The human brain in numbers: a linearly scaled-up primate brain

In the paper

“Among birds, those that are larger-brained (corvids, parrots and owls) are also considered the most intelligent (Lefebvre et al., 2004).”

What the sources say

• Partly supported

  The “Neural and Cognitive Correlates of Innovation in Birds” section identifies corvids, parrots, and owls among larger-brained birds and reports a general association between brain size and cognitive measures. It does not establish that these three groups are considered the most intelligent; in particular, the passage listing owls does not link them to superior cognition. Page numbers are unavailable in our copy. (LLM, conf=0.84)

  cites Lefebvre et al., 2004 sourceExperimental — accuracy not yet measured

In the paper

“Absolute brain mass and number of neurons, left out of the encephalization equation, must clearly be taken into consideration, since the “exceeding number of neurons” in a large brain should necessarily be larger than that in a smaller brain of same EQ (Herculano-Houzel, 2007).”

What the sources say

• Partly supported

  The ‘Implications for Volume Comparisons Across Orders’ section says that comparisons of brain size should consider neuronal counts directly, not just brain volume. The paper also relates larger primate brain structures to greater numbers of neurons. It does not address encephalization quotient (EQ) or establish that a larger brain has more ‘exceeding’ neurons than a smaller brain with the same EQ; no page numbers are available in our copy. (LLM, conf=0.86)

  cites Herculano-Houzel, 2007 sourceExperimental — accuracy not yet measured

In the paper

“we have suggested that, rather than humans having a larger brain than expected, it is the great apes such as orangutans and, more notably, gorillas that have bodies that are much larger than expected for primates of their brain size (Herculano-Houzel et al., 2007).”

What the sources say

• Partly supported

  Split into 2 atoms — 0 confirmed, 2 partially supported, 0 disputed, 0 misattributed, 0 unverifiable. Expand the claim to see per-atom verdicts.

  cites Herculano-Houzel et al., 2007 sourceExperimental — accuracy not yet measured

In the paper

“primate brain size increases approximately isometrically as a function of neuron number, with no systematic change in neuronal density or in the non-neuronal/neuronal ratio with increasing brain size”

What the sources say

• Supported

  The Discussion states that primate brain size increases linearly with neuron number, neuronal densities remain stable, and the nonneuronal-to-neuronal cell ratio does not correlate with brain mass. These findings support all three parts of the paraphrase. (LLM, conf=0.94)

  cites Herculano-Houzel et al., 2007 source

In the paper

“In the order Rodentia, we find that the brain increases in size faster than it gains neurons, with a decrease in neuronal densities which, in the presence of constant non-neuronal cell densities, implies that average neuronal size increases rapidly as neurons become more numerous (Herculano-Houzel et al., 2006).”

What the sources say

• Supported

  The Results report that brain mass increases faster than neuron number across the six rodent species, while neuronal density decreases and nonneuronal cell density varies very little with structure size. The Discussion says neurons increase in average size as their numbers increase; the paper treats that size change as an estimate rather than a direct measurement. (LLM, conf=0.93)

  cites Herculano-Houzel et al., 2006 source

In the paper

“Different mammalian orders have traditionally been pooled together in studies of brain allometry, as if their brains were built according to the same scaling rules (for example, Haug, 1987; Zhang and Sejnowski, 2000).”

What the sources say

• ?Not checked — no full text of the sourcecites Haug, 1987 source
• Supported

  Section 2 (Empirical Scaling Law; page not provided) analyzes 59 mammalian species together under a single gray–white matter power law. The Introduction also describes earlier scaling studies using data from different species. This paper therefore serves as an example of pooling mammalian species under a shared scaling rule, though it does not explicitly classify them by order. (LLM, conf=0.88)

  cites Zhang and Sejnowski, 2000 source

In the paper

“the cerebral cortex, despite varying in relative size from 42% (in the mouse) to 82% of brain mass (in the human), contains between 13 and 28% of all brain neurons in 15 of 18 species studied, ranging between 13% (in moles) and 41% (in the squirrel monkey; Herculano-Houzel et al., 2006, 2007; Sarko et al., 2009).”

