The Visual Map in the Corpus Callosum of the Cat

The corpus callosum conveys all the fibers that connect areas 17 and 18 in the 2 cerebral hemispheres of the cat. The purpose of the present study was to ascertain the organization of the visual field map described by these fibers in the corpus callosum. This was achieved by injecting anterograde and retrograde pathway tracers at known locations in the callosally connected zones of areas 17 and 18. The positions of the injection sites were varied systematically to include all visual field elevations represented along the marginal and posterolateral gyri. Overall, the results show (1) that callosal fibers projecting between the 2 marginal gyri, where the lower visual fields are represented, pass through the body of the corpus callosum; (2) that fibers connecting the junction of the marginal and posterolateral gyri in the 2 hemispheres, where central fields are represented, pass through the dorsal splenium of the corpus callosum; and (3) that fibers passing between the ventral portions of the 2 posterolateral gyri, where upper fields are represented, pass through posterior and ventral splenium. In addition, the density of visual fibers in the splenium is greater than in the body of the corpus callosum. Within the overall pattern, a finer arrangement exists, and it was possible, by comparison with the cortical visual field maps, to describe a map of visual field elevations in the corpus callosum. In this map, the representations of the different visual field elevations are not a simple reflection of the map in the cortex. The map of the lower fields contained in the body is spread out, whereas the map of the central and upper fields in the splenium is highly compressed. The high degree with which observations can be reproduced in different cats indicates that the map is stereotyped from one animal to another.