FUNCTIONAL ANATOMY OF PERCEPTUAL AND MEMORY SYSTEMS IN THE PRIMATE BRAIN

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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 ocipitotemporal 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. Whereas the areas along the pathway into the temporal lobe appear to be organized as a hierarchy, in which each area processes both color and form, the areas along the pathway into the parietal lobe 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. Data from cerebral blood flow studies indicate the existence in humans, as in monkeys, of two distinct visual processing pathways. In humans, processing of object identity and spatial location depends primarily on functional interactions between posterior cortical areas in the right hemisphere. Old subjects, compared to young, show reduced functional separation of the two pathways, suggesting recruitment of visual areas belonging to the "other" pathway in order to compensate for reductions in each pathway's processing efficiency.