Laminar Organization of Area V1
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The laminar organization of layer 4 of the primary visual area (area V1, area 17, striate cortex) was modified in human evolution, losing the dense band of cytochrome oxidase staining in layer 4A that is present in most anthropoid primates that have been examined. In most New World and Old World monkeys that have been studied, histochemical staining for cytochrome oxidase (CO) in the primary visual cortex yields two distinct bands of dark staining in layer 4: a thick band corresponding to layer 4C (and spanning both its main sublayers, 4C-alpha and 4C-beta) and a thinner band corresponding to layer 4A. These bands of CO labeling correspond to the location of terminations from the main layers of the lateral geniculate nucleus. However, Preuss et al. (2002) report that the chimpanzees (and one orangutan) they examined also lacked a dense band of CO labeling in layer 4A; that is, they resembled humans rather than Old World monkeys. This suggests that the modification of layer 4A that resulted in the loss of dense CO staining in humans is actually an ape-human specialization rather than a human specialization (i.e., a hominid rather than hominin specialization).
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