Visual Specialization and Brain Evolution in Primates [RSPB]
RA Barton 1998
Abstract: Several theories have been proposed to explain the evolution of species differences in brain size, but no concensus has emerged. One unresolved question is whether brain size differences are a result of neural specializations or of biological constraints affecting the whole brain. Here I show that, among primates, brain size variation is associated with visual specialization. Primates with large brains for their body size have relatively expanded visual brain areas, including the primary visual cortex and lateral geniculate nucleus. Within the visual system, it is, in particular, one functionally specialized pathway upon which selection has acted: evolutionary changes in the number of neurons in parvocellular, but not magno- cellular, layers of the lateral geniculate nucleus are correlated with changes in both brain size and ecological variables (diet and social group size). Given the known functions of the parvocellular pathway, these results suggest that the relatively large brains of frugivorous species are products of selection on the ability to perceive and select fruits using specifc visual cues such as colour. The separate correlation between group size and visual brain evolution, on the other hand, may indicate the visual basis of social information processing in the primate brain.
This article is required reading if you want to read his 2004 PNAS article since the PNAS article uses his previous findings to make speculative and receive counterintuitive results on the role of the parvocellular layers and magnocellular layers on binocularity. Both articles provide highly speculative hypotheses that can provide a long critique/thought/thinktank sessions. Nevertheless, these articles both provide information on the role of binocularity and speculates on whether the brain evolved due to more internal or external forces.