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 examined the functional areas that comprise this cortex in the macaque and explored their interconnections by the use of neuroanatomical tracing techniques in combination with physiological recording of neural activity. Our results indicate that a multiplicity of separate visual areas lie beyond the striate cortex (Vl) in the stream of information processing. These areas are organized into two divergent cortical pathways, each having Vl as the source of its initial input. One, an occipitotemporal pathway, enables the recognition of objects, while the other, an occipitoparietal pathway, mediates the appreciation of spatial relationships among objects as well as the visual guidance of movement. The areas along the occipitotemporal pathway (Vl, V2, V3, V4, and TEO and TE of the inferior temporal cortex) appear to be organized as a serial hierarchy, in which each area processes both color and form. 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. Data from cerebral blood flow studies indicate the existence in humans, as in monkeys, of two distinct visual processing pathways, although there may be cross-species differences in their anatomical locations. To establish the links of both the occipitotemporal and occipitoparietal pathways with the motor system, we have explored the projections of visual association cortex to the striatum. The targets of these projections are the tail of the caudate nucleus and ventral putamen, which project in turn to the substantia nigra, pars reticulata (SNr) and the globus pallidus (GP). Because of the known inputs of the SNr and GP to the supplementary motor and premotor cortex via the thalamus, the striatum could be a link by which the visual system influences behavior.