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Journal of Neuroscience, Vol 7, 1655-1671, Copyright © 1987 by Society for Neuroscience
Tactile discrimination of shape: responses of slowly adapting mechanoreceptor afferents to a step stroked across the monkey fingerpad
RH LaMotte and MA Srinivasan
The representation of shape in the responses of slowly adapting
mechanoreceptive afferent fibers (SAs) in monkeys was investigated. A
series of flat plates was 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 of a sinusoid. The height of the step was fixed at 0.5 mm, while
its width (half-cycle wavelength) was varied from 0 to 3.13 mm, resulting
in step shapes that varied in steepness and curvature. The steps fell into
2 categories, characterized as "steep" and "gradual." A servocontrolled
mechanical stimulator stroked each step across the distal fingerpad from
the high to the low side of the step and back, while maintaining the
contact force at 20 gm wt. Evoked action potentials in single SAs
innervating the fingerpads of anesthetized monkeys were recorded. Each SA's
response to a step provided a spatial response profile (discharge rate as a
function of step position) that reflected the distribution of curvature
across the step shape. All the major features of the SA response could be
consistently explained as being due to the sensitivity of the SA to the
amount and rate of change in skin curvature. The response profile was
altered by changes in stroke direction, step shape, and stroke velocity.
Differences in stroke direction (back and forth) were indicated by
differences in pattern of response: a "burst-pause-burst" for strokes from
high to low, and a "pause-burst-pause" for strokes from low to high; a
greater discharge rate in response to the step for low to high strokes, and
for some SAs, the reduction or absence of basal discharge in one of the
directions. The discharge rate during the burst for either direction of
stroking was greater for steep than for gradual steps, and increased, for a
given step shape, with increases in stroke velocity. Regardless of
differences in stroke velocity, steep steps were distinguished from gradual
steps by having narrower burst widths for low-to-high strokes and narrower
pause widths for high-to-low strokes. The same stimuli were delivered to
the human fingerpad, and the capacities of humans to discriminate between
the steps were measured. It was concluded that the spatial features of SA
responses, representing the widths of regions of active and inactive SA
populations, as well as the intensive feature of discharge rate, accounted
for the gross sensory discriminations of shape.(ABSTRACT TRUNCATED AT 400
WORDS)
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