Proceedings of The Physiological Society
King's College London (2011) Proc Physiol Soc 22, PC01
Skin stretch at the human wrist reduces the accuracy of tactile distance discrimination.
F. Cody1, A. Ali1, Y. Gbejuade1, D. Lloyd2
1. Faculty of Life Sciences, University of Manchester, Manchester, United Kingdom. 2. School of Psychological Sciences, University of Manchester, Manchester, United Kingdom.
Stretching the skin around the human wrist, e.g. by altering flexion-extension angle, has been shown to impair tactile localization (Cody et al., 2010). This effect is probably due to stretch-induced enlargement of the receptive fields (RFs) of local touch units. To test whether a related tactile discrimination task is similarly affected by skin stretching, we have compared the precision of tactile distance discrimination at two wrist angles. Twenty young adults (16 female, 4 male) were recruited. A 7-point linear stimulus array (0, 2.5, 5.0, 7.5, 10.0, 12.5 and 15.0mm) was marked in ink along the longitudinal axis of the shaved dorsal surface of the wrist of the non-dominant (test) side whilst the relaxed joint was supported at a neutral angle. A single pair of stimulus points (corresponding to loci 0 and 7.5mm) was marked on the contralateral, dominant (reference) wrist. In each trial, two brief, sequential tactile stimuli were applied, respectively, to the 0 and 7.5mm loci of the reference wrist using a von Frey hair (rating 70mN). Shortly after, a pair of sequential stimuli was applied to two loci of the 7-point array on the test side. The separations of these test stimulus pairs, in the neutral posture, ranged from 0mm (both stimuli at 0mm) to 15mm (stimuli at 0 and 15mm). The subject, eyes closed, was asked to state whether the separation distance (variable) between the two stimuli on the test side was “longer” or “shorter” than the separation distance (constant 7.5mm) on the reference side. Each test separation, ordered randomly, was repeated 10 times and the probability of the response “longer” (relative to reference separation distance 7.5mm) was calculated. The interval of uncertainty (IU, a measure of discriminatory threshold for separation distance) was estimated from standard psychophysical functions (probability of judgement “longer” versus test separation distance). Discriminatory acuity for separation distance was determined in this way under (1) baseline conditions (test wrist supported at neutral angle, no applied skin stretch) and (2) skin-stretched conditions (test wrist supported at 70deg. flexion). Statistical analysis indicated that IU for discrimination of separation distance was significantly greater (less accurate) under skin-stretched than baseline (neutral) conditions (Wilcoxon 2-tailed test, p = 0.048). We interpret this finding as suggesting that tactile distance discrimination is strongly dependent upon the RF dimensions of regional touch units in an analogous manner to that previously demonstrated for localization of stimuli.
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