ELECTROPHYSIOLOGICAL EVIDENCE FOR OVERLAPPING DOMINANT AND LATENT INPUTS TO SOMATOSENSORY CORTEX IN SQUIRREL-MONKEYS

1. The pattern of reorganization in area 3b of adult primates after me dian or ulnar nerve section suggests that somatic afferents from the d orsum of the hand, carried by the radial nerve, have preferential acce ss to the cortical territories normally expressing glabrous inputs car ried by the median and ulnar nerves. A likely mechanism underlying pre ferential access is preexisting, but silent radial nerve inputs to the glabrous region of cortex. 2. We tested this by comparing the effects of electrical stimulation of median or ulnar versus radial nerves, on responses in the hand representation of area 3b. Laminar current sour ce density and multiunit activity profiles were sampled with the use o f linear array multicontact electrodes spanning the laminae of area 3b . Data were obtained from three squirrel monkeys anesthetized during r ecording. 3. Compared with colocated median or ulnar nerve responses, the radial nerve response had 1) an initial short-latency response in the middle laminae that was subtle; there was a small transmembrane cu rrent flow component without a discernable multiunit activity correlat e; and 2) a laminar sequence and distribution of activity that was sim ilar to those of the median or ulnar nerve responses (i.e., initial ac tivation of the middle, followed by upper and lower laminae), but the significant current how and multiunit response to radial nerve stimula tion occurs 12-15 ms later. 4. Normal corepresentation of nondominant dorsum hand (radial) inputs with the dominant (median or ulnar) inputs in the glabrous hand surface representation provides a clear vehicle for the biased patterns of reorganization occurring after peripheral n erve section. The initial, ''subtle'' activity phase in the nondominan t response is believed to reflect intracortical inhibition, and the la ter ''significant'' response phase, a rebound excitation, possibly com pounded by an indirect or extralemniscal input. The spatiotemporal pat tern of nondominant input is proposed to play a role in normal somatos ensory perception.