, 687).In fact, combination changes observed after tDCS at different frequencies seem to indicate that the cortical networks oscillating at the theta frequency during encoding of hippocampus-dependent memories are functionally linked to the networks that oscillate at the slow oscillation frequency during subsequent SWS to consolidate these memories.

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§ About sleep's role in memory > 2. The relation between SWA and memory benefits

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§ About sleep's role in memory > 1. The concept

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§ About sleep's role in memory > 1. The concept

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§ About sleep's role in memory > 1. The concept

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§ About sleep's role in memory > 1. The concept

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§ About sleep's role in memory > 1. The concept

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§ About sleep's role in memory > 1. The concept

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§ About sleep's role in memory > 1. The concept

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§ About sleep's role in memory > 1. The concept

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§ About sleep's role in memory > 2. Critical issues

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§ About sleep's role in memory > 3. SWA enhances subsequent encoding Importantly in the synaptic homeostasis theory, sleep-associated synaptic down-scaling primarily serves to renormalize synaptic weights in networks that, in the course of the encoding of information during prior wakefulness, were potentiated to close to saturation.Therefore, rather than support the consolidation of memory, synaptic down-scaling during SWS is expected to ease the encoding of new information during subsequent wakefulness.Indeed, there is convergent evidence for such action in humans and rats, i.e., an impairment of learning when they were deprived of sleep (195,328,500,634,759,1291,1345).Fittingly, in rats, sleep deprivation also hampered the induction of LTP in the hippocampal regions (170, 778, 1161, 1179), the expression of plasticity-related genes (496) as well as excitability of the hippocampal neurons (801, 802).Selective deprivation of REM sleep appears to be sufficient for producing an impairment in LTP induction (269,270,596,636,743,970,1010).In humans, the capacity to encode episodic memories deteriorates across the day and is restored after a period of sleep (759).Prior sleep deprivation impairs verbal learning ability and reduces temporal lobe activation during learning (328,329).Interestingly, the ability to encode pictures after a nap was already substantially diminished when SWA during napping was selectively attenuated by contingent mild acoustic stimulation leaving the gross sleep architecture intact (1232, 1233).The impairing effect on picture encoding was accompanied by reduced hippocampus activation during learning.Surprisingly, no change in activation in relevant neocortical areas was found, and suppressing SWA also did not impair the learning of a motor skill task.Complementary effects, i.e., enhanced encoding on hippocampus-dependent declarative tasks in the absence of changes in motor skill learning, were revealed after enhancing sleep SWA by transcranial direct current stimulation (oscillating at the 0.75 Hz slow oscillation frequency) (39).However, no signs of improved encoding were revealed after a drug-induced enhancement of SWA by sodium oxybate (1297).After 36 h of total sleep deprivation, encoding of emotional pictures was strongly impaired, with this effect sparing negative emotional pictures (1291).Interestingly, amygdala responses during the viewing of aversive pictures increased after 35 h of sleep deprivation (1345).Collectively, these initial data corroborate the idea that SWA during sleep refreshes capacities for the encoding of information possibly by a synaptic down-scaling-like mechanism.It remains unclear why the effect apparently predominates in hippocampal networks that encode information in declarative tasks, although the hippocampus itself does not generate slow oscillations (597).

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§ About sleep's role in memory > 1. Spindle generation, fast and slow spindles

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§ About sleep's role in memory > 3. SWA enhances subsequent encoding Importantly in the synaptic homeostasis theory, sleep-associated synaptic down-scaling primarily serves to renormalize synaptic weights in networks that, in the course of the encoding of information during prior wakefulness, were potentiated to close to saturation.Therefore, rather than support the consolidation of memory, synaptic down-scaling during SWS is expected to ease the encoding of new information during subsequent wakefulness.Indeed, there is convergent evidence for such action in humans and rats, i.e., an impairment of learning when they were deprived of sleep (195,328,500,634,759,1291,1345).Fittingly, in rats, sleep deprivation also hampered the induction of LTP in the hippocampal regions (170, 778, 1161, 1179), the expression of plasticity-related genes (496) as well as excitability of the hippocampal neurons (801, 802).Selective deprivation of REM sleep appears to be sufficient for producing an impairment in LTP induction (269,270,596,636,743,970,1010).In humans, the capacity to encode episodic memories deteriorates across the day and is restored after a period of sleep (759).Prior sleep deprivation impairs verbal learning ability and reduces temporal lobe activation during learning (328,329).Interestingly, the ability to encode pictures after a nap was already substantially diminished when SWA during napping was selectively attenuated by contingent mild acoustic stimulation leaving the gross sleep architecture intact (1232, 1233).The impairing effect on picture encoding was accompanied by reduced hippocampus activation during learning.Surprisingly, no change in activation in relevant neocortical areas was found, and suppressing SWA also did not impair the learning of a motor skill task.Complementary effects, i.e., enhanced encoding on hippocampus-dependent declarative tasks in the absence of changes in motor skill learning, were revealed after enhancing sleep SWA by transcranial direct current stimulation (oscillating at the 0.75 Hz slow oscillation frequency) (39).However, no signs of improved encoding were revealed after a drug-induced enhancement of SWA by sodium oxybate (1297).After 36 h of total sleep deprivation, encoding of emotional pictures was strongly impaired, with this effect sparing negative emotional pictures (1291).Interestingly, amygdala responses during the viewing of aversive pictures increased after 35 h of sleep deprivation (1345).Collectively, these initial data corroborate the idea that SWA during sleep refreshes capacities for the encoding of information possibly by a synaptic down-scaling-like mechanism.It remains unclear why the effect apparently predominates in hippocampal networks that encode information in declarative tasks, although the hippocampus itself does not generate slow oscillations (597).

