Journal of Neuroscience, Vol 9, 1280-1293, Copyright © 1989 by Society for Neuroscience
Spatial and temporal factors determining afferent fiber responses to a grating moving sinusoidally over the monkey's fingerpad
AW Goodwin, KT John, K Sathian and I Darian-Smith
Department of Anatomy, University of Melbourne, Parkville, Victoria, Australia.
Gratings of alternating grooves and ridges were moved sinusoidally across
the fingerpads of anesthetized monkeys, while responses were recorded from
individual slowly adapting afferents (SAs), rapidly adapting afferents
(RAs), and Pacinian afferents (PCs) in the median nerve. The stimulus
comprised 2 spatial variables, namely, groove width (G) and ridge width
(W), and 2 temporal variables, namely, the peak speed of movement (S) and
the peak temporal frequency (F) at which successive spatial cycles of the
grating pass over a point in the receptive field. The responses of all 3
fiber types were determined by only 1 spatial variable, G, and only 1
temporal variable, F. Changes in W or S affected responses only if there
was a concomitant change in either G or F. Responses were phase-locked to
the occurrence of successive spatial cycles of the grating, and we have
used the number of impulses elicited by a single spatial cycle as the
fundamental measure of response. An equation of the form I = cGaexp(-b
square root of F) describes the responses of all 3 fiber types. For SAs,
the effect of groove width was greater (a = 2.64) than for RAs and PCs (a =
0.924 and 1.05, respectively). The reduction in response with frequency was
most marked for SAs (b = 0.262), and greater for PCs (b = 0.167) than for
RAs (b = 0.130). From the equation, the instantaneous response during the
entire sinusoidal cycle was reconstructed as well as a second measure, the
mean cyclic response. These 2 measures behaved differently with changes in
the stimulus parameters. The temporal properties of the fibers, as revealed
by gratings, may appear to be in conflict with those established by
vibratory threshold studies; in fact, they are compatible with
suprathreshold responses to vibrating probes.
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