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

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... s 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. In primates, corticocortical projections generally terminate in columnar regions that lie orthogonal to the pial surface. Within these columns, the terminals are often concentrated in particular laminae (Goldman and Nauta, 1977; Jones et al., 1979; Innocenti, 1986 ). The projections from two different (heterotopic) cortical regions (in the same or opposite hemispheres) to a common cortical target appear to terminate in columns that either interdigitate or overlap, depending on the regions and the target concerned (Goldman-Rakic and Schwartz, 1982; Selemon and Goldman-Rakic, 1988) . When the projections overlap, the terminals from each region seem to be concentrated in complementary laminae within the same column (Selemon and Goldman-Rakic, 1988) . The terminal fields of converging projections from heterotopic areas thus appear to be largely segregated within common cortical targets. Little information exists on the relationship between the terminal fields of converging corticocortical projections from the same (homotopic) regions of the left and right hemispheres. Such information might help further elucidate general principles of cortical connectivity and address the issue of how the outputs of the two hemispheres may be integrated. We have examined this relationship by labeling the projections from homotopic areas in the left and right hemispheres with different anterograde tracers [WGA-HRP and tritiated amino acids ( 3 H-AAs)] in the same animal. Comparison of adjacent sections processed for the respective tracers has allowed us to examine the relationship between the ipsi-and contralateral terminal fields in common cortical targets. The areas chosen for injection, the principal sulcal cortex (PS) and the supplementary motor area (SMA), both have bilateral conical projections (