The laminar and size distribution of commissural efferent neurons in the cat visual cortex

The laminar and size distribution of commissural efferent neurons were studied in the cat visual cortex including area 17, 18, 19 and the lateral suprasylvian cortex (the lateral wall of the middle suprasylvian gyrus) by means of retrograde transport of horseradish peroxidase. Single or multiple injections of the enzyme were made unilaterally along the medial part of the lateral gyrus (the adjoining parts of area 17 and 18) or along the lateral wall of the middle suprasylvian gyrus. In the contralateral cerebral cortex (area 17, 18, 19 and lateral suprasylvian cortex) the majority of peroxidase labeled commissural efferent neurons was identified in layer III with a smaller population in other layers except layer I. The size distribution of labeled cells of layer III showed that all sizes of neurons contribute to interhemispheric connections, ranging from small cells which are comparable to those predominating in layer III of the acommissural medial area 17 to the characteristic large pyramides of layer III at the area 17/18 boundary. There was a certain difference in the pattern of size distribution of commissural efferent neurons of layer III between area 17, 18, 19 and the lateral suprasylvian cortex. Labeled neurons in area 17, 18, 19 showed a dominant peak in the histograms of cell size, while in the lateral suprasylvian cortex they were more evenly distributed in a wide size range.