What the sources say

• Partly supported

  The Results (page not provided) report that cortical mass increases with brain size while the cortex contains a relatively stable 17.8 ± 3.4% of brain neurons across six rodent species. This supports the general pattern, but the paper does not establish the quoted 42%–82% mass range or the neuron percentages across 18 species, including humans, moles, and squirrel monkeys. (LLM, conf=0.88)

  cites Herculano-Houzel et al., 2006 sourceExperimental — accuracy not yet measured
• Partly supported

  In “Fractional Distribution of MBR and Neurons” (page unavailable), the paper reports that the cerebral cortex contains 18.9–41.8% of brain neurons in the primates examined, with 41.8% in the squirrel monkey (Saimiri). It does not establish the claimed 15-of-18-species distribution or the figures for moles, mice, and humans; its reported cortical mass range is 66.8–79.9% for the examined primates. (LLM, conf=0.88)

  cites Herculano-Houzel et al., 2007 sourceExperimental — accuracy not yet measured
• ?Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  cites Sarko et al., 2009

In the paper

“an informal survey with senior neuroscientists that we ran in 2007 showed that most believed that the number of cells in the human brain was indeed already known: that we have about 100 billion neurons, outnumbered by about 10 times more glial cells – but none of the consulted scientists could cite an original reference for these numbers”

In the paper

“the so-called overdeveloped human cerebral cortex holds only 19% of all brain neurons, a fraction that is similar to that found in other mammals.”

In the paper

“the human cerebral cortex, which represents 82% of brain mass, holds only 19% of all neurons in the human brain”

In the paper

“An examination of the cellular composition of the cerebellum of orangutans and one gorilla shows that the sizes of the cerebellum and cerebral cortex predicted for these species from the number of cells in the cerebellum match their actual sizes, which suggests that the brain of these animals indeed is built according to the same scaling rules that apply to humans and other primates (Herculano-Hou”

What the sources say

• ?Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  cites Herculano-Houzel and Kaas, in preparation

In the paper

“the absolute number of neurons in the rodent and primate cerebral cortex does increase much faster in larger brains compared to the number of neurons in the combined brainstem, diencephalon and basal ganglia, and is accompanied by a similarly fast increase in the number of neurons in the cerebellum (Figure 5).”

In the paper

“compared to rodents, and probably to whales and elephants as well, it is built according to the very economical, space-saving scaling rules that apply to other primates”

In the paper

“led two authors to conclude that the “factors that correlate better with intelligence (across species) are the number of cortical neurons and conduction velocity, as the basis for information processing” (Roth and Dicke, 2005).”

What the sources say

• ?Not checked — no full text of the sourcecites Roth and Dicke, 2005 source

In the paper

“Many original articles, reviews and textbooks affirm that we have 100 billion neurons and 10 times more glial cells (Kandel et al., 2000; Ullian et al., 2001; Doetsch, 2003; Nishiyama et al., 2005; Noctor et al., 2007; Allen and Barres, 2009), usually with no references cited.”

What the sources say

• ?Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  cites Kandel et al., 2000
• ?Not checked — no full text of the sourcecites Ullian et al., 2001 source
• ?Not checked — no full text of the sourcecites Doetsch, 2003 source

In the paper

“These authors estimated the number of neurons in the human brain at about 85 billion: 12–15 billion in the telencephalon (Shariff, 1953), 70 billion in the cerebellum, as granule cells (based on Lange, 1975), and fewer than 1 billion in the brainstem.”

What the sources say

• ?Not checked — no full text of the sourcecites Williams and Herrup (1988) source
• ?Not checked — no full text of the sourcecites Shariff, 1953 source
• ?Not checked — no full text of the sourcecites Lange, 1975 source

In the paper

“among economically built primate brains, it is the largest, hence containing the most neurons.”

In the paper

“since different cellular scaling rules apply to the brains within these orders, brain size can no longer be considered a proxy for the number of neurons in the brain.”

In the paper

“the only possible explanation for the ubiquitous quote of 10 times more glial than neuronal cells in the entire human brain would be the presence of nearly one trillion glial cells in the remaining structures alone – an unlikely scenario, since these structures represent <10% of total brain mass.”

What the sources say

• ?Not checked — no full text of the sourcecites Andersen et al., 1992 source

In the paper

“Monkeys, for instance, possess brains that are much smaller than those of ungulates, but the higher cognitive and behavioral flexibility of monkeys over ungulates is anecdotally evident to any observer who compares the ingenious and complex abilities of macaques to those of cows or horses, even though the latter have 4–5× larger brains than macaques.”

In the paper

“Across species, the number of neurons and their relative abundance in different parts of the brain is widely considered to be a determinant of neural function and, consequently, of behavior (Williams and Herrup, 1988).”

What the sources say

• ?Not checked — no full text of the sourcecites Williams and Herrup, 1988 source

In the paper

“This is evidence that the cellular scaling rules considered here from a set of primate species also extend to primates as a whole, and can be used to infer the expected cellular composition of the human brain”

In the paper

“Across insectivore species, on the other hand, the cerebellum increases linearly in size as a function of its number of neurons (as in primates), while the cerebral cortex increases in size hypermetrically as it gains neurons (as in rodents; Sarko et al., 2009).”