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§ About sleep's role in memory > 1. Spindle generation, fast and slow spindles

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§ About sleep's role in memory > D. Sharp Wave-Ripples

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§ About sleep's role in memory > D. Sharp Wave-Ripples

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§ About sleep's role in memory > 2. Relationship between spindles and memory Many studies in humans and animals have indicated a robust association between spindle activity and memory processing during sleep.Effects appear to be particularly consistent for the classic fast spindles.However, not many studies differentiated both types of sleep spindles.Intense learning of declarative memories (word pairs or virtual maze) increases the number of spindles (12-15 Hz) during subsequent sleep, particularly in the early part of the night (432, 812).In another similar study, only participants who exhibited enhanced spindle activity (11.5-16Hz) after word pair learning (in comparison with sleep after a nonlearning control session) also showed a significant overnight improvement of verbal memory (1037).Encoding of a difficult list of abstract words produced an increase in the power and density of spindles (11.27-13.75Hz) compared with sleep after encoding a list of easier, concrete words (1044).In rats, robust increases in sleep spindles (12)(13)(14)(15) were observed after learning odor-reward associations (354) and after avoidance training (397).Furthermore, indicators of sleep spindle expression consistently correlated with the amount of overnight retention of declarative memories (86,213,214,235,446,562,822,1016,1024,1037,1044,1057).Interestingly, spindle activity (11-15 Hz) also correlated with signs of overnight lexical integration of newly learned information, suggesting that spindles contribute to the integration of new memories into existing neocortical knowledge networks (1175).Moreover, spindles (13-15 Hz) during a postlearning nap predicted sleepdependent memory improvement for contextual aspects of episodic memories known to most closely depend on the hippocampal function (1230).Using combined EEG/fMRI recordings, Bergmann et al. (84) observed conjoint reactivations in relevant neocortical and hippocampal regions that occurred in temporal synchrony with spindle events (12-14 Hz) during non-REM sleep after learning of facescene associations.The strength in reactivations covaried with spindle amplitude.Together these findings provide first hints that spindles are implicated in the hippocamponeocortical exchange of memory information mediating active system consolidation during sleep.Interestingly, spindle activity following exposure to a novel spatio-tactile experience in rats predicted immediate early gene activity (Arc expression) in the somatosensory cortex during later REM sleep, suggesting that neocortical parts of representations become tagged for later synaptic strengthening during spindles (989). Spindles also appear to be involved in the consolidation of skills, especially of simple motor (e.g., pursuit rotor task, simple sequential finger tapping) and visuomotor skills (1114).Increases of spindles (13)(14)(15) and stage 2 sleep are particularly observed following training of tasks like figure tracing (59, 394 -396, 399, 837, 920, 921, 1173, 1174), which were also revealed to be vulnerable to stage 2 sleep deprivation (1112, 1117).In some studies the increase in spindle activity was topographically restricted to the cortex areas most strongly involved in skill performance, i.e., in the contralateral motor cortex after motor tapping training (862) and in the parietal cortex after training in a visuospatial skill (214).Also, correlations between different parameters of spindle activity or stage 2 sleep and overnight improvements in motor skills have been consistently reported (562, 862, 1216, 1286, 1287), and these correlations were revealed for fast rather than slow spindles, in the studies distinguishing these two spindle types (59,962,1174). In spite of the quite robust correlational evidence for the involvement of spindles in memory processing, a causal role of spindles in memory consolidation has not yet been demonstrated, as this requires the selective experimental manipulation of spindle activity.Approaches to increase spindle activity pharmacologically by administration of GABA A receptor agonistic drugs do not appear to be suitable in this regard, as they concurrently decrease SWA (see sect.VA2).

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§ About sleep's role in memory > E. Slow Oscillations, Spindles, and SW-R Interact The fine-tuned temporal relationship between the neocortical slow oscillations, thalamic spindles, and hippocampal SW-Rs originates from a top-down control of the slow oscillation on the two other events.In addition to the neural synchronization in the neocortex, the synchronizing effects of slow oscillations spread to thalamic and hippocampal as well as other brain regions involved in off-line memory consolidation.At the neocortical level, the slow oscillation in terms of EEG rhythms strongly modulates the amplitude of faster beta and gamma frequencies which is reduced to a minimum during the hyperpolarizing down-phase of the slow oscillation (224,240,828,829).Generation of spindles in the thalamus and generation of SW-Rs in the hippocampus are distinctly suppressed during the down-phase of the slow oscillation, and this is followed by a rebound in spindle and SW-R activity during the succeeding depolarizing up-state (66, 215, 597, 829 -832, 925, 1088).As for spindles, only the classic fast spindles display the strong phase synchronization with the emergent depolarizing upstate of the slow oscillation.Slow frontal spindles, in contrast, tend to follow fast spindles by 200 -500 ms and thus occur already in the up-to-down state transition of the slow oscillation (38,826).As for the modulation of SW-Rs, this appears to be entirely driven by cortical inputs, as the hippocampus itself does not generate slow oscillations (597).However, membrane potentials with a slight delay (of ϳ50 ms) follow the up-and down-states of neocortical slow oscillations, particularly in dentate gyrus and CA1 (502, 503, 1333).In parallel, neocortical up states might timelock spontaneous activity in the medial entorhinal cortex, a major gateway between the neocortex and hippocampus (501).There is also evidence for a slow oscillation modulation of locus coeruleus burst activity with preferential locus coeruleus firing during the down-to-up state transition of the slow oscillation (353, 727). Importantly, prior learning appears to strengthen the topdown control of slow oscillations on spindles and ripples.In humans, intense learning of vocabulary did not only increase the slope of the down-to-up state transition of the slow oscillation during succeeding non-REM sleep, but concurrently enhanced fast spindle activity, with this increase concentrating on the up-states of the slow oscillation, whereas no changes were observed in hyperpolarizing down-states (826, 829).Interestingly, these analyses also revealed that learning promoted the occurrence of trains of several succeeding slow oscillations.In these trains, fast spindles were not only driven by the depolarizing slow oscillation up-state but appeared to feed themselves back to enforce the succeeding slow oscillation, as well as the likelihood of associated slow frontal spindles.The enhancement of such slow oscillation-spindle cycles might be a key mechanism whereby fast spindles initiate the consolidation of newly learned materials during sleep (826).Learning also increases hippocampal SW-Rs (355, 955), and it is likely that learning-induced increases in slow oscillations strengthen parallel to the synchronization of enhanced SW-R activity and the depolarizing up-state of the slow oscillation, which remains to be demonstrated. SW-Rs are also temporally coupled to spindles (1075, 1088) which can only be partly explained by the common driving impact of the slow oscillation on both phenomena (829).Event-correlation histograms derived from intracranial recordings in rat and humans revealed that ripples are associated with a rise in spindle activity that starts shortly before ripple onset and then outlasts the ripple (215, 216, 829, 832).The rise in spindle activity was even more persistent (up to 2 s) when the rats had performed on a learning task prior to sleep (829).Moreover, fine-grained temporal analyses revealed that the co-occurrence of spindles and ripples leads to the formation of so-called "spindle-ripple events" where individual ripple events become temporally nested into succeeding troughs of a spindle (216, 1075, 1317).As ripples accompany assemble reactivations in the hippocampus, spindle-ripple events might represent a mechanism serving the sequenced transfer of reactivated memory information towards neocortical sites (827, 1087).Importantly, the formation of spindle-ripple events is restricted to classic fast spindles, which (like SW-Rs) are driven by the depolarizing down-to-up state transition of the slow oscillation.In contrast, the slow frontal spindles typically occur 200 -500 ms later in the slow oscillation cycle (i.e., at the up-to-down state transition), and thus also tend to follow hippocampal SW-Rs with the same delay (216).In contrast to fast spindles, the neocortex might be "functionally deafferented" from its hippocampal inputs during frontal slow spindles (925).Taken together, these data suggest a looplike scenario during sleep after learning. While thalamo-cortical sleep spindles enforce the generation of hippocampal ripples, ripple events in turn feed back to sustain the ongoing generation of spindle activity (828), and possibly also slow oscillation activity, although this is presently unclear (635, 1087).Such looplike coordination goes along with an enhanced formation of spindle-ripple events whereby ripples (together with the reactivated hippocampal memory information they carry) are fed exactly into the excitatory phases of the spindle cycle.By still reaching neocortical networks during the depolarizing up-phase of the slow oscillation, the spindle-ripple event may thus serve as an effective mechanism for transferring hippocampal memory information towards neocortical long-term stores.Indeed, fast spindles do not only phase-lock hippocampal ripples but also neocortical gamma-band activity as an indicator of coherent information processing in local neocortical networks (52).Spindle-gamma coupling might be linked to facilitate synaptic plastic processes underlying the storage of information in neocortical circuitry (1012). In sum, there is now growing evidence for a dialogue between neocortex and hippocampus mediating the system consolidation of hippocampus-dependent memory, which is orchestrated by a fine-tuned interaction between oscillatory field potential activities.In this dialogue, the neocortical slow oscillation provides a top-down temporal frame that synchronizes the reactivation of hippocampal memories with the simultaneous occurrence of fast spindles to enable the formation of spindle-ripple events.Spindle-ripple events occurring during the slow oscillation up-states, conversely, provide a bottom-up mechanism for the transfer of reactivated memory information from the hippocampus to the neocortex where they might effectively support plastic synaptic processes underlying the storage of this information.

