The anatomical substrate of callosal messages from SI and SII in the cat.

Horseradish peroxidase (HRP) was injected into the first (SI) or second (SII) somatosensory areas of 21 adult cats. The radial and tangential (normal and parallel to the pial surface, respectively) distribution and morphology of the callosal neurons were studied. HRP injections were combined with single unit recording in the contralateral cortex in order to determine which part of the somatosensory periphery is represented within the regions containing callosal neurons, the callosal (efferent) zones, in SI and SII. The callosal zone of SI extends over the trunk and part of the forepaw representation. In the forepaw and hindlimb representations callosal neurons projecting only to the contralateral SII are found, while in the trunk representation callosal neurons projecting to contralateral SI or SII are found. The callosal zone in SII extends widely throughout the forepaw representation in this area and projects to the contralateral SII but not to SI. In both SI and SII the callosal neurons are mainly located in layer III. A few of them are also found in layer VI. They are very rare in other layers. Callosal neurons in layer III are mostly pyramidal but exceptionally stellate; in layer VI they are pyramidal, triangular, and occasionally stellate. These data indicate that transformations of the cortical somatosensory maps are achieved in the message sent through the corpus callosum. These transformations are i) determined by the extent and location of the callosal zones and perhaps by the distribution of callosal neurons within them, ii) different in different areas, iii) different in a same area, according to the cortical targets to which they are conveyed. The existence of callosal connections originated from areas of distal forepaw representation supplies a possible anatomical substrate for those types of intermanual transfer of tactile learning which depend upon the integrity of the corpus callosum.