The callosal connexions of the primary somatic sensory cortex in the monkey

The callosal connexions of the primary somatic sensory cortex, SI, of the monkey have been studied with axonal degeneration methods after the placement of lesions of varying size in the cortex of one hemisphere and after section of the corpus callosum. For the correlation of the distribution of the degeneration with the cytoarchitectonic subdivisions of SI and with their boundaries, planar reconstructions of the extents of the subdivisions and of area 5 were made. The extent of area 5 is surprisingly large, being about the same as SI, and area 3a can be recognized as a distinct subdivision along the entire medio-lateral extent of SI. The callosal fibres end in narrow, irregular bands aligned in the medio-lateral dimension and there are accentuations at the boundaries of the cytoarchitectural subdivisions. In the representations of the trunk and face, the bands of degeneration are present across the entire antero-posterior extent of SI and with increases at the boundaries, while in the limb regions the degeneration becomes restricted to the boundaries. It is suggested that the callosal connexions of the somatic sensory cortex, like those in the visual and auditory areas, are connecting those parts of the cortex in the two hemispheres that are concurrently activated by a peripheral stimulus. The parts of SI that are devoid of callosal connexions are related to the distal limbs. The callosal connexions are homo- and heterotopical; an architectonic subdivision within the callosally connected regions projects to the same and other architectonic subdivisions at the same medio-lateral level in the opposite hemisphere; the cortex containing the representation of the caudal trunk near the post-central dimple is connected with the same region in the other hemisphere and with that of the separate representation of the caudal trunk in the posterior part of the cingulate sulcus, while the representation of the occipital region at the post-central dimple is connected both with the homotopical site in the other hemisphere and with the other representation of this part of the periphery at the level of the lower end of the intraparietal sulcus.