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 functional areas that comprise this cortex in the macaque and exploring their interconnections by the use of neuroanatomical tracing techniques in combination with electrophysiological recording of neural activity. Our results indicate that a multiplicity of separate visual areas lie beyond the primary visual cortex, or striate cortex (Vl), in the stream of information processing. These areas are organized into two divergent corticocortical pathways, each having Vl as the source of its initial input. One, an occipitotemporal pathway, enables the visual recognition of objects, while the other, an occipitoparietal pathway, mediates the appreciation of the spatial relationships among objects as well as the visual guidance of movements toward objects in space. The visual areas along the occipitotemporal pathway (Vl, V2, V3, V4, and areas TEO and TE of the inferior temporal cortex) appear to be organized primarily as a serial hierarchy, in which each area processes both color and form information. By contrast, the areas along the occipitoparietal pathway (Vl, MT, and MT's projection zones in parietal cortex) process the direction of stimulus motion. Because a major component of this pathway extends anteriorly within the superior temporal sulcus, the neural mechanisms underlying visuospatial function may be more extensive than previously thought. To establish the links of both the occipitotemporal and occipitoparietal pathways with the motor system, we have been exploring the projections of visual association cortex to the striatum. So far, we have found that TE, TEO and V4 all project to the tail of the caudate nucleus and ventral putamen, an arrangement that contrasts with the pattern of projections from the occipitotemporal pathway to the limbic system, which arise from TE only. The presence of direct projections to the striatum but not to the limbic system from V4 and TEO may explain the ability of monkeys with TE lesions to acquire visual habits but not visual memories.