The effect of force and conformance on tactile intensive and spatial sensitivity

The effect of force on intensive and spatial processing was examined with three measures of tactile sensitivity. One of the measures based on intensive cues is the smooth-grooved (SM/GV) task, and the two other measures based on spatial cues are the grating orientation and gap detection tasks. Measures were made at two locations that vary in sensitivity and in the density of innervation of the primary afferent fibers, the right index fingerpad and the palmar surface of the proximal phalanx (fingerbase). At each location, psychometric functions were generated for each of the three measures for two forces (50 and 200 g). The results indicated that increasing force led to marked improvement on the task that relied on intensive cues; however, on the tasks that relied on spatial cues, force had no effect on performance. Biomechanical measures were made of the depth to which the skin invades the grooves of the contactors (conformance) at the two test sites, with the two forces, and with different groove widths. Conformance was found to be a joint function of force and groove width. Further, performance on the SM/GV task could be predicted by the amount of conformance. The psychophysical results are consistent with the view that increasing conformance increases neural activity in the primary afferent fibers, and that this increase in neural activity improves SM/GV performance, but has little effect on the quality of the spatial image.