The organization of ipsilateral cortical connections of the rat primary somatic sensory area (SI) was analyzed following small injections of multiple fluorescent tracers in the same case, into two or three SI body representations identified electrophysiologically. Labeling patterns were studied in tangential cortical sections and in flattened reconstructions from coronal sections. The cytochrome oxidase staining in tangential sections served as a control for injection location and to position labeling patterns found within granular portion of SI.
The results show that most connections made with SI are reciprocal. Their topographical organization show different degrees of precision in the different areas. Homotypical and heterotypical connections were defined, the latter being more evident within the granular portion of SI. The findings: (1) were consistent with subdividing rat SI into four distinct areas with each having its own pattern of connections, (2) revealed two topographically organized regions in parietal cortex lateral to SI called second somatosensory (SII) and parietal ventral (PV) areas, (3) confirmed a topographical pattern in motor cortex and suggested an organization for connections between SI and an agranular medial field, and (4) demonstrated three more regions in parietal cortex connected to SI: posterior to SI called parietal medial; lateral to PV called parietal rhinal; posterior to SII called parietal lateral. Differences were noted in the distinctions between and within the maps when label distributions were plotted separately from supra- and infragranular layers. These findings agree with previous parcellations of the rat SI (Chapin et al., 1987: J. Comp Neurol 263:326–346), squirrel PV and SII (Krubitzer et al., 1986: J. Comp Neurol 250:403–430), and the organization of rat corticospinal neurons in many of the same areas (Li et al., 1990: Somat Motor Res 7:315–335).