Interhemispheric integration: I. Symmetry and convergence of the corticocortical connections of the left and the right principal sulcus (PS) and the left and the right supplementary motor area (SMA) in the rhesus monkey.

The relationship between the termination zones of projections from paired homotopic areas in the frontal lobe was examined in the cerebral cortex of the macaque monkey. Injections of WGA-HRP and tritiated amino acids were made in topographically matched regions of the principal sulcus (PS) or the supplementary motor area (SMA) in each hemisphere, such that the projections from the same area on each side were differentially labeled in the same animal. Adjacent sections through the cortical regions that received bilateral inputs from these areas were processed for the respective tracers, permitting the relationship between the converging projections to be defined. Comparison of the cortical connections of the left and right PS or of the left and right SMA yielded two major findings. First, only minor differences in the topographic distribution and strength of connections of homotopic areas were observed, providing little evidence of asymmetry in the connections of either the PS or the SMA in the macaque. Second, with the exception of interdigitation observed in a portion of the dorsal bank of the PS, the cortical projections from both the left and the right PS and SMA converged (overlapped) in common columnar territories. These termination patterns allow for a remarkable degree of interhemispheric integration.