Tactile discrimination of shape: responses of slowly and rapidly adapting mechanoreceptive afferents to a step indented into the monkey fingerpad.

The representation of shape in the responses of monkey cutaneous mechanoreceptive afferents to steps of varying shape vertically indented into the fingerpad was studied. A series of flat plates was used, each with a step change in thickness in the middle so that one-half of the plate was thicker than the other. The cross-sectional shape of the step approximated that of a half-cycle sinusoid, 0.5 mm high, that was varied in half-cycle wavelength (step width) and hence in steepness and curvature. The steps fell into 2 categories, characterized as "steep" and "'gradual." Evoked action potentials were recorded from single, slowly adapting and from rapidly adapting Meissner corpuscle mechanoreceptive afferent fibers (SA and RA, respectively) innervating the fingerpad of the anesthetized monkey while each step was indented at a succession of lateral positions across the fiber's receptive field. The responses of each SA provided a spatial response profile (number of evoked impulses as a function of step position) that was directly related to the variation in curvature across the step. The rate of discharge was greatest under the sharpest (convex) portion of the step, least under the adjacent concave portion, and intermediate under the flat portions of the steps. The results indicated an exquisite sensitivity of the SA, even during the ramp phase of vertical indentation, to the changes in skin curvature. The spatial response profile remained relatively undistorted over time during the ensuing steady phase, while the contrast between the peak and the minimum response improved. RAs responded only during the ramp phase and with fewer responses, and gave rise to a poorly modulated spatial response profile, even though half of the RAs tested showed limited sensitivity to the amount or rate of change of skin curvature. It was hypothesized that RA responses are predominantly influenced by the vertical velocity of the most sensitive spot in the receptive field. When the same step stimuli were applied to the human fingerpad, the capacities of humans to discriminate differences in step shape were found to correlate with the discriminability of SAs, as opposed to the considerably poorer discriminability of RAs. It is concluded that information concerning the local curvature and hence the shape of objects indenting the skin is primarily coded by the SAs. In the 2 preceding papers (LaMotte and Srinivasan, 1987a, b), we investigated the responses of SAs and RAs to the same sinusoidal steps stroked back and forth across the fingerpad under constant compressional force.(ABSTRACT TRUNCATED AT 400 WORDS)