Functional characteristics of the parallel SI- and SII-projecting neurons of the thalamic ventral posterior nucleus in the marmoset

The functional organization of the primate somatosensory system at thalamoc ortical levels has been a matter of controversy, in particular, over the ex tent to which the primary and secondary somatosensory cortical areas, SI an d SII, are organized in parallel or serial neural networks for the processi ng of tactile information. This issue was investigated for the marmoset mon key by recording from 55 single tactile-sensitive neurons in the lateral di vision of the ventral posterior nucleus of the thalamus (VPL) with a projec tion to either SI or SII, identified with the use of the antidromic collisi on technique. Neurons activated from the hand and distal forearm were class ified according to their peripheral source of input and characterized in te rms of their functional capacities to determine whether the direct thalamic input can account for tactile processing in both SI and SII. Both the SI- and SII-projecting samples contained a slowly adapting (SA) class of neuron s, sensitive to static skin displacement, and purely dynamically sensitive tactile neurons that could be subdivided into two classes. One was most sen sitive to high-frequency (greater than or equal to 100 Hz) cutaneous vibrat ion whose input appeared to be derived from Pacinian sources, while the oth er was sensitive to lower frequency vibration (less than or equal to 100 Hz ) or trains of rectangular mechanical pulse stimuli, that appeared to recei ve its input from rapidly adapting (RA) afferent fibers presumed to be asso ciated with intradermal tactile receptors. There appeared to be no systemat ic differences in functional capacities between SI- and SII-projecting neur ons of each of these three classes, based on receptive field characteristic s, on the form of stimulus-response relations, and on measures derived from these relations. These measures included threshold and responsiveness valu es, bandwidths of vibrational sensitivity, and the capacity for responding to cutaneous vibrotactile stimuli with phase-locked, temporally patterned i mpulse activity. The analysis indicates that low-threshold, high-acuity tac tile information is conveyed directly to both SI and SII from overlapping r egions within the thalamic VP nucleus. This direct confirmation of a parall el functional projection to both SI and SII in the marmoset is consistent w ith our separate studies at the cortical level that demonstrate first, that tactile responsiveness in SII largely survives the SI inactivation and sec ond, that SI responsiveness is largely independent of SII. It therefore rei nforces the evidence that SI and SII occupy a hierarchically equivalent net work for tactile processing.