Visual cortex in the macaque contains at least two processing streams, which have been variously termed the dorsal and ventral, motion and color/form, or where and what pathways. The ventral pathway is much larger than the dorsal pathway and has several interesting features including responsivity to color and object shape, as well as highly developed specificity for faces.

Recently, a series of studies from our laboratory using visual cortex lesions in macaques has advanced our understanding of the functions of the ventral cortical pathway. These studies have shown that lesions at one of two levels in this pathway (in area V4 or in inferotemporal cortex) cause distinctive losses of complex form and color discriminations, whereas simple shape discriminations are spared by lesions at either level. The latter finding seems incompatible with the traditional view that inferotemporal cortex is critical to shape discriminations.

V4 lesions in macaques and ventral cortex lesions in humans also cause dramatic losses of noise masked contrast sensitivity, but only small losses of unmasked contrast sensitivity. This finding may reflect a disruption of feedback or top down processing in the ventral pathway, whose normal function is to identify interesting visual stimuli in cluttered environments.