Representation pattern in the second somatic sensory area of the monkey cerebral cortex

Abstract

The body representation in the second somatic sensory area of macaques has been studied by tracing with anatomical techniques the projections from defined parts of the body representation in the first somatic sensory area (SI) to their terminal regions within the lateral sulcus. The second somatic sensory area (SII), as identified in terms of cytoarchitecture and its connection with the thalamic ventrobasal complex, is the only region of the lateral sulcus to receive a projection from SI. The nearby retroinsular area and area 7b receive a projection from area 5. Within SII the face and head representations lie anteriorly, occupying the dorsalmost part of the insula and portions of the fronto‐parietal operculum. The digits, hand, and arm are represented posterior to the face and may take up the mediolateral extent of the parietal operculum in the region immediately in front of the posterior pole of the insula. The trunk representation is lateral to the arm representation, i.e., deep within the superior circular sulcus and on the dorsal insula. The hindlimb appears behind the trunk also occupying the superior circular sulcus in addition to the deepest 2–3 mm of the upper bank of lateral sulcus immediately posterior to the insula. Areas 3b, 1, and 2 each project to SII, and their projections appear to converge within the representation of a given body part. Injections of anterogradely transported tracers in SI label vertically oriented columnar arrays of terminal ramifications in SII, resembling those previously described for other cortico‐cortical projections within the sensory‐motor region. In experiments which combined anterograde and retrograde labeling, cells projecting from SII to SI formed columns exactly coinciding with the columns of anterogradely labeled axons terminating in SII. The cells of origin of cortico‐cortical projections emanating from SII formed two distinct laminar populations, one in the supragranular layers and the second mainly in layer VI. There is evidence for fiber terminations within layer VI mainly underlying the column formed by the terminal ramifications in layers I through IV.