Topography, cytoarchitecture, and sulcal patterns in primary somatic sensory cortex (SmI) prosimian primate, Perodicticus potto.

The topographic organization of the primary somatic sensory projection area (SmI) in relation to cytoarchitectural fields and sulcal patterns was examined in the prosimian primate Perodicticus potto. The area of cortex responding to low threshold (LT) cutaneous stimulation of the glabrous and hairy surfaces of the hand was determined by microelectrode mapping techniques, with standardized threshold stimuli for defining receptive fields. A single somatotopic projection of the two hand surfaces was found; the glabrous projection area is rostral to that of the hairy hand. Within both the glabrous and hairy areas, receptive fields on the distal digits are found anterior to those on the proximal hand. The glabrous hand projection area is coextensive with a dense granular area typical of koniocortex. The hairy hand area corresponds to a cytoarchitectural field which is less granular than the glabrous field. While koniocortex occupies the crown of the gyrus caudal to the coronally oriented sulcus, a large more rostral field, which contains both granule and large pyramidal cells, occupies the whole of the caudal bank of the sulcus. Force thresholds of many receptive fields (RFs) in Perodicticus were high both on the borders and within the LT area (perhaps because of the advanced age of these animals). However, the receptive field sizes for both the glabrous and hairy hand areas were of the same magnitude as those of Nycticebus (Carlson and FitzPatrick, '82). From the combined studies of three species of Lorisidae, Perodicticus, Galago (Carlson and Welt, '80), and Nycticebus (Carlson and FitzPatrick, '81), using similar mapping and stimulation techniques, both general and specific features of SmI hand area organization can be illustrated. A single projection of the glabrous and hairy hand is common to Perodicticus and Galago, but two glabrous projection areas are seen in Nycticebus. The projection area for the hand in Perodicticus is twice as large (relative to brain size) as in Galago and Perodicticus. The possible behavioral significance of increased differentiation of the hand area in Nycticebus and elaboration of the area in Perodicticus could be examined by study of hand use and tactile capacity in these same species.