Journal of Neuroscience, Vol 13, 2332-2341, Copyright © 1993 by Society for Neuroscience
Texture perception and afferent coding distorted by cooling the human ulnar nerve
JR Phillips and PB Matthews
University Laboratory of Physiology, Oxford, United Kingdom.
The roughness of standardized surfaces (embossed dot arrays or gratings)
was compared by scanning them with the little finger of either hand while
the ulnar nerve was cooled unilaterally at the elbow; both hands remained
warm. Across-hand comparison of roughness showed that a given surface felt
smoother on the cooled side. When the surfaces were adjusted to feel
equally rough, that on the cooled side would normally have felt appreciably
rougher. The effect of the nerve cooling on axonal conduction was monitored
during the psychophysical experiments by stimulating the ulnar nerve above
the cooled region and recording the EMG of an ulnar-innervated hand muscle.
During cooling, large myelinated axons remained unblocked, but prolongation
of their absolute refractory period to 5-10 msec left them unable to
transmit trains of impulses at high frequencies (Wedensky inhibition). By
varying the length of nerve cooled and the cooling temperature, it was
shown that the perceptual effects were not due to an increase in the normal
temporal dispersion of impulses transmitted by different-sized afferents.
The effect of increasing the absolute refractory period on the signals from
the various types of cutaneous afferent was modeled mathematically, using
earlier human single-fiber responses to dot arrays. It is concluded that
the reduction of perceived roughness with nerve cooling is due to Wedensky
inhibition, and that the percept of roughness is related to the local
contrast in the afferent spatiotemporal image.