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§ About sleep's role in memory > F. PGO Waves and Theta-Rhythm of REM Sleep

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§ About sleep's role in memory > E. Slow Oscillations, Spindles, and SW-R Interact The fine-tuned temporal relationship between the neocortical slow oscillations, thalamic spindles, and hippocampal SW-Rs originates from a top-down control of the slow oscillation on the two other events.In addition to the neural synchronization in the neocortex, the synchronizing effects of slow oscillations spread to thalamic and hippocampal as well as other brain regions involved in off-line memory consolidation.At the neocortical level, the slow oscillation in terms of EEG rhythms strongly modulates the amplitude of faster beta and gamma frequencies which is reduced to a minimum during the hyperpolarizing down-phase of the slow oscillation (224,240,828,829).Generation of spindles in the thalamus and generation of SW-Rs in the hippocampus are distinctly suppressed during the down-phase of the slow oscillation, and this is followed by a rebound in spindle and SW-R activity during the succeeding depolarizing up-state (66, 215, 597, 829 -832, 925, 1088).As for spindles, only the classic fast spindles display the strong phase synchronization with the emergent depolarizing upstate of the slow oscillation.Slow frontal spindles, in contrast, tend to follow fast spindles by 200 -500 ms and thus occur already in the up-to-down state transition of the slow oscillation (38,826).As for the modulation of SW-Rs, this appears to be entirely driven by cortical inputs, as the hippocampus itself does not generate slow oscillations (597).However, membrane potentials with a slight delay (of ϳ50 ms) follow the up-and down-states of neocortical slow oscillations, particularly in dentate gyrus and CA1 (502, 503, 1333).In parallel, neocortical up states might timelock spontaneous activity in the medial entorhinal cortex, a major gateway between the neocortex and hippocampus (501).There is also evidence for a slow oscillation modulation of locus coeruleus burst activity with preferential locus coeruleus firing during the down-to-up state transition of the slow oscillation (353, 727). Importantly, prior learning appears to strengthen the topdown control of slow oscillations on spindles and ripples.In humans, intense learning of vocabulary did not only increase the slope of the down-to-up state transition of the slow oscillation during succeeding non-REM sleep, but concurrently enhanced fast spindle activity, with this increase concentrating on the up-states of the slow oscillation, whereas no changes were observed in hyperpolarizing down-states (826, 829).Interestingly, these analyses also revealed that learning promoted the occurrence of trains of several succeeding slow oscillations.In these trains, fast spindles were not only driven by the depolarizing slow oscillation up-state but appeared to feed themselves back to enforce the succeeding slow oscillation, as well as the likelihood of associated slow frontal spindles.The enhancement of such slow oscillation-spindle cycles might be a key mechanism whereby fast spindles initiate the consolidation of newly learned materials during sleep (826).Learning also increases hippocampal SW-Rs (355, 955), and it is likely that learning-induced increases in slow oscillations strengthen parallel to the synchronization of enhanced SW-R activity and the depolarizing up-state of the slow oscillation, which remains to be demonstrated. SW-Rs are also temporally coupled to spindles (1075, 1088) which can only be partly explained by the common driving impact of the slow oscillation on both phenomena (829).Event-correlation histograms derived from intracranial recordings in rat and humans revealed that ripples are associated with a rise in spindle activity that starts shortly before ripple onset and then outlasts the ripple (215, 216, 829, 832).The rise in spindle activity was even more persistent (up to 2 s) when the rats had performed on a learning task prior to sleep (829).Moreover, fine-grained temporal analyses revealed that the co-occurrence of spindles and ripples leads to the formation of so-called "spindle-ripple events" where individual ripple events become temporally nested into succeeding troughs of a spindle (216, 1075, 1317).As ripples accompany assemble reactivations in the hippocampus, spindle-ripple events might represent a mechanism serving the sequenced transfer of reactivated memory information towards neocortical sites (827, 1087).Importantly, the formation of spindle-ripple events is restricted to classic fast spindles, which (like SW-Rs) are driven by the depolarizing down-to-up state transition of the slow oscillation.In contrast, the slow frontal spindles typically occur 200 -500 ms later in the slow oscillation cycle (i.e., at the up-to-down state transition), and thus also tend to follow hippocampal SW-Rs with the same delay (216).In contrast to fast spindles, the neocortex might be "functionally deafferented" from its hippocampal inputs during frontal slow spindles (925).Taken together, these data suggest a looplike scenario during sleep after learning. While thalamo-cortical sleep spindles enforce the generation of hippocampal ripples, ripple events in turn feed back to sustain the ongoing generation of spindle activity (828), and possibly also slow oscillation activity, although this is presently unclear (635, 1087).Such looplike coordination goes along with an enhanced formation of spindle-ripple events whereby ripples (together with the reactivated hippocampal memory information they carry) are fed exactly into the excitatory phases of the spindle cycle.By still reaching neocortical networks during the depolarizing up-phase of the slow oscillation, the spindle-ripple event may thus serve as an effective mechanism for transferring hippocampal memory information towards neocortical long-term stores.Indeed, fast spindles do not only phase-lock hippocampal ripples but also neocortical gamma-band activity as an indicator of coherent information processing in local neocortical networks (52).Spindle-gamma coupling might be linked to facilitate synaptic plastic processes underlying the storage of information in neocortical circuitry (1012). In sum, there is now growing evidence for a dialogue between neocortex and hippocampus mediating the system consolidation of hippocampus-dependent memory, which is orchestrated by a fine-tuned interaction between oscillatory field potential activities.In this dialogue, the neocortical slow oscillation provides a top-down temporal frame that synchronizes the reactivation of hippocampal memories with the simultaneous occurrence of fast spindles to enable the formation of spindle-ripple events.Spindle-ripple events occurring during the slow oscillation up-states, conversely, provide a bottom-up mechanism for the transfer of reactivated memory information from the hippocampus to the neocortex where they might effectively support plastic synaptic processes underlying the storage of this information.

