The pattern of interhemispheric connections and its relationship to extrastriate visual areas in the macaque monkey.

The distribution of interhemispheric connections was studied in extrastriate visual cortex of the macaque monkey. Callosal fiber terminations were identified by staining for anterograde degeneration following transection of the splenium of the corpus callosum. Retrogradely labeled cell bodies of callosal projection neurons were identified histochemically following application of horseradish peroxidase to the cut surface of the callosum. Results were displayed on unfolded, two-dimensional representations of the cortex, which permitted spatial and topological relationships between callosal recipient and callosal free cortical regions to be discerned readily. The overall pattern of callosal inputs to visual cortex can be subdivided into nine callosal recipient zones which surround seven callosal free regions in the occipital, temporal, and parietal lobes. This pattern provides reliable and useful landmarks for identifying the borders of at least five topographically organized extrastriate visual areas. There is a pronounced dorsoventral asymmetry in callosal projections, not recognized in previous studies, which reflects an asymmetry in the organization of visual areas in dorsal versus ventral halves of the occipital lobe. The pattern of callosal fiber terminations is mirrored by a very similar distribution of callosal projection neurons. There are significant regional differences in the laminar distribution of callosal projection cells, and these differences may reflect functionally distinct cortical subdivisions. A considerable degree of individual variability was found in the relationship of callosal connections to gyral and sulcal landmarks as well as in the fine structure of individual callosal recipient strips, suggesting that each animal has a unique "callosal fingerprint." These findings emphasize the usefulness of the callosal pathway in elucidating the functional organization of extrastriate visual cortex.