Theories of visual cortex organization in primates: areas of the third level.

This brief review has a few main points. (1) Early proposals on how extrastriate cortex is subdivided were inconsistent with each other, and differences in interpretation were not resolved. (2) Brodmann's proposal of two ring-like areas, 18 and 19, surrounding primary visual cortex gained great acceptance despite the lack of agreement among different investigators considering the same evidence. (3) The concepts of areas 18 and 19, transposed to signify V2 and V3, have had great impact on recent and even current theories of extrastriate visual cortex organization in primates. (4) Nevertheless, Brodmann's areas 18 and 19, as defined in humans and Old World monkeys, correspond to none of the fields currently proposed for these primates. (5) All or most mammals appear to have a V2, and there is now widespread complete agreement over the extent and organization of this area in all studied primates. V2 is commonly referred to as area 18 because of its correspondence to area 18 as defined by Brodmann is some mammals. Yet, we should recognize that V2 is about half the size of Brodmann's area 18 in Old World monkeys and humans. (6) Current concepts of V3 differ greatly from the ring-like area 19 of Brodmann. We question the validity and usefulness of retaining the concept of V3 in primates. Our proposal for DM and other visual areas along the outer border of V2 seems more consistent, not only with the evidence from New World monkeys, but with evidence from Old World and prosimian primates, and even mammals most closely related to primates (see Kaas and Preuss, 1993). In all of these primates and close relatives of primates, the evidence indicates that more than one field forms the outer border of V2.