Proceedings of The Physiological Society

University College London December 2005 (2006) Proc Physiol Soc 1, PC19

Poster Communications

Locognostic acuity differs between the vertical and horizontal axes of dermatomes of the human abdomen

Cody, Frederick William; Chadwick, Rebecca Denise; Dickenson, Sarah Elizabeth;

1. Faculty of Life Sciences, University of Manchester, Manchester, United Kingdom.

The precision of tactile point localization (locognosia) has been reported to vary between the transverse and longitudinal axes of the arm (Hamburger, 1980). We have investigated whether comparable directional differences in locognostic acuity are present in the abdomen, whilst additionally attempting to replicate Hamburgers findings for the upper limb. Twenty-nine (13 female, 16 male, aged 18-26 years) healthy, subjects participated. A 7 x 7 cross of stimulus points (13 points, 5 mm separation), was drawn on the shaved skin of (a) the dorsal surface of the non-dominant forearm and (b) the anterior aspect of the ipsilateral abdomen at the level of the T10 dermatome. The axes of the cross were orientated (a) transversely and longitudinally for the arm and (b) vertically and horizontally for the abdomen. In each trial, a brief tactile stimulus was first applied, with a von Frey hair (rating 150 mN), to the central locus of the cross (reference), followed, after 1s, by a stimulus to one of the 13 test loci. The test region was obscured from the subjects view. The subject was required to state the direction (e.g. 'more distal' for the longitudinal axis of the arm) of the test locus relative to the reference locus, using a 2-alternative, forced-choice procedure. The four region-axis combinations were tested separately in each subject. Each test locus received 10 stimuli (total 70 stimuli per axis). For each subject, at each test locus, the probability of a specified directional response was calculated. The interval of uncertainty (IU, a measure of locognostic discriminatory threshold) was estimated from standard psychophysical functions (probability of specified directional judgement versus stimulus locus). Statistical analysis (2-way, repeated measures ANOVA) of IU values indicated a significant main effect of axis (P < 0.001) whereas there was no significant (P = 0.128) effect of body region. Paired t tests, with Bonferroni correction, indicated that IU values (a) for the upper arm were smaller (i.e. locognosia more accurate; P < 0.001) in the transverse than longitudinal axis, confirming Hamburger's (1980) findings, and (b) for the T10 dermatome were smaller (P < 0.001) in the vertical than horizontal axis. Thus, differential, directional locognostic acuity is a feature of abdominal dermatomes, as well as the upper arm. We believe that this characteristic probably results from the receptive fields of local sensory neurons being oval-shaped and similarly aligned, which favours better tactile localization in the direction of their narrower dimension.

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