The response of SI directionally selective neurons to stimulus motion occurring at two sites within the receptive...

Data from two classes of primary somatosensory (SI) neurons (termed "direction-invariant" and "direction-variant") were analyzed to evaluate 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 arranged 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 response elicited by dual-site stimulation was usually much less than a linear summation of the responses elicited by independent stimulation of each site. For the direction-invariant neurons, when the two sites were arranged end to end and direction of motion at both sites was the same, directional sensitivity with dual-site stimulation most often matched or exceeded a vectorial sum of the sensitivities observed at each site when stimulated alone. In contrast, with the side-by-side arrangement, the level of directional sensitivity achieved with dual-site stimulation often failed to attain that predicted by vectorial summation of the sensitivities observed at each site. Instead, directional sensitivity under this dual-stimulus condition only approximated that attained with single-site stimulation at the more sensitive site. When noncorresponding directions of motion were presented at two sites within the RF (using either the end-to-end or side-by-side arrangement), direction-invariant neurons failed to respond differentially to opposing patterns 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 boundary that in these neurons separates regions with opposite directional preferences. With this arrangement, the differential response was greater when opposite directions of motion were applied to the two sites than it was when the same direction of motion was delivered at both sites. The observations suggest that for both groups of SI neurons, the magnitude of directional sensitivity is dependent on the same attributes of dual-site stimulation that influence cutaneous directional sensitivity--that is, on the spatial arrangement of and temporal delay between the two stimuli, and on the correspondence of their directions. 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.