Qualitative and quantitative analysis of the callosal projections to prefrontal, frontal motor, and parietal areas in the macaque monkey

In the macaque brain most cortical areas are connected through the corpus callosum to the same or different areas of the contralateral hemisphere (homotopic and heterotopic callosal connections). Many studies have described the callosal connectivity of several cortical areas, but the proportion of callosal vs. ipsilateral afferences and, except for frontal motor areas, of homotopic vs. heterotopic afferences is still unknown. We have analyzed qualitatively and quantitatively the distribution of callosal projecting neurons (CPNs) after neural tracer injections in prefrontal, frontal motor and opercular, and parietal areas (36 tracer injections in 20 macaques). The percentage of CPNs with respect to the total number of labeled neurons (ipsi + contra), with few exceptions, was at least 5% and though quite variable tended to be higher for prefrontal (~ 17%) and premotor (~ 14%), and lower for parietal (~ 9%) areas. For most areas, heterotopic afferences were richer than homotopic ones and for some (e.g., F6 and F3) the areal distribution of CPNs was almost similar to that of the ipsilateral labeling. Furthermore, in areas 24, F6, and F3 the amount of CPNs tended to be relatively high. Finally, the laminar distribution of CPNs could differ from that of the ipsilateral labeling and varied according to the projecting and/or the target area. The present data provide a general framework, though still not complete, of the interhemispheric connectivity in the primate brain and could be useful for a better understanding of the interhemispheric interactions in bimanual coordination, sensorimotor integration, and cognitive functions.