Discriminating smooth from grooved surfaces: effects of random variations in skin penetration.

The ability to discriminate a smooth surface from a grooved one depends on several variables, including the width of the grooves and the force with which the skin is contacted. It has been hypothesized that this smooth-grooved discrimination with statically presented stimuli is based on intensity cues, namely, the overall difference in perceived intensity between the smooth and grooved surfaces. To test this hypothesis, the perceived intensities of test stimuli were varied on a trial-by-trial basis by varying the depth of penetration the contactor was allowed to travel into the skin. As compared to a control condition in which stimuli were presented with the same average penetration and contrary to the hypothesis, random variations in penetration produced no decline in smooth-grooved performance. The total amount of conformance was an accurate predictor of sensitivity across various penetrations and across two test sites (distal finger pad and finger base). It appears that subjects are making absolute rather than comparative judgments in the smooth-grooved task. A recently developed continuum mechanical model of the responses of first-order mechanoreceptive afferents to static stimuli provided both a good fit to the data and indicated what aspect of the peripheral neural image was relevant for discriminating smooth surfaces from grooved surfaces.