Electrophysiological examination of the representation of the face in the suprasylvian gyrus of the ferret: a correlative study with cytoarchitecture.

Using high-resolution microelectrode mapping methods, we explored the organization of the face representation within the primary somatosensory cortex of ferrets, finding evidence for at least two and probably four representations of the face distributed consecutively from anterior to posterior along the long axis of the suprasylvian gyrus. Examination of the cytoarchitecture (Rice et al., this issue) revealed that these four areas corresponded to four different cytoarchitectonic fields within the crown of the suprasylvian gyrus. The two central, most completely defined representations were oriented so that the dorsal cutaneous surfaces of the face were represented on the lateral side of the gyrus, while the perioral and ventral surfaces were represented on the medial side. The rostral-to-caudal organization within these two representations was reversed; the glabrous rhinaria were represented at the opposite ends of the maps, and penetrations progressively further away from the cortex serving the rhinaria encountered neurons activated by sites progressively more caudal on the face. Receptive fields obtained more rostrally on the gyrus suggested another reversal, implying a third representation. A small area with large receptive fields near the caudal and medial border of the two central maps suggested the presence of a fourth representation. Since the projections of adjacent skin surfaces overlapped considerably, cortical sites serving a particular cutaneous surface were illustrated as enclosed areas that overlapped the territories of other, adjacent representations. The results of this study and of others suggest a need for a re-evaluation of the hypothesis establishing a homology between the representation found in area 3b of primates and that of the primary somatosensory area in nonprimates.