Neuronal activity in the primary somatosensory cortex in monkeys (Macaca mulatta) during active touch of textured surface gratings: Responses to groove width, applied force, and velocity of motion

In a descriptive survey of primary somatosensory cortex (SI) responses during active touch, two monkeys (Macaca mulatta) were trained to stroke their fingertips over pairs of gratings with constant ridge (250 μm) and varying groove (500-2,900 μm) width (roughness) and to identify the smoother (smaller groove). Speed of hand motion and applied force level during the stroke were monitored and recorded. Transdural single-unit recordings were obtained from areas 3b and 1 of SI while animals performed the task. The statistical sample consisted of 164 single units. Most cells in the sample responded in some fashion during the stroke, including brief increased or decreased activity to 1-mm ridges (touch strips) placed between surface pairs and at ends of each block to serve as touch sensors. These peristroke response patterns were described briefly. Most cells (153/164) responded to grating contact. Three types of responses to groove width were characterized. 1) Response was proportional to groove width in many cells. There was a vigorous response to roughest and none to smoothest surface. Mean firing rates for these cells appeared linearly related to groove width. 2) Graded responses were seen with smaller response differences to the same groove width range as in 1. Responses of some cells of types 1 and 2 were uncorrelated with variations in applied force and velocity of stroke. 3) There were inverse responses to groove width. Greater responses occurred during contact with smoother surfaces. Many cells were influenced by a combination of changes in groove width, force, and/or velocity. Activity of a small sample of cells in area 1 with slowly adapting (SA) response properties was an almost exclusive positive function of variations in force level. Unlike SAs in 3b, responses of these cells were uncorrelated with alterations in groove width or stroke velocity. Velocity effects were almost always associated with response to groove width. Positive velocity cells coded temporal period. Significant velocity effects were not evident in graded or inverse graded cells. Negative force and velocity effects were due to shifts in behavioral strategy. Periodicity related to the spatial period of the grating was found in autocorrelograms of a small number of cells. Finally, responses of some cells were unaffected by changes in groove width, force, or velocity. Some of these were affected by contact with touch strips. Others responded in undifferentiated fashion to the stroke, and their function remains unresolved. Overall, there was a continuum of response patterns. Subgroups of cells were not distinct. Responses of peripheral receptors change over the range of force and velocity used in active touch. Yet changes in these variables have minimal affect on perceived roughness, suggesting that they are 'filtered' out to provide perceptual constancy of texture. One function of SI might be to process mixed input from the periphery to produce unambiguous reflections of surface roughness and applied force. The range of mixed groove, force, and velocity responses found in SI might represent stages of cortical processing.