ANATOMY OF THE PRIMATE VISUAL SYSTEM

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To better understand the role of visual association cortex in perception and memory, we have been examining the multiple functional areas that comprise this cortex in the macaque and exploring the complex circuitry of their interconnections. In these studies, we have utilized neuroanatomical tracing techniques combined with electrophysiological recording as well as newly developed histological stains that, for the first time, clearly distinguish among the multiple visual areas. Our results indicate that the primary visual area, striate cortex, is the source of two cortical projection systems. The first system begins with the striate projections to the second and third visual areas, V2 and V3. Both V2 and V3 project in turn to V4. These three prestriate areas are arranged in adjacent belts that nearly surround the striate cortex, and, like the striate cortex, each belt contains a topographic map of the visual field. Area V4 projects in turn to the inferior temporal cortex. The second system begins with both striate and V2 projections to area MT, which is also topographically organized. However, in contrast to V4, which appears to provide a major link forward from striate cortex into the temporal lobe, our results on MT indicate that it provides a major link forward from striate cortex into the parietal lobe via its projections to four additional areas in the superior temporal and intraparietal sulci. Thus, one system of projections out of striate cortex is directed ventrally into the temporal lobe, while a second is directed dorsally into the parietal lobe. On the basis of evidence from our neurobehavioral work, we propose that these two systems mediate object vision and spatial vision, respectively. We have begun an analysis of human brain material with the goal of identifying visual areas homologous to the ones that we have delineated in the macaque.