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§ About sleep's role in memory > 2. GABA

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§ About sleep's role in memory > 2. GABA

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§ About sleep's role in memory > 2. GABA

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§ About sleep's role in memory > 1. Effects of wake-versus sleep-promoting signals on LTP and LTD

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§ About sleep's role in memory > 1. Effects of wake-versus sleep-promoting signals on LTP and LTD

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§ About sleep's role in memory > 3. Hormonal modulation associated with sleep and SWS, and memory processing SWS is associated with the inhibition of glucocorticoid release from the hypothalamus-pituitary-adrenal (HPA) sys-tem and a profound surge in the release of growth hormone releasing hormone (GHRH) and GH from the somatotropic axis (122, 123, 1271).While activation of the latter system might support hippocampal memory processing through brain-borne GHRH (430, 509), inhibition of the former system affects limbic regions mainly via cortisol feedback that is downregulated during SWS.Low cortisol concentrations during SWS-rich sleep benefit declarative memory consolidation by preventing activation of glucocorticoid receptors that mediate an inhibitory influence on hippocampal LTP and output from CA1 (648, 649, 666).Accordingly, increasing glucocorticoid levels during sleep by postlearning administration of cortisol or dexamethasone impaired consolidation of memory for word pairs and for the temporal order in stories (932, 933, 1323).In addition, spontaneously increased nocturnal cortisol levels around midnight were found to be correlated with impaired retrieval of declarative memories in healthy subjects and patients with primary insomnia (58).However, a certain basal release of cortisol is necessary during SWS to sufficiently occupy mineralocorticoid receptors, which bind cortisol with a distinctly higher affinity and support the transition of early into late LTP in hippocampus.Accordingly, lowering cortisol levels in humans during early nocturnal SWS below baseline levels by pharmacological blockade of cortisol synthesis impaired sleep-dependent declarative memory consolidation (1272). Melatonin is a circadian hormone that is released by the pineal gland during the night and, in diurnal animals and humans, substantially contributes to the entrainment of sleep to the night-time phase (212).Melatonin inhibits hippocampal LTP (223,877,1171,1307) and has also been found to impair acquisition of hippocampus-dependent spatial memories in rats (372) and of an active avoidance task in diurnal zebrafish (971), although divergent findings were obtained in humans (997).Whereas these observations point to an impairing influence of melatonin on the encoding of memory, there is preliminary evidence from fMRI experiments in humans that melatonin might enhance processes of hippocampal memory consolidation during sleep: parahippocampal activity patterns during retrieval of word-pair memories were found following a nap showed some similarity with those obtained when napping was replaced with administration of melatonin (471).

