Evidence that the different classes of relay cells of the cat's lateral geniculate nucleus terminate in different layers of the striate cortex.

Small electrophoretic injections of horseradish peroxidase (HRP) were made with recording microelectrodes in different layers of area 17 of the cat. The sizes and locations of labeled relay cells in the lateral geniculate nucleus (LGNd) were then studied. Layer I was found to receive inputs mainly from small LGNd cells (13--21 micrometer). These cells were located in the parvocellular C laminae and the MIN. Layer II and the upper portion of layer III were not found to receive LGNd afferents. Lower layer III was found to receive inputs mainly from small to medium-sized relay cells. These cells were located in the A laminae and the parvocellular C laminae. Layer IVab and the layer III--IV border region were found to receive afferents from predominantly large LGNd neurons (greater than 20 micrometers). These cells were located in alminae A, A1, C, and the MIN. Layer IVc was found to receive inputs from mostly small to medium-sized cells (15--23 micrometer). Nearly all of these cells were in the A laminae although some small cells in laminae C were observed following layer IVc injections which involved the layer IV--V border region. Layer V, away from the layer IV--V border region, was not found to receive LGNd afferents. Finally, layer VI was found to receive inputs from both large and small relay cells. These cells were all located in the A laminae. In the light of previous findings, these results suggest that the terminals of W, X, and Y cell axons are segregated within the cat's striate cortex: W cells projecting to layer 1, the deeper portion of layer III and the layer IV--V border region; X cells projecting heavily to layer IVc with a minor projection to layer VI and possibly also to lower layer III; Y cells projecting heavily to layer IVab and the layer III-IV border region with a smaller projection to layer VI.