Bilateral Receptive Fields in Cortical Area SII: Contribution of the Corpus Callosum and Other Interhemispheric Commissures

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Abstract
The corpus callosum contributes to the interhemispheric transfer of somatosensory information. Since the somatosensory pathways are essentially crossed, a number of studies have postulated that the corpus callosum may be responsible for the presence of bilateral receptive fields (RFs) in cortical area SII. Moreover, subcortical structures, as well as some of the other commissures, may also contribute to the bilateral nature of these cells. In order to assess the relative importance of the corpus callosum, this study compared the RF properties of cells in area SII of callosum-sectioned cats to normal cats, using single-cell recordings. Results showed that the corpus callosum makes an important contribution to the bilateral activation of cells in SII, since the proportion of cells with bilateral RFs found in callosum-sectioned cats was less than half that obtained in normal cats. The decrease in the proportion of bilateral RFs was found for all body regions with the exception of the face. However, the substantial number of bilateral RFs remaining in callosotomized cats indicates that this structure is not the sole contributor to the bilateral activation of cells in SII. In order to determine whether this residual bilateral activation might be mediated by the other interhemispheric commissures, a group of cats was subjected, besides the callosotomy, to the additional transection of their subcortical commissures, including the anterior, posterior, habenular, and intertectal commissures, as well as the massa intermedia. When this group of deep-split cats was compared to the callosotomized group, the results indicated that the contribution of the other commissures to bilateral activation is negligible, since approximately the same proportion of bilateral RFs was encountered in the two groups. The relative importance of the callosal contribution to bilateral RFs of different body regions is discussed with respect to the roles commonly attributed to this structure.