Associational and commissural connections of area 19 and of the lateral suprasylvian visual area (LS) of the cat were studied by means of an axonal anterograde transport technique. Multiple injections of L-[5-3H] proline were placed, in different animals, in area 19 and in two different subdivisions of LS (PMLS, PLLS). Serial histological sections of the whole brains were processed by autoradiographic technique after long (8-12 days) and short (30 hrs) survival times. Cortical areas labelled by accumulation of radioactive material were correlated with electrophysiological and cytoarchitectonic maps of cortical areas. Results showed a number of projections of areas 19 and LS not previously described. Area 19 sends ipsilateral projections to area 5 and to the medial bank of the anterior suprasylvian sulcus. The postero-medial (PMLS) and postero-lateral (PLLS) subdivisions of LS project to ipsilateral areas 5, 6, 7, 20, 21, 35, to the suprasylvian fringe (SF), to the cingulate gyrus and to the insular cortex. Contralaterally, new projections are found from area 19 to areas 20 and 5, and from LS to areas 19, 20, 21, 5, 7, SF to the cingulate gyrus and to the insular cortex. Results lead to conclude that the intra-modal and cross-modal interactions between cortical areas processing visual input are more complicated than previously assumed. The crucial role of the cat's LS in the cortico-cortical distribution of the visual message in emphasized.
