Theories of Visual Cortex Organization in Primates

Abstract

For nearly the last 100 years, many neuroscientists have attempted to determine how neocortex is subdivided into areas of functional significance. Accurate concepts of how cortex is subdivided into areas are obviously important as guides for further study and for the interpretation of previous results. More relevant experimental data have been gathered for visual cortex of primates than possibly any other cortical system and current proposals are much more valid than earlier proposals. However, areas can be difficult to define, especially by a single feature. Thus, modern maps of visual cortex still differ in many ways, and we do not have a comprehensive outline that includes different primate taxa. Nevertheless, some conclusions are well supported, and further progress can be expected as issues come into focus. The present review has a few main points. (1) While early concepts of how visual cortex is subdivided were inconsistent with each other, and have been shown to be incompatible with recently collected experimental data, these concepts continue to have impact and influence. In particular, the concept of V3 has persisted as an extension of Brodmann's portrayal of area 19. It now seems clear that the region of cortex along the outer border of V2 contains a number of visual areas in primates, none of which are like area 19 of Brodmann (1909). Proposals that retain a V3 generally do so in a greatly modified form. Rather than submit V3 to such modifications, the concept of V3 as an equivalent of area 19 should be abandoned. (2) Of the proposed extrastriate visual areas, only two, V2 and MT, are recognized by all investigators regardless of primate taxa. Nevertheless, other areas are likely to be basic to all primates, and DM is one of these fields. A number of other visual areas have been reasonably well defined in some monkeys, but the designations have not been fully accepted. Further study is important and can be expected to lead rapidly to widespread agreement for some of these fields. (3) The importance of identifying areas and modules held in common across taxa has not been fully appreciated. The significance of having evidence for areas in several primate taxa is that these fields are likely to be retained in branches of evolution, such as humans, where experimental approaches are limited. In addition, theories of cortical organization for different species should be compatible with each other within the framework of evolutionary change. Thus, comparisons across species serve as a further means of evaluating theories for particular species. (4) While it once seemed that New World and Old World monkeys differed greatly in how extrastriate visual cortex is organized, this conclusion has not been supported by more recent comparative studies. Primate taxa undoubtedly differ in many ways, but presently we have a better understanding of how primates are similar rather than different in extrastriate cortex organization. [References: 160]