The Human Frontal Lobes: Not Relatively Large but Still Disproportionately Important? A Commentary on Barton and Venditti

Abstract

not appropriately correct for the pattern of allometry between the frontal lobes and the rest of the brain. The whole neocortex becomes disproportionately large as brain size increases because white matter, which contains nerve fibres connecting different structures, expands much more rapidly than grey matter or neuron number. Although the ballooning of white matter probably reflects size-related constraints on maintaining connectivity, it can produce the appearance that the cortex – or a specific cortical region – is selectively enlarged when viewed as a proportion of total brain size [Barton and Venditti, 2013]. A second problem affecting some previous studies is the failure to acknowledge the shared evolutionary history of living species. Shared ancestry leads to the expectation that more closely related species should resemble each other more than more distant relatives, and renders comparative data non-independent. Using comparative methods that correct for this non-independence, Barton and Venditti [2013] re-visit several datasets to address the issue of allometry and resolve the debate over the size of the human frontal lobes. To do so they examine the relationship between the size of frontal and nonfrontal structures across non-human primates and assess whether humans fall withIt is widely accepted that there was something exceptional about human cognitive evolution. Among extant primates we are behaviourally and cognitively distinct [Laland and Brown, 2011]. This distinction evolved during our descent from an ape-like ancestor, was shaped by natural selection, and must have a proximate basis in neural adaptation and specialisation. Identifying these adaptations has long been a goal of evolutionary biologists [Striedter, 2005]. An often-cited candidate is the expansion of the frontal regions of the neocortex which, it has been argued, became exceptionally large during human evolution [Rilling, 2006]. Recent work, however, suggests that this was not the case [Barton and Venditti, 2013]. Previous claims for a human-specific enlargement of the frontal lobes have, in fact, been inconsistent [see Rilling, 2006]. Some studies have claimed that there was global expansion of the frontal lobe, whilst others have argued for a specific expansion of particular regions (e.g. the prefrontal cortex), tissue types (e.g. white matter), or hemispheres. Some have shown the pattern was human-specific, others a characteristic of great apes, or even apes in general. Barton and Venditti [2013] argue this ambiguity is a product of using different measures of frontal lobe size, including some that do Published online: September 6, 2013