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Journal of Neuroscience, Vol 7, 1682-1697, Copyright © 1987 by Society for Neuroscience
Tactile discrimination of shape: responses of slowly and rapidly adapting mechanoreceptive afferents to a step indented into the monkey fingerpad
MA Srinivasan and RH LaMotte
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)
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