Proceedings of The Physiological Society
Cardiff University (2009) Proc Physiol Soc 17, PC34
The accuracy of tactile localization is reduced by skin stretch at the human wrist
F. Cody1, R. Idrees1, D. Spilioti1, E. Poliakoff2
1. Faculty of Life Sciences, University of Manchester, Manchester, United Kingdom. 2. School of Psychological Sciences, University of Manchester, Manchester, United Kingdom.
The precision of tactile point localization (locognosia, defined by Hamburger, 1980) is greatest for body regions (e.g. hand) whose touch units have small receptive fields (RFs). However, the skin is an elastic organ that is continuously distorted as our limbs move, with fluctuations in RF dimensions. To date, possible associated locognosic changes have not been reported. Therefore, we have tested the hypothesis that tactile localization accuracy is reduced when the skin is distended, with concurrent expansion of RFs. Twenty-five (22 female, 3 male, aged 18-21 years) healthy subjects participated. A 7-point linear stimulus array (5mm point separation) was drawn on the shaved skin of dorsal surface of the non-dominant wrist, along its longitudinal axis. In each trial, a brief tactile stimulus was first applied, with a von Frey hair (rating 150mN) to the central locus (reference) followed by an identical stimulus (test) to one of the 7 test loci. The subject stated the direction (“more distal” / “more proximal”) of the test relative to the reference stimulus. Each test locus received 10 stimuli and the probability of a “more distal” directional response was calculated. The interval of uncertainty (IU, a measure of locognosic discriminatory threshold) was estimated from standard psychophysical functions (probability of directional judgement versus stimulus locus). Spatial locognosic acuity was quantified under baseline conditions (wrist at 0deg) and during application of background skin stretch by (1) controlled flexion (Wrist-Bend, small (35deg) and large (70 deg) amplitude) of the relaxed joint and (2) matched direct pulling on the skin (Skin-Pull, small and large amplitude) using wires attached to adhesive pads. Statistical analysis of IU using a 2 (amplitude of skin stretch, small, large) x 2 (mode of skin stretch, Wrist-Bend, Skin-Pull) repeated-measures ANOVA showed a significant main effect of stretch amplitude (F(1,24) = 11.123, p = .003) but no main effect of stretch mode (F(1,24) = 2.306, p = .142). Paired t-tests indicated that IU was significantly greater (less accurate) than baseline for the two large amplitude stretch conditions (Wrist-Bend t(24) = 2.445, p = .022; Skin-Pull t(24) = 2.120, p = .045) but did not differ significantly (p > .25) from baseline for either of the small stretch conditions. We interpret our observations as (1) supporting the long-held assumption that tactile localization depends primarily upon the RF dimensions of regional touch units and (2) suggesting that tonic activation of non-cutaneous proprioceptors during joint positioning exerts rather little modulatory effect.
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