Organization of intracortical and commissural connections in somatosensory cortical areas I and II in the raccoon.

The organization of intracortical and callosal projecting cell bodies was examined in somatosensory representation areas I (SI) and II (SII) of the raccoon by use of horseradish peroxidase (HRP) or horseradish peroxidase-wheat germ agglutin (HRP-WHA). HRP and HRP-WHA were injected into commissurally and noncommissurally connected subdivisions of SI and SII. Injection sites in SII were identified electrophysiologically. Results were obtained from transverse sections in which the HRP was visualized with the aid of the substrates dihydrochlorobenzidine or tetramethyl benzidine in the presence of hydrogen peroxidase. The principal findings were the following: (1) there are reciprocal connections between SI and SII; (2) in SI the intracortically projecting cell bodies and terminals are located primarily in sulcal cortex; (3) intracortically projecting neurons in SI are located primarily in layers III whereas in SII they are located principally in layers III and V; (4) there are connections between disparate areas within SI; and (5) there are intracortical connections between callosum-connected and acallosal regions in SII. These results are discussed with regard to the results of mapping studies of the SI, the significance of intracortical connections to the formation of sulci in SI, and the possible roles of nonhomotopic connections in the intermanual transfer of learning.