Investigating the wire fraction of the neuropil in primate cerebral ortex

Abstract

Whether the neuropil is a static, optimally wired entity, whose components must be balanced in a certain way, is an open question. Are the proportions of the components of the neuropil consistent across different mammalian cortices, especially in primates where the cerebral cortex is complexly organized? This question is interesting because the actual biological underpinnings of complex behaviours and intelligence in big-brained primates remain enigmatic and why they seem qualitatively different from other animals in terms of their cognitive abilities. Understanding changes that may have occurred in the brain, especially at the level of neuropil organization, during the evolution in primates is important to our growing understanding of the intellectual abilities and behaviours exhibited by members of this group. The current series of quantitative studies was aimed at investigating variations in the proportionality of the “wire fraction” in three primate species, the olive baboon (Papio anubis), vervet monkey (Cercopithecus aethiops) and the common chimpanzee (Pan troglodytes), in a range of higher and lower order cortical areas, using a newly developed method that involves standard and immunohistochemical staining techniques to reveal and quantify the various profiles of the fine structures of the cerebral cortex. The results of these studies demonstrate clear layer differences in the wire fraction of the cerebral cortex, and for the most part, consistency in the neuropil wire fraction of the same layer across areas of the cerebral cortex within and between individuals of the same species; however, differences in the wire fraction of the neuropil were associated with changes in brain size. It is apparent that the neuropil is not static, as wiring “optimality” changes with layers and brain size and this has functional implications regarding neuronal processing and behavioural outcomes. The adaptive rationale adopted by evolutionary psychology studies to explain behaviours may be erroneous, as adaptation does not always explain sufficiently the emergence of complex behaviours related to brain size increases, especially in primates.