Effects of Transcranial Electrical Stimulation on Cognition

“Reis and coworkers showed that repetitive tDCS over 5 consecutive days combined with a motor learning protocol resulted in increasing effects on performance, lasting for at least 3 months after training.(44) For right-handed individuals, anodal stimulation of the left primary motor cortex seems to be most effective, r…”

“Fregni and colleagues explored the effects of anodal, sham, or cathodal stimulation of the left dorsolateral prefrontal cortex on performance in a 3-back letter task.(28) They describe increased accuracy in task performance selectively under anodal tDCS.”

“Interestingly, stimulation for some minutes induced aftereffects, which lasted for at least some hours after the stimulation.(8) Similar effects can be obtained by noninvasive stimulation with direct currents in humans.”

“spontaneous cortical activity stimulation polarity dependently.(8,9) Anodal tDCS enhanced cortical activity and excitability, while cathodal stimulation had the opposite effect.”

“1-640 Hz) with random noise distribution.(7) The frequency range covers specific physiological human brain oscillations, by which tRNS possibly induces LTP-like cortical plasticity via augmenting the activity of neuronal sodium channels.”

“sleep.(51,52) For naturally available material, Fiori and colleagues describe a positive effect of anodal tDCS over Wernicke’s area on picture naming.”

“tACS over M1 facilitated motor sequence learning only when applied at alpha frequency, while high ripple frequency (140 Hz) stimulation had no effect on the same learning protocol.(6,74) Alpha oscillations are associated with the inhibition of irrelevant stimuli during cognitive tasks,(75) and this may explain the fac…”

“With regard to behavior in risk-taking tasks, it was reported that anodal tDCS of the right or left dorsolateral prefrontal cortex, which are involved in risky decision making, resulted in more cautious behavior, when combined with cathodal stimulation of the contralateral prefrontal cortex, thus altering hemispheric…”

“In another study of the same group, however, only anodal stimulation of the right combined with cathodal tDCS of the left dorsolateral prefrontal cortex reduced risky behavior.(68) In a task involving social cognition, Knoch and coworkers explored the importance of the right prefrontal cortex for performance in the ul…”

“Flöel and colleagues explored the effect of tDCS on associative verbal learning and showed that selectively anodal tDCS over the left perisylvian area applied during learning improved performance.(46) In contrast, learning of verbal material was impaired by cathodal tDCS of the left dorsolateral prefrontal cortex.”

“Phosphene perception was more effective when tACS was applied in the beta frequency range in an illuminated condition, whereas tACS at alpha frequencies improved phosphene perception in a dark surrounding.(71) Since beta frequencies are predominant in illuminated surroundings, whereas alpha frequencies dominate under…”

“Zaehle and coworkers describe similar positive effects of anodal tDCS on response accuracy in a 2-back working memory task, while cathodal tDCS disturbed the performance.(30) Interestingly, anodal tDCS enhanced alpha and theta activity, while cathodal tDCS had reversed effects, thus offering a plausible physiological…”

“With distractors, however, the effects of tDCS were reversed.(10) These effects were explained by a noise-reducing effect of excitability-diminishing cathodal stimulation in the condition with distractors, whereas excitability-enhancing anodal stimulation was presumed to enhance performance via the increased activatio…”

“Dockery and colleagues describe a phase-specific effect of tDCS over the left dorsolateral prefrontal cortex on performance in the Tower of London task, which involves strategic planning.(66) Cathodal tDCS improved performance during the early acquisition phase of task performance, probably due to its reducing effect…”

“For motor learning, it is thought that within the motorrelated cortical networks, which include the primary motor cortex, premotor, and parietal association areas, the primary motor cortex is involved in early learning phases.(35) It was shown that for a motor sequence learning task, as well as for visuomotor coordina…”

“In the latter study, the accuracy of an orientation discrimination task was significantly increased by tRNS in the high-frequency range (100-640 Hz).(76) In contrast, application of tRNS on right dorsal lateral prefrontal cortex impaired categorical learning in a prototype distortion task.”

“In accordance, anodal tDCS of this region improved performance of respective tasks in 2 studies.(61,62) With regard to the impact of false memories on task performance, the left anterior temporal cortex is thought to be involved.”

