Direct projection from the dorsal lateral geniculate nucleus to the prestriate cortex in macaque monkeys.

The enzyme horseradish peroxidase (HRP) was separately injected into striate, prestriate, inferotemporal, and parietal cortices in 19 macaque monkeys, and the lateral geniculate nucleus (LGN) was examined for retrograde transport. Labeled LGN cells were identified only in the animals, with HRP injections into the striate and prestriate cortex. Following injections into either of these regions, labeled cells were found in both parvocellular and magnocellular regions of the ipsilateral LGN only, in keeping with the topographic relation of HRP injection sites in the cortex to labeled areas in the LGN. It was also found that (1) labeled LGN cells were less numerous in both laminar and interlaminar zones following HRP injection into the prestriate cortex, whereas following HRP injection into the striate cortex labeled cells were found almost exclusively in the laminae, and localized to a wedge-shaped region; (2) following HRP injection into the prestriate cortex, the mean sizes of the labeled parvocellular and magnocellular cells, estimated in projected diameter, were almost the same, these means being significantly larger than the mean size of labeled parvocellular cells and much smaller than that of labeled magnocellular cells following HRP injection into the striate cortex; (3) the shapes of the labeled LGN cells following HRP injection into the prestriate cortex were ovoid, fusiform, or triangular (or multipolar), whereas those following HRP injection into the striate cortex were uniformly ovoid or round. The above findings following HRP injections into the prestriate cortex in normal monkeys were confirmed by HRP injections into the prestriate cortex of monkeys whose striate cortex had been removed several months prior to the injection; labeled cells were found in confines of areas of retrograde degeneration in the LGN and their labeling pattern was the same as that in intact animals. It was concluded that in macaque monkeys, just as in the cat, a geniculoprestriate projection system exists; it was suggested that there are two parallel system of visual information processing from the LGN to the prestriate cortex, a direct one and in indirect one through the striate cortex.