Conclusions

The evidence summarized in the previous sections strongly supports the existence of functionally distinct pathways or systems in the extrastriate visual cortex that follow anatomically separate routes. Some authors have gone beyond this to propose that a direct correspondence exists between the retinocortical P and M pathways and the temporal and parietal visual systems, respectively (e.g., Kandel, 1991 ). This proposal is based on two major premises. First, the anatomical segregation of the P and the M geniculocortical afferents is maintained within the striate cortex (area V1). In addition, efferent corticocortical projections, from area V1 to a number of extrastriate areas follow anatomically separate routes. For instance, magnocellular input from V1 layer IVB is directed to area MT but not to area V4. This segregation is also maintained within the projections to a single cortical relay area (i.e., area V2). Second, the anatomical connections between the cortical pathways (e.g., Felleman & Van Essen, 1991 ) are not as extensive as the connections within a given pathway. Connections between pathways probably serve a modulatory function and do not carry the main flow of neural signals between the hierarchically organized cortical areas.