The distribution of commissural terminations in somatosensory areas I and II of the grey squirrel

The topographic distribution of somesthetic interhemispheric projections was studied in grey squirrels using the Fink-Heimer technique follow- ing large aspiration lesions of' the corpus callosum. On the day of perfusion, receptive fields were determined for microelectrode recording sites in the first, S I, and second, S 11, somatosensory areas of cortex, and small electrolytic lesions were made in order to identify some of these sites in prepared brain sections. The cortex was then separated from the rest of the brain, flattened, and cut parallel, so that with the aid of the reference lesions, the total pattern of degeneration could be related to a surface view of the brain and to previous electrophysiological maps of S I (Sur et al., '78) and S I1 (Nelson et al., '79). The results show that callosal terminations are unevenly distributed in S I and S 11, and suggest that there are several categories of callosal inputs to S I. A major region of terminations is in the architectonically distinct "unresponsive zone" within SI, and perhaps in other similar, but narrower, specialized zones within and bordering S I, as previously described in rats (Ryugo and Killackey, '75; Wise and Jones, '76, '78 Akers and Killackey, '78). Other callosal projections terminate within the responsive regions of S I. These regions include at least some of the representation of the body midline, most clearly the midline of the representations of the upper and lower face, as well as regions unrelated to the midline of the body. Most or all of the SI cortex responsive to stimuli away from the midline on the upper and lower lips, the mystacial vibrissae, and the glabrous forepaw was almost free of direct callosal terminations. Except for a central core region, most of S I1 appears to receive a moderate distribution of callosal inputs. Patterns of callosal connections in parietal cortex have been studied in a range of mamma- lian species with the consistent observation that terminations are unevenly distributed within the first (S I) and second (S 11) somato- sensory representations (for review see Akers and Killackey, '78). Many of these studies have attempted to relate the observed distribution of terminations to prevailing concepts of how the body is represented in these two fields, with the result that several different, although not nec- essarily incompatible, concepts of the func- tional roles of callosal connections have evolved. Two different explanations were used to account for the conclusion that the represen- tations of the distal limbs have few or no callosal connections in S I and S 11. The earlier view was that the separate maps of the contra- lateral body surface in each hemisphere are connected along the representations of the dor- sal midline of the body through the corpus callosum so as to form continuous or joined rep- resentations (Ebner and Myers, '62, '65; Ebner, '69; Jones, '67; Jones and Powell, '68, '69a, '73; Pandya and Vignolo, '69; Karol and Pandya, '71; Jones et al., '75; Shanks et al., '75) in a manner analogous to the joining of the visual hemifield representations along the vertical meridian. An alternative explanation for this conclusion is that commissural terminations are devoted to those parts of the two hemirepre- sentations that have integrated or coordinated sensory-motor functions, while callosal connec- tions are lacking in parts of S I and S I1 with highly independent or "lateralized functional roles (Jacobson, '70; Yorke and Caviness, '75).