Further observations on corticofrontal connections in the rhesus monkey

Abstract

The frontal lobe connections of the post-Rolandic sensory association areas are investigated. Our results indicate that the caudal portion of the superior temporal gyrus (area 22), the lateral peristriate belt (areas 18, 19), and the superior parietal lobule and the rostralmost portion of the inferior parietal lobule (areas 5 and 7), all project to periarcuate cortex, while the middle portion of area 22, caudal inferotemporal cortex (area 20), and the middle portion of the lower bank of the intraparietal sulcus, all have connections predominantly to prearcuate cortex. In contrast, rostral area 22 and the rostral inferotemporal cortex (area 21) project primarily to the orbital surface, and the middle portion of area 7 projects to the mid-principal sulcus. Those regions that projects to periacuate cortex are termed first association areas (AA1, VA1, SA1), those that project primarily to prearcuate cortex are designated second association areas (AA2, VA2, SA2), while those that project mainly to the orbital surface or the mid-principal sulcus are called third association areas (AA3, VA3, SA3). It was also found that the caudalmost portion of area 7 has a distinct projection pattern, connecting with the dorsal prearcuate cortex — areas 8B and 46. Additionally, it was observed that the connections from the association areas of different sensory modalities appear to overlap in specific areas of frontal cortex. Projections from the first association areas seem to converge in the periarcuate zone (bimodal overlap is noted between VA1 and AA1 in the arcuate concavity, and SA1 and VA1 dorsal to the arcuate sulcus), while those from the second association areas overlap in the ventral prearcuate cortex (area 46), where both bimodal and trimodal overlap is observed.