What the sources say

• ?Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  cites Sarko et al., 2009

In the paper

“These findings corroborated previous studies describing neuronal density decreasing and the glia-to-neuron ratio increasing with increasing brain size across mammalian taxa (Tower and Elliot, 1952; Shariff, 1953; Friede, 1954; Tower, 1954; Hawking and Olszewski, 1957; Haug, 1987; Reichenbach, 1989; Stolzenburg et al., 1989).”

What the sources say

• ?Not checked — no full text of the sourcecites Tower and Elliot, 1952 source
• ?Not checked — no full text of the sourcecites Shariff, 1953 source
• ?Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  cites Friede, 1954

In the paper

“our quantitative studies on the cellular scaling rules that apply to different mammalian orders have shown that this assumption is invalid and therefore should no longer be applied”

In the paper

“With a brain of only 52 g, the behavioral, social and cognitive repertoire of the capuchin monkey is outstanding compared to the capybara, a giant Amazonian rodent (MacDonald, 1981), even though the latter has a larger brain of 75 g.”

What the sources say

• ?Not checked — no full text of the sourcecites MacDonald, 1981 source

In the paper

“Capuchin monkeys, for instance, have much larger EQs than gorillas (Marino, 1998), but are outranked by these in cognitive performance (Deaner et al., 2007).”

What the sources say

• ?Not checked — no full text of the sourcecites Marino, 1998 source
• ?Not checked — no full text of the sourcecites Deaner et al., 2007 source

In the paper

“if primates have big brains merely because they have small bodies, we cannot presume that this represents an evolutionary trend driven by cognitive demands”

What the sources say

• ?Not checked — no full text of the sourcecites Deacon, 1997 source

In the paper

“Brain size varies across mammals by a factor of approximately 100,000 (Tower, 1954; Stolzenburg et al., 1989).”

What the sources say

• ?Not checked — no full text of the sourcecites Tower, 1954 source
• ?Not checked — no full text of the sourcecites Stolzenburg et al., 1989 source

In the paper

“compared to other mammalian orders, primate encephalization is the result of a shift in postcranial growth processes, not a modification of brain growth (Deacon, 1997).”

What the sources say

• ?Not checked — no full text of the sourcecites Deacon, 1997 source

In the paper

“Compared to mammals as a whole, humans have the largest EQ found for any mammal, of between 7 and 8 (Jerison, 1973)”

What the sources say

• ?Not checked — no full text of the sourcecites Jerison, 1973

In the paper

“like in other non-anthropoid primates, the human brain mass represents about 2% of body mass.”

In the paper

“a generic rodent brain of human-sized proportions, weighing 1.5 kg: such a brain would have only 12 billion neurons, and a much larger number of 46 billion non-neuronal cells”

In the paper

“the distribution of cortical mass in humans may differ from that in other primates, endowing particularly relevant regions such as area 10 with relatively more neurons in the human cortex (Semendeferi et al., 2001).”

What the sources say

• ?Not checked — no full text of the sourcecites Semendeferi et al., 2001 source

In the paper

“although gorillas and orangutans overlap or exceed humans in body size, their brains amount to only about one-third of the size of the human brain.”

In the paper

“even if compared to anthropoid primates only, humans still have an EQ of over 3, a value that is larger than that obtained for any other primate or cetacean (Marino, 1998).”

What the sources say

• ?Not checked — no full text of the sourcecites Marino, 1998 source

In the paper

“The very concept of encephalization presupposes that not only the brain scales as a function of body size, but that all brains scale the same way, such that the only informative (and sufficient) variable is brain size and its deviation from the expected.”

In the paper

“its relatively large cerebral cortex, which accounts for 82% of brain mass.”

In the paper

“according to this notion, small-brained animals with very large EQs should be expected to have more cognitive abilities than large-brained animals with smaller EQs.”

In the paper

“By maintaining the average neuronal size (including all arborizations) invariant as brain size changes, primate brains scale in size in a much more space-saving, economical manner compared to the inflationary growth that occurs in rodents, in which larger numbers of neurons are accompanied by larger neurons.”

In the paper

“This assumption, which was justifiable by the lack of direct estimates of the neuronal composition of the brain of different species, is so widespread that it implicitly or explicitly underlies most comparative studies to date (for example, Haug, 1987; Finlay and Darlington, 1995; Barton and Harvey, 2000; Clark et al., 2001).”

What the sources say

• ?Not checked — no full text of the sourcecites Haug, 1987 source
• ?Not checked — no full text of the sourcecites Finlay and Darlington, 1995 source
• ?Not checked — no full text of the sourcecites Barton and Harvey, 2000 source

In the paper

“the human brain exhibits a further modification in that it continues to grow as though in a larger body (Deacon, 1997).”

