Brain Size Cortex Folding Toro 2008

Jun 18, 2010 11:31 AM

Roberto TORO Nottingham

Tomas Paus  McGill  et al  McGill, Montreal Neuro Inst

from Cereb Cortex   2008   great paper.

During evolution, the mammalian cerebral cortex has expanded

disproportionately to brain volume. As a consequence, most

mammals with large brains have profusely convoluted cortices.

The human cortex is a good example of this trend, however, given

the large variability in human brain size, it is not clear how cortical

folding varies from the smallest to the largest brains. We analyzed

cortical folding in a large cohort of human subjects exhibiting a 1.7-

fold variation in brain volume. We show that the same

disproportionate increase of cortical surface relative to brain

volume observed across species can be also observed across

human brains: the largest brains can have up to 20% more surface

than a scaled-up small brain. We introduce next a novel local

measure of cortical folding, and we show that the correlation

between cortical folding and size varies along a rostro-caudal

gradient, being especially signiﬁcant in the prefrontal cortex. The

expansion of the cerebral cortex, and in particular that of its

prefrontal region, is a major evolutionary landmark in the

emergence of human cognition. O

See figure 1 for answer to how big is brain and how big surface area of cortex

and how does it vary across age in teenagers.

Surface area of cortex =  2400 cm2

Volume is about 1070  cm3  (total hemispheric volume).

2356  last page

The increase in brain size is one of the most evident features

of human brain evolution. Among mammals, primates have

particularly large brains in relation to body size, and humans

have the largest brains among all primates (Jerison 1973;

Passingham 1973). Genes involved in the regulation of brain

size seem to be among the fastest evolving in primates, and

especially in the lineage leading to humans (Dorus et al. 2004;

Vallender and Lahn 2006). In humans, nonfunctioning muta-

tions of some of these genes, Microcephalin, Abnormal Spindle-

like Microcephaly-associated, and Sonic Hedgehog, cause

different types of microcephaly—a severe reduction of total

brain size to levels typical of great apes or Australopithecines,

accompanied by an overall simpliﬁcation of the folding pattern

The increase in prefrontal folding with brain size may be

another aspect of the link between the size of different brain

structures and their time of development during ontogenesis

(Finlay and Darlington 1995; Finlay et al. 2001). By analyzing

the volume of different brain structures across species, Finlay

and Darlington (1995) observed that the rostral telencephalic

structures exhibit the steepest increase in volume relative to

body size. The more caudal structures of the rhombencephalon

start their development earlier than rostral structures such as

the cerebral cortex, which take also longer to reach maturation.

It is possible that the differences in prefrontal folding that we

observed, and perhaps also those observed among primates, can

be a consequence of the prolongation of this rostro-caudal

gradient into the cerebral cortex. This idea is supported by the

fact that human gyriﬁcation lasts up to 1 month longer in the

rostral than in the caudal regions of the cerebral cortex

(Armstrong et al. 1995).

Under the hypothesis of mosaic evolution, the greater

elaboration of the prefrontal cortex in humans may be seen

as resulting from the selection of the cognitive functions it

supports. It is also possible, however, that the availability of

more cortical surface leads to the further specialization of this

region by allowing a larger elaboration of the corresponding

cytoarchitectonic ﬁelds or the emergence of new cytoarchi-

tectonic structures, which may then support the reﬁnement of

its associated functions. Brodmann’s area 10, which occupies

a large part of the prefrontal cortex, is indeed greatly expanded

in humans compared with other primates (Semendeferi et al.

2001; Allman et al. 2002; Jerison 2006), and spindle neurons,

a type of neuron in the anterior cingulate cortex unique to

humans and great apes, appear to relay especially to area 10

(Allman et al. 2002). The coordinated expansion of functionally

related structures, such as the medial nuclei of the thalamus,

should also follow from the enlargement of the prefrontal

cortex. Experiments showing the plasticity of the brain in cases

The cranial volume of the Homo genus has more than

doubled during the 2 million years that separate us from Homo

habilis, and the cortical surface may have expanded at least in

the same proportion. Our results suggest that this growth should

lead to a natural expansion of the prefrontal cortex. This

expansion may have allowed for the greater specialization of the

cortical regions related today to goal-directed sequential

behavior (planning) and divergent (creative) thinking, and may

have been a key process for the emergence of human cognition.

Brain Size Cortex Folding Toro 2008

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