Does absolute brain size matter?

In recent years it has become unfashionable to talk about absolute brain size as a measure of cognitive capacity. The common assumption is that it is only meaningful to consider brain size if body size, or some relative measure, is taken into account. The idea is that since the brain, like any other organ, scales with body size, the validity of the use of brain size as a measure of intelligence or information processing capacity rests upon the size of the brain relative to the size of the body.

A review by Marino of an article by Sherwood et al. in PNAS suggests that in ignoring absolute brain size we may have thrown out the baby with the bath water. The Sherwood paper addresses the general question of whether human brains should best be thought of as large hominoid brains, or, alternatively, as a singularly endowed product of evolution somewhat apart from the rest of primate brain evolution. They indeed find that the human frontal cortex displays a higher ratio of glia to neurons than in other primates. However, and importantly, this relative difference is predicted by the allometric scaling inherent in the enlargement of the human brain. In other words, overall or absolute brain size constitutes a key factor in the ratio of glia to neurons. The authors suggest that the greater numbers of glia in the human neocortex may be due to the increased energetic costs of larger dendritic arbors and longer fiber projections within the context of the large human brain. The bottom line is that the human brain conforms to the general mammalian pattern of higher glia–neuron ratios with larger brains.

In brain areas key to specific human abilities, such as area 44( language production) and area 32 (theory-of-mind tasks in humans) Sherwood et al. find no significant species differences and suggest that the energetics of frontal cortex, even in these regions, have been largely conserved over the past 25 million years of primate brain evolution. Their overall conclusion is striking: "... human cognitive and linguistic specializations have emerged by elaborating on higher-order executive functions of the prefrontal cortex ... that evolved earlier in the primate lineage".

The fundamental insight supported by Sherwood et al. is that the human brain is not unique or anomalous, rather it is a product of changes in brain anatomy that are well predicted by scaling expectations for any nonhuman anthropoid primate. There is a growing body of evidence for this conclusion. For instance, several studies have shown that the human frontal cortex occupies the same proportion of total cortex in humans as it does in great apes. Similarly, the human brain possesses the degree of cortical gyrification expected for a primate of our brain size.

While this may be the emerging consensus there is also evidence from MRI scans of 11 different primate species that reach opposite conclusions, namely that: (1) that the human neocortex is significantly larger than expected for a primate of our brain size, (2) that the human prefrontal cortex is significantly more convoluted than expected for our brain size, and (3) that increases in cerebral white matter volume outpace increases in neocortical gray matter volume among anthropoid primates. (see Rilling and Insel, "The primate neocortex in comparative perspective using magnetic resonance imaging." Journal of Human Evolution Volume 37, Issue 2 , August 1999, Pages 191-223.)