The Evolution of Parallel Visual Pathways in the Brains of Primates

Vision is the dominant sense in primates. This article attempts to reconstruct a reasonable scenario as to how parallel visual pathways might have evolved in primates by comparing key factors that might distinguish this group evolutionarily. The focus is on visual information channels from the eye through the lateral geniculate nucleus (LGN) of the thalamus to cortex since these pathways may have become uniquely specialized in primate evolution. We defend the position that magnocellular (M) and parvocellular (P) retinogeniculocortical pathways are homologous across primates and therefore probably existed in the mammalian common ancestor of primates. Whether homologues to these visual pathways can be found in extant mammals remains controversial, but evidence suggests that functionally similar pathways can be identified in a range of mammals. Even for the less well-researched koniocellular (K) pathway, data exist suggesting an early evolutionary history. Only among primates, however, is the evidence strong enough to support homology. We also present data suggesting that the common ancestor to primates was dichromatic and that early primates may even have been diurnal given the existence of genes for at least two cone types in all primates. We also review evidence for homologies between ocular dominance pathways and other properties. In addition, we review evidence for the evolutionary history of cortical hierarchies of visual areas and conclude that only a few areas can be considered homologous across primates and even fewer across mammals. In the final section, we provide a summary and also outline questions that should be addressed in order to arrive at more definitive conclusions concerning the evolution of parallel visual pathways. We also outline some practical strategies for answering some of these questions.