Intra- and interhemispheric projections of the precentral, premotor and arcuate areas in the rhesus monkey

Abstract

Summary The intra- and interhemispheric connections of various parts of the precentral gyrus and rostrally adjacent frontal areas are studied with silver impregnation techniques in the rhesus monkey. The subdivisions of the precentral motor area (MI) are found to be connected ipsilaterally in a topographic manner with the premotor area, MII, primary somatosensory area (SI) and the second sensory area (SII). The areas in MI representing the hand and foot do not project to the opposite hemisphere whereas face-, trunk-, and limb-girdle areas do so, and their projections are directed to both homotopical and non-homotopical areas, including topographic projections to MII. These non-homotopical projections form an arch anterior to the projection-free hand and foot areas representing the dorsal aspect of the body from face to occiput and from thoracolumbar to sacral segment of the trunk. The premotor and arcuate regions can be differentiated on the basis of their intrahemispheric connectivity. The premotor area sends projections to its immediate vicinity (the prefrontal cortex MI and MII), to the superior parietal lobule including the adjacent upper bank of the intraparietal sulcus and to the rostral part of the inferior parietal lobule. In contrast, the projections of areas within the concavity of the arcuate sulcus are directed to the prefrontal cortex, to the caudal part of the inferior parietal lobule including the adjacent lower bank of the intraparietal sulcus and to the cingulate gyrus. Likewise, the interhemispheric connections of the premotor areas and the arcuate areas are mainly found in the homotopic regions except for a small area above the caudal tip of the principal sulcus. The non-homotopic projection of the premotor areas are directed to MI, MII, and prefrontal cortex, whereas those from the arcuate areas are observed in the prefrontal cortex only. The possible significance of these projections is discussed.