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§ About sleep's role in memory > 1. Effects of wake-versus sleep-promoting signals on LTP and LTD

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§ About sleep's role in memory > 1. Effects of wake-versus sleep-promoting signals on LTP and LTD

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§ About sleep's role in memory > 3. Genetic studies on sleep-dependent memory consolidation

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§ About sleep's role in memory > 1. Stability and heritability of sleep

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§ About sleep's role in memory > 3. Genetic studies on sleep-dependent memory consolidation

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§ About sleep's role in memory > 2. Song learning in birds

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§ About sleep's role in memory > 1. Filial imprinting

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§ About sleep's role in memory > 2. Song learning in birds

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§ About sleep's role in memory > 3. Ocular dominance plasticity

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§ About sleep's role in memory > 2. Song learning in birds

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§ About sleep's role in memory > 3. Ocular dominance plasticity

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§ About sleep's role in memory > 3. Ocular dominance plasticity

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§ About sleep's role in memory > 3. Ocular dominance plasticity

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§ About sleep's role in memory > 3. Ocular dominance plasticity

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§ About sleep's role in memory > B. Early Development: Human Studies

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§ About sleep's role in memory > 3. Children extract more explicit knowledge from implicitly trained tasks than adults Alternatively, the lack of sleep-dependent gains in skill performance in children might be ascribed to a competitive interaction between declarative and procedural memory systems (385, 1318).Particularly in the initial stages of training, contributions of explicit (i.e., conscious) learning mechanisms involving prefrontal-hippocampal circuitry can be essential for skill acquisition, with the parallel storing of procedural and hippocampus-dependent declarative aspects favoring interactions and competitions between the two memory systems (16,43,937,1042), which can result in an impaired implicit task performance (e.g., slowing of reaction times) (1064, 1135).Importantly, this competitive interaction between the memory systems might extend to processes of consolidation during sleep (143, 147, 273, 1000).On the basis of the assumption that SWS predominantly supports consolidation of hippocampus-dependent declarative memory, it was argued that sleep in children, because of its great amounts of SWS, preferentially strengthens hippocampus-dependent explicit aspects in a skill representation, thereby hampering implicit perfor-mance.A recent study indeed confirmed a striking superiority of sleep in 8-to 11-yr-old children, compared with adult's sleep, to extract explicit declarative sequence knowledge from a coarse motor SRTT that was trained under implicit conditions (i.e., without being aware of the underlying sequence) before sleep (1322).On the generation task after retention sleep requiring the subjects to deliberately generate the eight-element sequence of cue positions underlying the SRTT trained before sleep, the children clearly outperformed the adults.The children's performance was also distinctly better than in control children who performed the generation task after training before the sleep interval.Superior explicit knowledge after sleep correlated with the amount of SWA in both age groups, in line with studies in adults indicating that SWS is critically involved in the extraction of explicit knowledge (1347). Consistent with the notion that sleep in children particularly benefits explicit task aspects and the extraction of abstract knowledge, 15-mo-old infants already showed a sleep-dependent benefit for extraction of grammatical rules during language learning (469).The results were confirmed in a second study of this group employing an extended retention interval of 24 h (589).Both studies used the infant's orienting response towards the auditory word-strings to assess memory recall.Orienting is a well-known hippocampus-dependent function ( 1119), suggesting that the retrieval test tapped hippocampus-dependent declarative aspects of the memory, although this issue deserves further examination.

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§ About sleep's role in memory > 1. Hippocampus-dependent declarative memory

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§ About sleep's role in memory > 2. Procedural memory With regard to procedural memory, several studies relying on different versions of the SRTT revealed age-related impairments in the formation of motor skill memories during sleep.Comparing older (45-80 years) and younger adults (18 -24 years) on the SRTT under implicit and explicit learning conditions, Spencer et al. (1127) found that only the younger but not the older subjects distinctly improved in performance in both task versions across overnight sleep, compared with performance changes across a daytime retention period of wakefulness.Another study revealed sleep-dependent performance gains on a deterministic SRTT in middle-aged (35-55 years), but not in 55-70 years old participants (1324).In contrast, using a finger sequence tapping task, one study reported that elderly subjects (60 -79 years) maintained their performance across 24 h including sleep, while their performance was impaired after 12 h of wakefulness, suggesting a role for sleep in optimal motor consolidation also in the elderly (1214).However, the overnight improvements typically observed in young subjects did not occur in these elderly participants.Sleep also did not benefit the performance of older (50 -75 years) subjects on a continuous motor tracking task where the moving target to be tracked could follow a repeated sequence or move randomly similar to the SRTT (1079 -1081).Interestingly, age-matched stroke patients in this study did show an improvement in tracing accuracy selectively across the sleep retention interval for both explicit and implicit performance conditions.Finally, sleep also did not improve performance in a second-order probabilistic SRTT in old subjects (60 -80 years) (852).However, sleepdependent benefits are also not consistently revealed in younger subjects for this task, possibly due to the great complexity of the underlying sequence rules associated with a higher degree of implicitness at learning (852,1122,1123). To what extent the age-related decreases in sleep benefits for skill learning are related to specific alteration in sleep, in particular of SWS, is presently unclear as most of the relevant studies in aged subjects did not include polysomnographic recordings.REM sleep does not appear to be involved as neither experimental REM sleep deprivation nor REM sleep augmentation (i.e., the rebound after deprivation) produced any significant performance change in aged subjects on a procedural mirror tracing task (570).Nevertheless, like in young adults, high cholinergic activation during REM sleep seems to contribute to sleep-dependent consolidation of procedural memory also in aged individuals, as administration of the acetylcholinesterase inhibitor donezipil in aged subjects, together with an increase in REM density, significantly enhanced sleep-dependent gains in mirror tracing performance (570, 961).The contributions of impaired SWS to the age-related decline in procedural memory consolidation remain to be specified in future studies.

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§ About sleep's role in memory > 1. Hippocampus-dependent declarative memory

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§ About sleep's role in memory > 1. Hippocampus-dependent declarative memory

