LAMINAR PATTERNS OF GENICULOCORTICAL PROJECTION IN CAT

The cortical afferents from individual laminae of the dorsal lateral geniculate nucleus (LGN) were studied using both light microscopy and EM autoradiography. In area 17 the A geniculate laminae (A and A1) had 2 main bands of projection, one extending from the bottom of IVc to the deepest cells in layer III, and one in layer VI. The C geniculate laminae projected in 2 dense bands to the upper and lower borders of layer IV, thus bracketing the A laminae projection, though with some overlap. The C laminae projected to the superficial half of layer I, which the A laminae did not. Conversely, while the A laminae projected to layer VI, the C laminae did not. The 2 sets of laminae also showed differences in the areas to which they projected. The A geniculate laminae projected to areas 17 and 18, where the C geniculate laminae had a more extensive projection, including areas 17, 18, 19 and other areas on the suprasylvian gyrus. The laminar organization of the projection to area 18 was similar to that found in area 17. At the EM level the geniculate terminals made Gray''s type 1 synapses, for the most part onto dendritic spines. Labeled terminals were found in all the projection bands seen in the light microscope. The implications of these findings for the connectivity of cells in layer IV were discussed. The presence of labeled terminals in layer VI, which contains the cells of origin of the corticogeniculate pathway, suggested that the recurrent loop to the LGN is mediated monosynaptically. The afferents from each geniculate lamina were segregated into patches, about 500 .mu.m wide, which probably form the anatomical basis for ocular dominance columns.