THE RESPONSE OF SI DIRECTIONALLY SELECTIVE NEURONS TO STIMULUS MOTIONOCCURRING AT 2 SITES WITHIN THE RECEPTIVE-FIELD

Data from two classes of primary somatosensory (SI) neurons (termed '' direction-invariant'' and ''direction-variant'') were analyzed to eval uate their capacity to process the directional information provided by two moving (i.e., brushing) stimuli delivered to nonoverlapping skin sites within the receptive field (RF). The stimulus sites were arrange d either end to end or side by side on the skin. The two stimuli were delivered at the same time (i.e., simultaneously) or asynchronously in precisely defined orders. For both classes of neurons, and with both the end-to-end and side-by-side dual-stimulus arrangements, the respon se elicited by dual-site stimulation was usually much less than a line ar summation of the responses elicited by independent stimulation of e ach site. For the direction-invariant neurons, when the two sites were arranged end to end and direction of motion at both sites was the sam e, directional sensitivity with dual-site stimulation most often match ed or exceeded a vectorial sum of the sensitivities observed at each s ite when stimulated alone. In contrast, with the side-by-side arrangem ent, the level of directional sensitivity achieved with dual-site stim ulation often failed to attain that predicted by vectorial summation o f the sensitivities observed at each site. Instead, directional sensit ivity under this dual-stimulus condition only approximated that attain ed with single-site stimulation at the more sensitive site. When nonco rresponding directions of motion were presented at two sites within th e RF (using either the end-to-end or side-by-side arrangement), direct ion-invariant neurons failed to respond differentially to opposing pat terns of dual-site stimulation. For the direction-variant SI neurons, a particular end-to-end arrangement of the two sites within the RF was studied: Sites were identified on opposite sides of the within-RF bou ndary that in these neurons separates regions with opposite directiona l preferences. With this arrangement, the differential response was gr eater when opposite directions of motion were applied to the two sites than it was when the same direction of motion was delivered at both s ites. The observations suggest that for both groups of SI neurons, the magnitude of directional sensitivity is dependent on the same attribu tes of dual-site stimulation that influence human cutaneous directiona l sensitivity-that is, on the spatial arrangement of and temporal dela y between the two stimuli, and on the correspondence of their directio ns. The effects of dual-site stimulation on the behavior of these two neuron populations appear to be in good agreement with the hypothesis that they subserve a function in tactile motion perception.