The cortical projections of foveal striate cortex in the rhesus monkey.

1. The cortical projections of the foveal and extrafoveal parts of the striate cortex have been compared, using conventional degeneration techniques, as well as combinations of anatomical methods. While both foveal and extrafoveal striate cortex share a common pattern of projections (to areas V2, V3 and the visual area in the medial part of the posterior bank of the superior temporal sulcus), foveal striate cortex was found to have an additional projection (to part of the cortex of the fourth visual areas, V4). The latter projection includes the posterior lip of the inferior occipital sulcus which, on anatomical grounds, is regarded as the ventral extension of V4. 2. Anatomical studies using double tracers were employed to clarify the nature of the projections from the striate cortex and from V2 to V4. In one such experiment, tritiated proline was injected into extra-foveal striate cortex and a small lesion was made in that part of V2 receiving a direct projection from the region of the striate cortex into which the radioactive tracer was injected. Only degenerating fibres (due to the lesion), and no radioactive label, was found in V4. Such an experiment showed that, unlike foveal striate cortex, the projections from extrafoveal striate cortex to V4 are not direct, but through V2. 3. In another type of anatomical experiment using double tracers, the corpus callosum was sectioned and tritiated proline was injected into foveal striate cortex. Such an experiment allowed a more accurate determination of the extent of V4, as judged from its callosal connexions, to which foveal striate cortex projects. 4. Considering the projections of V1 to areas V2, V3 and the visual area in the medial part of the posterior bank of the superior temporal sulcus, and considering the differences in the projections of foveal and extrafoveal striate cortex, it is suggested that, among other functions, the striate cortex acts as a distribution centre for the information coming over the retino-geniculo-cortical pathways, parcelling this information out to different visual areas of the prestriate cortex for further analysis.