Homotopic and heterotopic callosal afferents of caudal inferior parietal lobule in Macaca mulatta

Abstract We have examined callosal‐axon neurons giving rise to homotopic and heterotopic callosal projections to caudal inferior parietal lobule (area PG) in Macaca mulatta , identifying these neurons by means of retrograde axonal transport of horseradish peroxidase. The labeled neurons in the homotopic region occur predominantly in layers IIIB and V. A moderate number are seen also in layer VI, a smaller number in layer IV, and rare cells occur in layer II. These neurons occupy a region very similar in outline to the injection area, and though variable in density in the horizontal plane are continuously distributed in this plane. The heterotopic neurons are seen in the contralateral cingulate gyrus, continuing caudally into medial parietal cortex, in the cortex of the superior temporal and occipitotemporal sulci, in the caudal superior temporal gyrus, and in the caudal inferior parietal lobule, behind the homotopic area. These same regions on the ipsilateral side contain labeled neurons of origin of ipsilateral association projections to area PG. For other ipsilateral association regions (e.g., frontal lobe), no corresponding contralateral heterotopic labeling was found. A review of the literature on heterotopic callosal connections allows tentative generalization of this conclusion: The callosal heterotopic connections of a particular cortical area are made with regions which on the ipsilateral side have association connections with that area, though usually not with all of such regions.