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 (V1) in the stream of information processing. These areas are organized into two divergent cortical pathways, each having V1 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 (V1, V2, V3, V4, and TEO and TE of the inferior temporal cortex) appear to be organized as a hierarchy, in which each area processes both color and form. By contrast, the areas along the occipitoparietal pathway (V1, MT, and MT's projection zones in parietal cortex) process the direction of stimulus motion. Visual information about object identity and spatial location, initially processed in separate cortical pathways, appears to be integrated in cortex located within the superior temporal sulcus. The subcortical connections of the two pathways, however, remain highly segregated. Whereas parietal but not temporal cortex projects to the pons and superior colliculus, temporal but not parietal cortex is reciprocally connected with the amygdala, indicating the importance of spatial information for the visual guidance of movement and of object identity information for object-reward associations. 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 precise anatomical locations.