Cortical and Subcortical Connections of Visual Cortex in Primates

Over the last several years, considerable progress has been made in understanding the connections of the visual system in primates. Much of this progress has been the result of applications of the relatively new and powerful autoradiographic and histochemical tracing methods that reveal connections in great detail and clarity. The major limitation on further understanding of visual system connections does not seem to be technical at this time, but rather it is our incomplete knowledge of the functional subdivisions of the primate visual system. It is difficult to study connections of parts of the brain for which the organization and divisions into separate areas or nuclei are still unclear. Quite different schemes of cortical organization have been proposed for visual cortex of New World (5–7) and Old World monkeys (96, 111, 116–118) and neither of these schemes includes all of visually responsive cortex. For example, neither organization deals with subdivisions of inferotemporal cortex, a visually responsive region of cortex that in Old World monkeys has received detailed attention (33, 34, 59), without completely resolving the issue of number and boundaries of subdivisions. Efforts have been made to show similarites in cortical organization between Old and New World primates (1, 95, 99), but how similar or dissimilar the two groups of primates are presently remains uncertain.