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 their interconnections by the use of neuroanatomical tracing techniques in combination with electrophysiological recording. Our results indicate that the primary visual area (V1), or striate cortex, is the source of two divergent corticocortical pathways: one, an occipitotemporal pathway, which enables the visual recognition of objects; the other, an occipitoparietal pathway, which mediates the appreciation of the spatial relationships among objects. The visual areas along the occipitotemporal pathway (V1, V2, V3, V4, and areas TEO and TE of the inferior temporal cortex) appears to be organized primarily as a serial hierarchy, in which each area processes several different stimulus attributes in parallel. This pathway is modality-specific throughout its extent, unlike the occipitoparietal pathway, in which the later stations are polysensory. Whereas area V4 provides a major link forward from striate cortex into the temporal lobe, our results on visual area MT indicate that it provides a major link forward from striate cortex into the parietal lobe via its projections to three separate areas in the superior temporal and intraparietal sulci. However, MT does not provide the sole route by which visual information from striate cortex reaches the parietal lobe. Other potential pathways are provided by several additional visual areas located in occipito-parietal cortex. Most of these areas receive inputs representing predominantly the peripheral visual field, which presumably reflects the importance of such inputs for spatial vision. By contrast, the predominance of central visual field inputs to V4 and TEO in the temporal lobe presumably reflects the importance of these inputs for object vision.