Hominoid-specific calretinin-immunopositivity of the optic radiation (geniculocalcarine tract).

It has been proposed that the optic radiation (OR) of primates is specifically revealed with parvalbumin immunohistochemistry. To test this proposition, the immunohistochemical expression of three calcium-binding proteins (CaBPs, parvalbumin, calbindin, and calretinin), was investigated in the OR (also known as the geniculocalcarine tract) of five primate species, including a Strepsirrhini (Galago moholi-southern lesser galago), a Platyrrhini (Saimiri boliviensis-black-capped squirrel monkey), a Cercopithecidae (Macaca nigra-crested macaque) and two Hominoidea (Hylobates lar-lar gibbon, Pan troglodytes-chimpanzee). The OR of the southern lesser galago did not reveal substantial immunostaining of any of the CaBPs investigated. The black-capped squirrel monkey, and crested macaque evinced strong intensity parvalbumin-immunostaining of the OR. In contrast, the OR of the lar gibbon and chimpanzee presented with strong intensity calretinin-immunostaining, with weak to moderate parvalbumin-immunostaining. These results indicate that the neurochemistry of the OR differs between the major primate lineages, although the trajectory of the OR through the white matter, including the temporal loop, is consistent across primates. While it is unclear precisely what effect this differing CaBP neurochemistry has on the processing of visual information, it is possible that these differences modulate axonal excitability or signal fidelity in the OR of hominoids when compared to other primates.