Tactile discrimination of shape: responses of rapidly adapting mechanoreceptive afferents to a step stroked across the monkey fingerpad

Abstract

Responses of rapidly adapting Meissner corpuscle mechanoreceptive afferent fibers (RAs) to steps of varying shape stroked across the distal fingerpad were recorded from anesthetized monkeys. A series of flat plates were used, each having an increase in thickness (a step) 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. The width (half-cycle wavelength) of the sinusoidal step was varied from 0 to 3.13 mm, producing a series of step shapes that differed in steepness and curvature. These steps could be broadly categorized into 2 groups, "steep" and "gradual." Each step was stroked back and forth under constant compressional force, using a servocontrolled mechanical stimulator. The RA's response to a step provided a spatial pattern of action potentials in which the occurrence of each impulse corresponded to a position of the step on the skin. This response consisted of a single "burst" of impulses to the sinusoidal portion of the step. Changes in stroke direction, step shape, or velocity of stroking primarily affected the RA discharge rate during the burst, and, less consistently, the spatial width of the burst. For a given step shape and stroke velocity, the discharge rate was greater for strokes from the low to the high side of the step than for strokes from the high to the low side. Discharge rate was greater for steep than for gradual steps and, for a given step, it increased with stroke velocity. All the major features of the responses were interpreted as being due predominantly to the sensitivity of the RA to vertical velocity at the most sensitive spot on its receptive field, together with a sensitivity to the rate of change in skin curvature at that spot. RA discharge rate distinguished not only the gross differences between steep and gradual steps, but also some of the finer differences in sharpness among steep steps. From a comparison with the human capacity for tactile discrimination of the steps, it was concluded that RAs, through their discharge rates, provide primarily "intensive" information about the sharpness of shapes.

Tactile spatial sensitivity and anisotropy

“…Increasing the surface area of the contactor, while holding force constant, results in less conformity of the skin to the edges of the grooves. The curvature of the skin to an edge is positively correlated with the response rate of the primary afferent fibers (LaMotte & Srinivasan, 1987a, 1987b. In some additional observations, higher forces were used, 400 g of force, and the threshold dropped from 8.62 mm to 3.50 mm.…”

Representation of curved surfaces in responses of mechanoreceptive afferent fibers innervating the monkey's fingerpad

“…Our data show that as the curvature of the stimulus changes, the shape of the profile changes correspondingly. This explains how humans are able to scale and discriminate the curvature of objects in contact with the fingerpad using only cutaneous receptors (LaMotte and Srinivasan, 1987a;Srinivasan and LaMotte, 1987;Goodwin et al, 199 1;Goodwin and Wheat, 1992a). An increase in contact force results in an upward scaling of the response profiles, explaining how humans are able to scale the contact force using cutaneous afferents (Goodwin and Wheat, 1992b).…”

“…In cases where the fingers move over the object or where the object is scanned over the skin as in the studies of LaMotte and Srinivasan (1987b), the RAs probably play an important role in improving shape discrimination. Since 80% of the PCs did not respond and since they have a low innervation density (Kumamoto et al, 1993) they are probably not significant for our stimuli.…”

“…Most psychophysical studies of tactile shape perception have used active touch, which results in input from all the above receptors (Lederman and Klatzky, 1987;Roland and Mortensen, 1987). A recent study used passive touch in which only the cutaneous receptors in the fingerpads could convey information about the stimuli; these experiments showed that information about the local shape of contacted objects is accurately signaled (LaMotte and Srinivasan, 1987a;Srinivasan and LaMotte, 1987). The first clear indication, from neural recordings, that shape affects cutaneous mechanoreceptor responses was the observation of edge enhancement in slowly adapting type I afferents (Vierck, 1979;Phillips and Johnson, 1981a;Johansson et al, 1982).…”

Aging and Curvature Discrimination From Static and Dynamic Touch

“…from immediate contact, when our hands and fingers come into actual physical contact with object surfaces [9][10][11]. This physical contact stimulates slowly-and rapidly-adapting sensory mechanoreceptors within the skin [12][13][14]; the resulting patterns of cutaneous activity eventually produce activation in a variety of areas within the cerebral cortex [15], leading to conscious awareness and perception. It is very important to note, however, that cutaneous activity resulting from simple contact is not the only source of information available to support the tactile perception of object shape.…”