Callosal and intrahemispheric connectivity of the prefrontal association cortex in rhesus monkey: Relation between intraparietal and principal sulcal cortex

Horseradish peroxidase (HRP) histochemistry and double labeling with the fluorescent dyes nuclear yellow (NY) and fast blue (FB) were used to examine and compare the laminar and tangential arrangement of ipsilateral (associational) and contralateral (callosal) neurons and their relative density in three regions of prefrontal granular cortex: Walker's area 46 (principal sulcus), area 8A (superior limb of the arcuate sulcus), and area 11 (lateral orbital sulcus). In all three prefrontal regions, neurons with ipsilateral projections were labeled following injections of tracers into the intraparietal sulcus (IPS) and neurons with callosal projections were sequentially or simultaneously labeled with injections into the contralateral principal sulcus (PS). Quantitative analysis indicates that associational and callosal neurons in prefrontal cortex are distinct cell populations with strikingly similar organization including (1) common topography; (2) common laminar positions in layers III, IV, and V; (3) two- to three-fold higher densities in supragranular than infragranular layers; (4) common morphologies including a high proportion of nonpyramidal soma in the deeper cortical layers; (5) common uneven tangential distribution reminiscent of the interdigitation of their terminal fields; and (6) common subpopulations differing on the basis of terminal arbors. These findings indicate that the posterior parietal cortex and the prefrontal cortex form part of an integrated neural system important for spatiotemporal behaviors.