Multi-synaptic cortical organization for stereopsis of macaque

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The primate visual system is characterized by two principal rules : parallel (dorsal and ventral) pathways and hierarchic signal processing. It is well accepted that the information for stereopsis is processed not only in the dorsal cortical areas but also ventral cortical areas. However, multi-synaptic architecture of these two pathways is poorly understood. Within these parallel pathways, MT and V4 are recognized as being situated at a similar hierarchical rank. To re-evaluate the hierarchic relationship between MT and V4, the patterns of retrograde transneuronal labeling in V1 and LGN were analyzed after rabies virus injections into MT and V4 of macaque monkeys. The present results suggest that MT and V4 may be posited at different hierarchic ranks in signal processing in the dorsal and ventral pathways by way of LGN and layer 4C of V1. Moreover, there may be another pathway linking LGN and MT or LGN and V4. It is likely that converging M and P inputs from LGN may contribute to signal processing in these "short-cut" pathways.