Associated and commisural connections of visual areas 17 and 18 were studied in the light of the latest knowledge about the anatomo-functional organization of the cat's visual cortices. Injections of L-[5-3H] proline were placed, in different animals, in area 17 or 18 of the right hemisphere. Serial histological sections of the whole brain were processed by autoradiographic technique, after long (8-16 days) or short (30 hours) survival times. Cortical areas labelled by axonally transported radioactive material were then correlated with electrophysiological and cytoarchitectonical maps. Areas 17 and 18 were found to send associational projections to area 19, to the anterior and posterior subdivisions of the lateral suprasylvian visual area and to two regions lying on the crown of the suprasylvian gyrus (areas 21A and 20). In addition, area 18 sends projections to ipsilateral areas 17, 7 and 5. Contralaterally, projections from areas 17 and 18 reach the homonymous areas near the margin of the other. Homotopical callosal connections of area 17 affect the band of the contralateral 17 in which part of the ipsilateral visual hemi-field is represented. Heterotopical commisural projections from the areas studied affect areas 19 and LS in regions adjoining the central vertical meridian representations. These results suggest that, on an anatomical basis, the interactions between the cortical targets of the classical geniculo-striate and the estrageniculo-striate systems are more complicated than previously believed. Furthermore, the geniculo-striate system supplies some visual input to non-visual areas through area 18.
