Cortical areas within the lateral sulcus connected to cutaneous representations in areas 3b and 1: A revised interpretation of the second somatosensory area in macaque monkeys

Cortical connections between various body representations in areas 3b and 1 and lateral parietal cortex were examined in 18 macaque monkeys. We injected tracers (Fast Blue, Diamidino Yellow, Horseradish Peroxidase, and Rhodamine Dextran), alone or in combination, into closely related cutaneous responsive sites, e.g., adjacent digits. Separated patches of labeling were found across the parietal operculum and insula for all injected locations. On the basis of cytoarchitectural criteria, the labeled regions include the second somatosensory area (SII), retroinsular area (Ri) and granular insula (Ig). Assuming the connections are homotopical from physiologically identified body representations in primary somatosensory cortex, the labeling patterns in SII include complete anterior and posterior body maps. The orientation of the body is erect in the posterior and supine in the anterior SII region. Area 3b has greater density of connections with anterior SII. The maps are mirror images aligned along the distal extremities. The anterior-posterior (A-P) length of the "SII region" exceeds 7 mm; it extends in the coronal plane from the fundus of the lateral sulcus to surface cortex near the anterior tip of the intraparietal sulcus. Two additional topographically organized maps are likely in Ri. These are "worm-like" body maps oriented along the A-P axis and joined at the head representation. Connections with the center of Ig are not somatotopically organized. The diversity of somatosensory areas in lateral parietal cortex revealed by the labeled connections was discussed in reference to prior mapping of SII in monkeys and was compared to reports of multiple areas in this region of cortex in other species.