Visual cortex organization in primates: theories of V3 and adjoining visual areas.

After years of experimentation and substantial progress, there is still only limited agreement on how visual cortex in primates is organized, and what features of this organization are variable or stable across lines of primate phylogeny. Only three visual areas, V1, V2, and MT, are widely recognized as common to all primates, although there are certainly more. Here we consider various concepts of how the cortex along the outer border of V2 is organized. An early proposal was that this region is occupied by a V3 that is as wide and as long as V2, and represents the visual hemifield as a mirror image of V2. We refer to this notion as the classical V3 or V3-C. Another proposal is that only the dorsal half of V3-C exists, the half representing the lower visual quadrant, and thus the representation is incomplete (V3-I) by half. A version of this proposal is that V3-I is discontinuous, extremely thin in places, and highly variable across individuals, much as a vestigial or degenerate structure might be (V3-IF-incomplete and fragmented). A fourth proposal is that there is no V3. Many results suggest that a series of visual areas border V2, none of which has the characteristics of V3. Alternatively, the possibility exists that primate taxa differ with regard to visual areas bordering V2. Currently, much of the supporting evidence for a classical V3 comes from fMRI studies in humans, much of the evidence for a series of bordering areas comes from New World Monkeys and prosimian galagos, and much of the evidence for a V3-I or V3-IF comes from macaque monkeys. Possibly all these interpretations of visual cortex organization are valid, but each for only one of the major groups of primate evolution. Here, we suggest that none of these interpretations is correct, and propose instead that a modified V3 (V3-M) exists in a similar form in all primates. This V3-M is smaller and thinner than V3-C, discontinuous in the middle, but with comparable dorsal and ventral halves representing the lower and upper visual hemifields, respectively. Because the evidence for V3-M is limited, and it stems in part from our ongoing but incomplete comparative studies of V1 connections in primates, this suggestion requires further experimental evaluation and it remains tentative.