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§ About sleep's role in memory > VIII. SLEEP-DEPENDENT MEMORY CONSOLIDATION IN THE IMMUNE SYSTEM A. Memory Formation in the Immune System The immune system forms long-lasting memories for an antigen in a multistep process.The mounting of a so-called adaptive immune response is at the core of this process.Once antigens invade the organism at certain barriers, they are taken up and processed by antigen presenting cells (APC).APC carry the antigenic information to secondary lymphoid organs where they present fragments of the processed antigen to naive T cells, which become only activated if they express the specific cognate receptor that can recognize this particular antigenic epitope.The T cell recognizing its antigen, together with the APC, form the so-called immunological synapse.Activation of the T cell is linked to the release of different proinflammatory cytokines by APC, among which interleukin-12 (IL-12) is central for the subsequent differentiation of the activated T cell into T-helper 1 (Th1) cells.T-cell growth, proliferation, and differentiation is additionally supported by the release of IL-2, whereas the release of IL-10 in this context plays an antagonistic role favoring differentiation of Th2 cells and the recruitment of immediate but preformed humoral immune responses.Th1 cells also support the differentiation of B cells and their production of antigen-specific antibodies.Whereas most effector cells differentiated during acute infection do not survive for a long time after the infection, there is a subset of differentiated T and B cells that maintains the antigenic memory for the long-term.These memory cells mediate a speedy and more effective immune response upon reencounter of the antigen (13,1258). There are apparent differences between memory formation in the immune system and in the central nervous system.Most obvious, in the immune system cells migrate to act in different tissues and body compartments and, unlike neurons, they often show strong proliferation in response to (antigenic) stimulation.These conditions may partly explain why memory formation in the immune system takes much longer (several days to weeks) than in the central nervous system.Nevertheless, immunological memory, like neurobehavioral memory, is formed as part of an adaptive organismic response to an environmental stressor, and shares the basic subprocesses of encoding, consolidation, and retrieval (641, 695).Also, consolidation includes a cellcell interaction in which the antigenic information is transferred from a temporary to a long-term store, with both stores represented by different cellular networks.Specifically, in this conceptual approach (see FIGURE 9A), encoding in the immune system would denote the uptake of antigenic information by APC.Consolidation would refer to processes at the immunological synapse that is subsequently formed between APC and T cells (338, 473) and comprise the transfer of the antigenic information from the APC, only serving as temporary store, to the T cells serving as longterm store.Recall would refer to the faster and facilitated immune response when the antigen is re-encountered.

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§ About sleep's role in memory > 2. Sleep supports APC-T cell interactions There is converging evidence that sleep promotes Th1 immune responses as an immediate sequel of APC-T cell interactions.Thus sleep in humans enhances the production of IL-12 by monocytes and premyolid dendritic cells (pre-mDC) which represent the most important precursors of mature professional APC circulating in blood (307, 696).Concurrently, sleep diminishes the production of anti-inflammatory IL-10 by monocytes (696).Furthermore, several reports indicated an enhancing influence of sleep on T-cell production of IL-2 which acts to further promote T-cell growth, proliferation, and differentiation (120, 595), although other studies failed to reveal such effects when examining IL-2 production in specific T-cell subpopulations (109,308).Sleep increased proliferation of Th cells as well as the activity of natural regulatory T (nTreg) cells (109).Concurrent upregulation of nTreg activity might reflect a counterregulatory response preventing overshooting of Th1 responses.Moreover, early nocturnal sleep increased the ratio in IFN-␥/IL-4 production by Th cells, indicating a shift in the Th1/Th2 cytokine balance towards preponderant production of Th1 cytokines (308).In patients with allergic rhinitis, wheal reactions to a skin prick test and spontaneous IgE production of peripheral blood mononuclear cells were increased after a night of total sleep deprivation, indicating a shift from Th1 to prevailing Th2 immune defense (637).Finally, there is evidence that sleep increases blood concentrations of IL-7, a cytokine fostering differentiation of T memory cells (76).Together, these findings speak for the notion that sleep specifically benefits APC-T cell interactions towards the development of predominant Th1 immune responses.

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§ About sleep's role in memory > 2. Sleep supports APC-T cell interactions There is converging evidence that sleep promotes Th1 immune responses as an immediate sequel of APC-T cell interactions.Thus sleep in humans enhances the production of IL-12 by monocytes and premyolid dendritic cells (pre-mDC) which represent the most important precursors of mature professional APC circulating in blood (307, 696).Concurrently, sleep diminishes the production of anti-inflammatory IL-10 by monocytes (696).Furthermore, several reports indicated an enhancing influence of sleep on T-cell production of IL-2 which acts to further promote T-cell growth, proliferation, and differentiation (120, 595), although other studies failed to reveal such effects when examining IL-2 production in specific T-cell subpopulations (109,308).Sleep increased proliferation of Th cells as well as the activity of natural regulatory T (nTreg) cells (109).Concurrent upregulation of nTreg activity might reflect a counterregulatory response preventing overshooting of Th1 responses.Moreover, early nocturnal sleep increased the ratio in IFN-␥/IL-4 production by Th cells, indicating a shift in the Th1/Th2 cytokine balance towards preponderant production of Th1 cytokines (308).In patients with allergic rhinitis, wheal reactions to a skin prick test and spontaneous IgE production of peripheral blood mononuclear cells were increased after a night of total sleep deprivation, indicating a shift from Th1 to prevailing Th2 immune defense (637).Finally, there is evidence that sleep increases blood concentrations of IL-7, a cytokine fostering differentiation of T memory cells (76).Together, these findings speak for the notion that sleep specifically benefits APC-T cell interactions towards the development of predominant Th1 immune responses.

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§ About sleep's role in memory > 2. Procedural memory With regard to procedural memory, several studies relying on different versions of the SRTT revealed age-related impairments in the formation of motor skill memories during sleep.Comparing older (45-80 years) and younger adults (18 -24 years) on the SRTT under implicit and explicit learning conditions, Spencer et al. (1127) found that only the younger but not the older subjects distinctly improved in performance in both task versions across overnight sleep, compared with performance changes across a daytime retention period of wakefulness.Another study revealed sleep-dependent performance gains on a deterministic SRTT in middle-aged (35-55 years), but not in 55-70 years old participants (1324).In contrast, using a finger sequence tapping task, one study reported that elderly subjects (60 -79 years) maintained their performance across 24 h including sleep, while their performance was impaired after 12 h of wakefulness, suggesting a role for sleep in optimal motor consolidation also in the elderly (1214).However, the overnight improvements typically observed in young subjects did not occur in these elderly participants.Sleep also did not benefit the performance of older (50 -75 years) subjects on a continuous motor tracking task where the moving target to be tracked could follow a repeated sequence or move randomly similar to the SRTT (1079 -1081).Interestingly, age-matched stroke patients in this study did show an improvement in tracing accuracy selectively across the sleep retention interval for both explicit and implicit performance conditions.Finally, sleep also did not improve performance in a second-order probabilistic SRTT in old subjects (60 -80 years) (852).However, sleepdependent benefits are also not consistently revealed in younger subjects for this task, possibly due to the great complexity of the underlying sequence rules associated with a higher degree of implicitness at learning (852,1122,1123). To what extent the age-related decreases in sleep benefits for skill learning are related to specific alteration in sleep, in particular of SWS, is presently unclear as most of the relevant studies in aged subjects did not include polysomnographic recordings.REM sleep does not appear to be involved as neither experimental REM sleep deprivation nor REM sleep augmentation (i.e., the rebound after deprivation) produced any significant performance change in aged subjects on a procedural mirror tracing task (570).Nevertheless, like in young adults, high cholinergic activation during REM sleep seems to contribute to sleep-dependent consolidation of procedural memory also in aged individuals, as administration of the acetylcholinesterase inhibitor donezipil in aged subjects, together with an increase in REM density, significantly enhanced sleep-dependent gains in mirror tracing performance (570, 961).The contributions of impaired SWS to the age-related decline in procedural memory consolidation remain to be specified in future studies.

