Further studies on the size specificity of commissural projecting neurons of layer III in areas 17, 18, 19 and the lateral suprasylvian area of the cat's visual cortex.

After injection of horseradish peroxidase (HRP) into the commissural area in the border region between areas 17 and 18, the lateral part of area 19 and the lateral suprasylvian area (LS) of cat's visual cortex, HRP-labeled neurons were distributed preferentially in layer III in each cortical region of the other hemisphere. Layer I was free from labeling in each cortical region. There were some real differences in the distribution of labeled neurons in other layers. In the area 17/18 border region, a significant population of labeled neurons was found in the upper layer IV. The labeling of neurons in deeper layers (V and VI) increased in area 19 and further in the LS. There was no great difference in the laminar distribution between neurons which appeared after HRP injections into the homotopical region and those after heterotopical injections on the other side. The commissural area, 17/18 border region and the LS, contained in layer III a larger number of medium-sized to large neurons than acommissural area 17 on the splenial gyrus, where layer III was filled with densely packed small neurons. The size distribution of layer III was unimodal with a marked skew towards the larger value in each cortical region. About 50% of labeled neurons in layer III were distributed in a narrow range at the border of two categories of cell sizes, viz small and medium-sized to large. Among the labeled neurons in layer III, 34.2-47.1% had larger cell sizes. Some of the smallest neurons in layer III were also labeled, however, they appeared only after HRP injections into the homotopical region of the cerebral cortex on the opposite side.