“For naturally available material, Fiori and colleagues describe a positive effect of anodal tDCS over Wernicke’s area on picture naming.(53) In principal accordance, anodal tDCS improved proper name retrieval for the pictures of famous individuals, when the anterior temporal lobes were stimulated.”

“Ferrucci and coworkers probed the involvement of the cerebellum on performance of the Sternberg task and described disturbed performance induced by both, anodal and cathodal tDCS.(34) Taken together, tDCS seems to be an efficient tool to alter working memory performance in healthy humans.”

“Excitability-diminishing cathodal tDCS reduced contrast perception, while anodal tDCS was without effect.(16) In another study, somewhat stronger and longer stimulation (15 minutes, 1 mA, electrode size 35 cm(2)) of the same area showed enhanced contrast sensitivity of central visual regions via anodal tDCS.”

“In a related study, again the speed of performance was the only parameter that improved for a 2-back but not for a 1-back letter task, when anodal tDCS was conducted before performance.(32) The reasons for these heterogeneous results are unclear.”

“In a task involving social cognition, Knoch and coworkers explored the importance of the right prefrontal cortex for performance in the ultimatum game.(69) In this game, a fixed monetary reward was to be split between 2 participants.”

“Cerruti and Schlaug probed the ability of tDCS to alter complex associative thought.(64) They report a specific beneficial effect of anodal tDCS applied to the left dorsolateral prefrontal cortex on the remote associates test, in which the volunteers had to identify a word that forms a compound noun with 3 other words…”

“For the effect of tDCS on grammar learning, de Vries and colleagues applied anodal tDCS to the Broca area during artificial grammar learning.(50) Anodal stimulation resulted in improved performance, as compared to a sham-stimulated group.”

“Similar effects can be obtained by noninvasive stimulation with direct currents in humans.(1-3,10,11) The after-effects of stimulation share some things in common with neuroplastic phenomena known from animal experimentation, such as dependency on the glutamatergic system and calcium channels.”

“In contrast, 20-minute anodal tDCS of the same area, when working memory (3-back letter and Sternberg task) was explored after the end of 20-minute tDCS, did not improve accuracy, but reduced the reaction times in the 3-back letter task in a complex time-dependent manner.(31) In a related study, again the speed of per…”

“Chi and Snyder describe performance enhancement with regard to solving an insight problem, when anodal tDCS of the right was combined with cathodal stimulation of the left anterior temporal lobe.(65) Dockery and colleagues describe a phase-specific effect of tDCS over the left dorsolateral prefrontal cortex on perform…”

“sleep.(51,52) For naturally available material, Fiori and colleagues describe a positive effect of anodal tDCS over Wernicke’s area on picture naming.”

“For the memorization of the localization of objects in a natural surrounding, Flöel and coworkers showed that anodal tDCS over the right temporoparietal cortex, which is involved in this type of task, improved memory consolidation, when performance was tested 1 week after learning.(57) Clark and colleagues explored th…”

“tACS over M1 facilitated motor sequence learning only when applied at alpha frequency, while high ripple frequency (140 Hz) stimulation had no effect on the same learning protocol.(6,74) Alpha oscillations are associated with the inhibition of irrelevant stimuli during cognitive tasks,(75) and this may explain the fac…”

“For the somatosensory modality, tactile sensations could be induced when S1 was stimulated by tACS, most effectively by stimulation within the alpha range frequency, followed by high gamma and beta stimulation.(73) The results of this study suggest that tACS might be suited not only to modulate but also to induce perc…”

“For right-handed individuals, anodal stimulation of the left primary motor cortex seems to be most effective, regardless of the performing hand.(45) The effect of tDCS on learning and retention of verbal material was explored in a couple of studies.”

“For modulation of attention, Bolognini and coworkers explored the effects of anodal tDCS applied to the posterior parietal cortex on multisensory field exploration.(27) Stimulation of the right parietal cortex improved visual exploration and orienting, as compared to sham stimulation, underscoring the causal involveme…”

“However, anodal tDCS applied to S1 resulted in improved spatial acuity of the contralateral index finger in another study.(20) For perception of painful stimuli, Antal and colleagues describe a diminishing effect of cathodal tDCS over the somatosensory cortex.”

