How Parallel are the Primate Visual Pathways?

The visual system, like all sensory systems, contains parallel pathways (see Stone 1 983). Recently, m uch emphasis has been placed on the relationship between two subcortical and two cortical pathways. It has been suggested that the cortical and subcortical pathways are continuous, so that distinct channels of information that arise in the retina remain segregated up to the highest levels of visual cortex. According to this view, the visual system comprises two largely independent subsystems that mediate different classes of visual behaviors. In this paper, we evaluate this proposal, which has far-reaching implications for our understanding of the functional organization of the visual system. The subcortical projection from the retina to cerebral cortex is strongly dominated by the two pathways (M and P pathways) that are relayed by the magnocellular and parvocellular subdivisions of the lateral geniculate nucleus (LGN) (see Shapley & Perry 1 986). The importance of these pathways is demonstrated by the fact that they include about 90% of the axons that leave the retinas (Silveira & Perry 1 99 1 ) and that little vision survives when both pathways are destroyed (Schiller et al 1 990a). The P and M pathways maintain their sharp anatomical segregation through the termination of the LGN projection in layer 4C of VI (striate cortex). The complex network of connections in primate extrastriate visual cor-