Second somatic sensory area in the cerebral cortex of cats: somatotopic organization and cytoarchitecture.

The cortex of the anterior ectosylvian gyrus and adjoining ectosylvian and suprasylvian sulci was explored with tungsten microelectrodes to determine the distribution of responses to light cutaneous stimulation in barbiturate-anesthetized cats. Recordings were spaced between 125 and 250 micrometers and, in several cases, nearly all of the somatic areas in this cortex were explored in the same brain. Four somatic sensory areas were identified on the basis of responses properties, sequences of receptive fields, and cytoarchitecture. The largest area, which occupied the rostral and medial two-thirds to three-fourths of the exposed, relatively flat portion of the anterior ectosylvian gyrus, was called the second somatic sensory area (SII). Receptive fields in SII were primarily from the contralateral side of the body; they were well defined and somatotopically organized into an erect representation of the body. The top of the head was located next to a similar representation of the periphery in a portion of the first somatic sensory area (SI). Individual distal digits and toes occupied discrete components of the SII map. Another representation for the distal forelimb and hindlimb was noted medially along the lateral bank of the anterior suprasylvian sulcus. Receptive fields and response properties in this region were equivalent to those seen in SII proper. However, only a crude anteroposterior, fore- to hindlimb topographical organization was noted, but with more distal parts of the limbs generally located closer to the fundus of the sulcus in this medial representation. As the cytoarchitecture in this medial region was similar to the rest of SII it was considered a medial subdivision of SII. A third, topographically organized zone was located lateral to SII largely within the upper bank of the anterior ectosylvian sulcus and adjoining lateral crest of the anterior ectosylvian gyrus. Large, stockinglike, contralateral receptive fields were common; ipsilateral components to the receptive fields were present. Some individual digit receptive fields were located in the rostral part of the forelimb zone within the anterior ectosylvian sulcus. This lateral somatic area is probably equivalent to a fourth somatic sensory area (SIV) recently identified by Clemo and Stein ('82). Posterior to the hindlimb zones of SII and medial to SIV was another region that responded to cutaneous plus auditory stimulation. There was no detectable topography in this area; nearly all of the receptive fields were large, frequently bilateral, and often involved the whole body or all four extremities. This area's cytoarchitecture was comparable to previous descriptions of the suprasylvian fringe (Rose, '49). The location and physiology of these four areas were discussed in reference to previous controversies regarding the topography of the body representation in SII and the location of an acallosal zone in this region of cortex.