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Journal of Neuroscience, Vol 15, 5582-5595, Copyright © 1995 by Society for Neuroscience
Tactile resolution: peripheral neural mechanisms underlying the human capacity to determine positions of objects contacting the fingerpad
HE Wheat, AW Goodwin and AS Browning
Department of Anatomy and Cell Biology, University of Melbourne, Parkville Victoria, Australia.
We measured the ability of humans to discriminate the positions of
spherical objects passively contacting the fingerpad. The discrimination
threshold averaged 0.55 mm for a moderately curved sphere (radius 5.80 mm)
and decreased to 0.38 mm for a more curved sphere (radius 1.92 mm); since
the receptor density is about 1 per mm2, these values are substantially
smaller than those predicted by the sampling theorem (referred to as
hyperacuity). To elucidate the underlying neural mechanisms, responses to
the same spheres and random sequences of stimuli were recorded from single
Merkel afferents (SAIs) and Meissner afferents (RAs) in anesthetized
monkeys. For multiple applications of identical stimuli, coefficients of
variation of responses were around 3%. Profiles of responses across the SAI
population were "hill-shaped." A change in position of the stimulus on the
skin resulted in a matching shift of the profile, evident over the whole
profile for the more curved sphere but ony at the skirts for the less
curved sphere. The shift in response profiles, relative to the standard
deviations, increased as the change in position increased, and was more
reliable for the more curved sphere. Responses were measured over four time
frames: 0.2, 0.3, 0.5, and 1.0 sec. Although responses increased with an
increase in integration time, so, too, did their standard deviations, so
that signal-to-noise ratios or the resolution in the SAI population was
bout the same at 0.2 sec as at 1.0 sec. Only half the RAs responded;
responses were small, but signalled reliable information about the position
of the stimulus.
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