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§ About sleep's role in memory > 2. Sleep supports APC-T cell interactions There is converging evidence that sleep promotes Th1 immune responses as an immediate sequel of APC-T cell interactions.Thus sleep in humans enhances the production of IL-12 by monocytes and premyolid dendritic cells (pre-mDC) which represent the most important precursors of mature professional APC circulating in blood (307, 696).Concurrently, sleep diminishes the production of anti-inflammatory IL-10 by monocytes (696).Furthermore, several reports indicated an enhancing influence of sleep on T-cell production of IL-2 which acts to further promote T-cell growth, proliferation, and differentiation (120, 595), although other studies failed to reveal such effects when examining IL-2 production in specific T-cell subpopulations (109,308).Sleep increased proliferation of Th cells as well as the activity of natural regulatory T (nTreg) cells (109).Concurrent upregulation of nTreg activity might reflect a counterregulatory response preventing overshooting of Th1 responses.Moreover, early nocturnal sleep increased the ratio in IFN-␥/IL-4 production by Th cells, indicating a shift in the Th1/Th2 cytokine balance towards preponderant production of Th1 cytokines (308).In patients with allergic rhinitis, wheal reactions to a skin prick test and spontaneous IgE production of peripheral blood mononuclear cells were increased after a night of total sleep deprivation, indicating a shift from Th1 to prevailing Th2 immune defense (637).Finally, there is evidence that sleep increases blood concentrations of IL-7, a cytokine fostering differentiation of T memory cells (76).Together, these findings speak for the notion that sleep specifically benefits APC-T cell interactions towards the development of predominant Th1 immune responses.

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§ About sleep's role in memory > 3. Effects of sleep on the migration of T cells and APC to lymphoid tissues Sleep appears to affect also the migratory pattern of leukocytes, although the trafficking of T cells as well as APC has not been studied thoroughly in the context of sleep.Cells centrally involved in the formation of adaptive immunity, like naive and central memory T cells, show a circadian rhythm with the highest numbers of circulating cells at night in humans.This circadian rhythm is mainly driven by cortisol which upregulates the chemokine receptor CXCR4 on these cells and thereby facilitates their redistribution to the bone marrow during daytime (305, 693).On top of this circadian rhythm, nocturnal sleep, compared with wakefulness, induces a slight but significant reduction in circulating naive and central memory T cells (695), which indeed might reflect a redistribution of these cells to lymph nodes that is specific to sleep.In rodents, counts of lymphocytes as well as percentages of Th cells showed a variation across the 24-h period that ran in parallel in blood, spleen, and lymph nodes, with peak counts reached during the rest period, suggesting a quick redistribution of T cells from the vascular compartment to secondary lymphatic tissues.Two studies, one in sheep (289) and the other in humans (351), provided more direct evidence that the efflux of lymphocytes from lymph nodes is reduced during sleep.In rats, lymphocyte counts in lymph nodes indeed increased after recovery sleep that followed a period of sleep deprivation (1354).The mechanisms underlying this sleep-induced homing of T cells to lymph nodes are not clear.Homing to the lymph node via high endothelial venules relies on adhesion molecules and chemokine receptors (CD62L, CCR7, and CD11a) expressed on the T-cell membrane.In addition to the suppression of cortisol release during early nocturnal SWS, sleep may ease T-cell trafficking to lymph nodes via sympathetic and peptidergic innervation of lymph nodes and influences on cytokine release (117,234,876). In combination, these data support the view that sleep favors an accumulation of T cells in lymph nodes, and there might be a parallel effect on APC migration (307, 627).These migratory effects probably occur in concert with synergistic actions of the circadian rhythm and eventually might serve to enhance the rate of APC-Th cell interactions in secondary lymphoid tissues during sleep.In contrast to sleep, wakefulness appears to support the recruitment of cells in circulating blood that have high cytotoxic effector potential, like proinflammatory monocytes, terminally differentiated cytotoxic T cells, and cytotoxic natural killer (NK) cells, and serve the immediate immune defense against pathogens invading the organism preferentially during the activity period (e.g., Refs.42, 287).

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§ About sleep's role in memory > 3. Effects of sleep on the migration of T cells and APC to lymphoid tissues Sleep appears to affect also the migratory pattern of leukocytes, although the trafficking of T cells as well as APC has not been studied thoroughly in the context of sleep.Cells centrally involved in the formation of adaptive immunity, like naive and central memory T cells, show a circadian rhythm with the highest numbers of circulating cells at night in humans.This circadian rhythm is mainly driven by cortisol which upregulates the chemokine receptor CXCR4 on these cells and thereby facilitates their redistribution to the bone marrow during daytime (305, 693).On top of this circadian rhythm, nocturnal sleep, compared with wakefulness, induces a slight but significant reduction in circulating naive and central memory T cells (695), which indeed might reflect a redistribution of these cells to lymph nodes that is specific to sleep.In rodents, counts of lymphocytes as well as percentages of Th cells showed a variation across the 24-h period that ran in parallel in blood, spleen, and lymph nodes, with peak counts reached during the rest period, suggesting a quick redistribution of T cells from the vascular compartment to secondary lymphatic tissues.Two studies, one in sheep (289) and the other in humans (351), provided more direct evidence that the efflux of lymphocytes from lymph nodes is reduced during sleep.In rats, lymphocyte counts in lymph nodes indeed increased after recovery sleep that followed a period of sleep deprivation (1354).The mechanisms underlying this sleep-induced homing of T cells to lymph nodes are not clear.Homing to the lymph node via high endothelial venules relies on adhesion molecules and chemokine receptors (CD62L, CCR7, and CD11a) expressed on the T-cell membrane.In addition to the suppression of cortisol release during early nocturnal SWS, sleep may ease T-cell trafficking to lymph nodes via sympathetic and peptidergic innervation of lymph nodes and influences on cytokine release (117,234,876). In combination, these data support the view that sleep favors an accumulation of T cells in lymph nodes, and there might be a parallel effect on APC migration (307, 627).These migratory effects probably occur in concert with synergistic actions of the circadian rhythm and eventually might serve to enhance the rate of APC-Th cell interactions in secondary lymphoid tissues during sleep.In contrast to sleep, wakefulness appears to support the recruitment of cells in circulating blood that have high cytotoxic effector potential, like proinflammatory monocytes, terminally differentiated cytotoxic T cells, and cytotoxic natural killer (NK) cells, and serve the immediate immune defense against pathogens invading the organism preferentially during the activity period (e.g., Refs.42, 287).