“With regard to other cortical areas, Berryhill and colleagues explored the effects of tDCS over right inferior parietal regions on an object recognition and recall working memory task.(33) Anodal, cathodal, or sham tDCS were performed before task performance.”

“Moreover, cathodal tDCS of the primary motor cortex reduced learning of action-related words, being in favor of an involvement of this area in learning this kind of material.(49) For the effect of tDCS on grammar learning, de Vries and colleagues applied anodal tDCS to the Broca area during artificial grammar learning.”

“In accordance, anodal tDCS of this area reduced encoding and retrieval of false memories.(63) Taken together, an increasing number of studies have been conducted in which the impact of tDCS on learning and memory formation was explored.”

“In accordance, anodal tDCS of this region improved performance of respective tasks in 2 studies.(61,62) With regard to the impact of false memories on task performance, the left anterior temporal cortex is thought to be involved.”

“Interestingly, anodal stimulation of the premotor cortex, which is involved in late sleepdependent consolidation of motor sequence learning,(41) was not effective during task learning, but was during rapid eye movement sleep-dependent consolidation.(42) This phase specificity of effective stimulation of cortical areas…”

“For visual memory performance, Chi and coworkers conducted a study which involved bilateral stimulation of the anterior temporal lobes during encoding and retrieval of a visual memory task.(55) They describe improved visual memory via right anodal/left cathodal stimulation but not under reversed stimulation polarities…”

“Cathodal tDCS of the same areas resulted in reversed effects.(26) For modulation of attention, Bolognini and coworkers explored the effects of anodal tDCS applied to the posterior parietal cortex on multisensory field exploration.”

“Interestingly, in a companion study, which used the same somatosensory testing procedures, cathodal stimulation of the primary motor cortex increased cold and mechanical detection thresholds as well as mechanical pain thresholds.(23) Taken together, the studies conducted in the field of somatosensory perception show a…”

“In a subsequent study, anodal tDCS over the auditory cortex improved temporal processing in the auditory domain of healthy humans, while cathodal stimulation resulted in reversed effects.(25) Finally, tDCS of occipital and temporal areas altered multisensory perception in a ‘‘sound-induced flash illusion task.”

“Penolazzi and colleagues explored the impact of bilateral frontotemporal stimulation on encoding of emotionally valenced pictures.(56) In this study, right anodal/left cathodal tDCS resulted in improved memory for emotionally pleasant, while left anodal/right cathodal stimulation increased the recall of emotionally un…”

“It was shown that for a motor sequence learning task, as well as for visuomotor coordination, excitability-enhancing anodal tDCS of the primary motor cortex performed during or immediately after task performance improved learning and early consolidation,(36-39) as well as intrinsic generalization of joint movements.(4…”

“Importantly, this effect was dosage dependent, and its size was larger for novices as compared to experienced participants, as shown in a consecutive study of this group.(59) Few studies were performed for other kinds of learning and memory formation.”

“In contrast, learning of verbal material was impaired by cathodal tDCS of the left dorsolateral prefrontal cortex.(47,48) Moreover, cathodal tDCS of the primary motor cortex reduced learning of action-related words, being in favor of an involvement of this area in learning this kind of material.”

“Loui and coworkers describe reduced auditory pitch matching ability, when cathodal tDCS was applied over areas involved in this perceptual task, namely inferior frontal and superior temporal areas.(24) In a subsequent study, anodal tDCS over the auditory cortex improved temporal processing in the auditory domain of he…”

“In another study, somewhat stronger and longer stimulation (15 minutes, 1 mA, electrode size 35 cm(2)) of the same area showed enhanced contrast sensitivity of central visual regions via anodal tDCS.(17) Different results between the studies might be caused by the variations in the stimulation protocols as well as by…”

“However, today there is at least a first approximation of an answer to this question for many of the most prevalent types of mental illness.”

“For instance, aftereffects of both TMS and tDCS have been related to long term depression (LTD) and potentiation (LTP) [11,12,14,38,72,78,79] depending on parameters which do not show any obvious link to intrinsic brain oscillations.”