What the sources say

• ?Not checked — no full text of the sourcecites Deacon, 1997 source

In the paper

“sheer size alone, or in relation to body size, is not an adequate parameter to qualify, or disqualify, the human brain as “special””

In the paper

“our standing among primates as the proud owners of the largest living brain assures that, at least among primates, we enjoy the largest number of neurons from which to derive cognition and behavior as a whole”

In the paper

“the enlargement of the cerebral cortex is not, in principle, an exclusive feature of the human brain: a similar expansion of the mass of the cerebral cortex, relative to the whole brain, is predicted by both the rodent and primate cellular scaling rules, irrespective of the number of neurons contained in the cortex (Table 2).”

In the paper

“if the same linear scaling rules are considered to apply to great apes as to other primates, then similar three-fold differences in brain size and in brain neurons alike apply to humans compared to gorillas, and to gorillas compared to baboons”

In the paper

“Azevedo et al. (2009), who found that the adult male human brain, at an average of 1.5 kg, has 86 billion neurons and 85 billion non-neuronal cells – numbers that deviate from the expected by 7 and 24% only.”

What the sources say

• ?Not checked — no full text of the sourcecites Azevedo et al. (2009) source

In the paper

“the human brain scales as a primate brain: this economical property of scaling alone, compared to rodents, assures that the human brain has many more neurons than would fit into a rodent brain of similar size, and possibly into any other similar-sized brain”

In the paper

“This generic primate brain should have a cerebral cortex of about 1.4 kg, containing 25 billion neurons, and a cerebellum weighing 120 g, with 70 billion neurons (Table 2).”

In the paper

“a hypothetical rodent brain with 86 billion neurons, like the human brain, would be predicted to weigh overwhelming 35 kg – a value that is way beyond the largest known brain mass of 9 kg for the blue whale, and probably physiologically unattainable”

In the paper

“it becomes increasingly likely that humans do not have truly unique cognitive abilities, and hence must differ from these animals not qualitatively, but rather in the combination and extent of abilities such as theory of mind, imitation and social cognition (Marino et al., 2009).”

What the sources say

• ?Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  cites Marino et al., 2009

In the paper

“although men have been reported to have more neurons in the cerebral cortex than women (Pakkenberg and Gundersen, 1997; Pelvig et al., 2008)”

What the sources say

• ?Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  cites Pakkenberg and Gundersen, 1997
• ?Not checked — no full text of the sourcecites Pelvig et al., 2008 source

In the paper

“Because of the diverging power laws that relate brain size and number of neurons across rodents and primates, the latter can hold more neurons in the same brain volume, with larger neuronal densities than found in rodents.”

In the paper

“the increase in cognitive abilities afforded by increasing the number of neurons in the brain can be expected to increase exponentially with absolute number of neurons, and might even be subject to a thresholding effect once critical points of information processing are reached”

In the paper

“Since neuronal density does not scale with brain size in primates, but decreases with increasing brain size in rodents, the larger the brain size, the larger is the difference in number of neurons across similar-sized rodent and primate brains.”

In the paper

“The human cerebral cortex, with an average 1233 g and 16 billion neurons, is slightly below expectations for a primate brain of 1.5 kg”

In the paper

“glial cells in the human brain are at most 50% of all brain cells, which is an important finding since it is one more brain characteristic that we share with other primates (Azevedo et al., 2009)”

What the sources say

• ?Not checked — no full text of the sourcecites Azevedo et al., 2009 source

In the paper

“Not only the extent of intraspecific variation is much smaller (on the order of 10–50%) than interspecific variation (which spans five orders of magnitude within mammals; Tower, 1954; Stolzenburg et al., 1989)”

What the sources say

• ?Not checked — no full text of the sourcecites Tower, 1954 source
• ?Not checked — no full text of the sourcecites Stolzenburg et al., 1989 source

In the paper

“while the human cerebellum, at 154 g and 69 billion neurons, matches or even slightly exceeds the expected (Table 2)”

In the paper

“According to one estimate, the false killer whale and the African elephant would have about 11 billion neurons in the cerebral cortex, despite their large size – and fewer neurons than the 11.5 billion estimated by the same method for the human cerebral cortex, though only marginally so (Roth and Dicke, 2005).”

What the sources say

• ?Not checked — no full text of the sourcecites Roth and Dicke, 2005 source

In the paper

“According to these rules, a generic primate brain of 1.5 kg should have 93 billion neurons, and 112 billion non-neuronal cells: glial cells, thus, should constitute at most half of all brain cells.”