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§ About sleep's role in memory > IX. CONCLUDING REMARKS

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§ About sleep's role in memory > IX. CONCLUDING REMARKS

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§ About sleep's role in memory > 3. Genetic studies on sleep-dependent memory consolidation

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§ About sleep's role in memory > 3. Genetic studies on sleep-dependent memory consolidation

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§ About sleep's role in memory > 3. Genetic studies on sleep-dependent memory consolidation

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§ About sleep's role in memory > 1. Memory processes

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§ About sleep's role in memory > Abstract Over more than a century of research has established the fact that sleep benefits the retention of memory.In this review we aim to comprehensively cover the field of "sleep and memory" research by providing a historical perspective on concepts and a discussion of more recent key findings.Whereas initial theories posed a passive role for sleep enhancing memories by protecting them from interfering stimuli, current theories highlight an active role for sleep in which memories undergo a process of system consolidation during sleep.Whereas older research concentrated on the role of rapid-eye-movement (REM) sleep, recent work has revealed the importance of slow-wave sleep (SWS) for memory consolidation and also enlightened some of the underlying electrophysiological, neurochemical, and genetic mechanisms, as well as developmental aspects in these processes.Specifically, newer findings characterize sleep as a brain state optimizing memory consolidation, in opposition to the waking brain being optimized for encoding of memories.Consolidation originates from reactivation of recently encoded neuronal memory representations, which occur during SWS and transform respective representations for integration into long-term memory.Ensuing REM sleep may stabilize transformed memories.While elaborated with respect to hippocampus-dependent memories, the concept of an active redistribution of memory representations from networks serving as temporary store into long-term stores might hold also for non-hippocampusdependent memory, and even for nonneuronal, i.e., immunological memories, giving rise to the idea that the offline consolidation of memory during sleep represents a principle of long-term memory formation established in quite different physiological systems.

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• AssumptionThe authors state this as an assumption: The waking brain is optimized for acute processing of external stimuli involving encoding and retrieval, while the sleeping brain provides optimal conditions for consolidation processes that integrate newly encoded memory into a long-term store.

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§ About sleep's role in memory > A. Sleep

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§ About sleep's role in memory > A. Sleep

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§ About sleep's role in memory > 3. The standard two-stage memory system Why does the consolidation of memory have to take place during sleep?The hypotheses that sleep serves memory consolidation is conceptually rooted in the standard two-stage memory system which is currently the most influential model of human memory, and has been developed as a solution to several key problems arising from simple associative network models of memory (175,780,800).The foremost of these problems is that although simple association networks are in fact able to store information very rapidly, as is the case in the declarative memory system , the uptake of new conflicting information has a strong tendency to erase the older memories, thus inducing so-called "catastrophic interference" (1005).The critical question is how the neuronal network can learn new patterns without simultaneously forgetting older memories, an issue that has also been referred to as the "stability-plasticity dilemma" (e.g., see Ref. 3).In addition, unstructured recurrent networks have been demonstrated to face essential capacity constraints (723).The two-stage memory formation mechanism first proposed by Marr (780) offers a solution to these problems.It assumes that memories are initially encoded into a fast learning store (i.e., the hippocampus in the declarative memory system) and then gradually transferred to a slow learning store for long-term storage (i.e., the neocortex).The fast learning store ensures quick and efficient encoding of memories, even in one attempt (one-trial learning).Yet, these representations are unstable and vulnerable to (retroactive) interference by newly encoded information.Over time, the information is gradually integrated in the slowly learning long-term store without overwriting older, more remote memories.It is assumed that by the repeated reactivation of the new memories during off-line periods like sleep, the slowly learning long-term store is trained and the new memories are gradually strengthened and adapted to preexisting long-term memories.The transformation new memory representations undergo in this system consolidation process comprises also the extraction of invariants and the development of prototypes and schemas, as the core of the newly learned information is reactivated more frequently than divergent details (734,800,1239).For the declarative memory system, the two-stage model has received strong support from lesion studies, indicating that lesions of the hippocampus abolish the ability to acquire new declarative memory and simultaneously produce a temporally graded retrograde amnesia where older memories remain intact (231,418).The time interval for a memory to reach a state of hippocampus-independent retrieval can vary from one day to several months or years, depending on the acquired information and the schemas preexisting in long-term memory (1210,1308).The standard twostage model of memory has been also successfully applied to nondeclarative kinds of memory, like procedural memory (668), suggesting that the offline reactivation of recent memories and their redistribution from a fast encoding temporary to a slowly learning permanent store could be a general feature of long-term memory formation.

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• AssumptionThe authors state this as an assumption: Memories are initially encoded into a fast learning store (the hippocampus in the declarative memory system) and then gradually transferred to a slow learning store for long-term storage (the neocortex).

§ About sleep's role in memory > A. Sleep

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§ About sleep's role in memory > 1. Memory processes

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§ About sleep's role in memory > 2. Memory systems

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§ About sleep's role in memory > 1. Memory processes