HEMISPHERIC DIFFERENCES IN MOTOR CORTEX EXCITABILITY DURING A SIMPLE INDEX FINGER ABDUCTION TASK IN MAN John G. Semmler and Michael A. Nordstrom. Department of Physiology, THE University of Adelaide, Adelaide, South Australia 5005, AUSTRALIA.
APStracts 4:332N, 1997.
ABSTRACT
Transcranial magnetic (TMS) and electrical (TES) stimulation was used to assess the contribution of the corticospinal pathway to activation of the first dorsal interosseous muscle (FDI) in each hand of 16 right-handed subjects. TMS was applied at relaxed threshold intensity while the subject performed isometric index finger abduction at seven force levels (0.5 N to 50% maximal voluntary contraction (MVC)). In a separate session, TES of equivalent intensity was applied to each hemisphere in 5 of these subjects while they performed the same force-matching protocol. In the resting state, mean threshold intensity for a muscle evoked potential (MEP) in FDI using TMS was similar for the hemispheres controlling the dominant and non-dominant hands. The size of the threshold MEPs in resting FDI following TMS and TES were also similar in each hand. With TMS, contraction-induced facilitation of the MEP in FDI was significantly larger when the non-dominant hand was used for index finger abduction. In the pooled data, the non-dominant/dominant ratio of MEP areas (normalised to the maximum M wave) ranged from 1.7 in the weakest contraction (0.5 N) to 1.1 in the strongest (50% MVC). Eight subjects had significant differences between hands in favour of the non-dominant hand, while in two subjects contraction-induced facilitation of MEPs was larger in the dominant hand. In five subjects for whom detailed motor unit data were available from a previous study, lateral differences in MEP facilitation were positively correlated with differences in FDI motor unit synchronization between hands. With TES, contraction-induced facilitation of the MEP was similar in each hand, suggesting that spinal excitability was equivalent on both sides. For the group of five subjects tested with both stimulation techniques, contraction-induced facilitation of the MEP was significantly larger following TMS than that obtained with TES when the contraction was performed with the non-dominant hand, but not when the dominant hand was used to perform the task. We conclude that the extent of corticospinal neuron involvement in the command for simple index finger abduction in right-handed subjects is generally greater when the non-dominant hand is used, compared with the same task performed with the dominant hand. 

Received 25 March 1997; accepted in final form 18 November 1997.
APS Manuscript Number J.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 12 December 1997