The effect of transcranial magnetic stimulation on movement selection. (Short Report).

Objective: To attempt to replicate previous findings that showed an influence of transcranial magnetic stimulation of the cortical motor areas on the selection of motor programmes on the contralateral side.

Methods: Healthy volunteers were asked to choose to make a right or left index finger extension movement freely after hearing the click produced by transcranial magnetic stimulation. The stimulation was applied to the motor areas (test), including the motor cortex, vertex, and prefrontal cortex, and in the air (control).

Results: There was no preference for choosing the hand cantralateral to the stimulation site, in either test or control trials.

Conclusions: Previous results could not be reproduced. Simple magnetic stimulation of the motor areas is insufficient to affect voluntary selection of movement.

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Repetitive transcranial magnetic stimulation over the motor cortex (Ml) or the supplementary motor area can disrupt the motor programme for movement sequences, (1,2) suggesting a role of Ml as well as the midline frontal region in implementation of a motor programme. Thus modulation of Ml excitability could influence movement selection. This concept is supported by previous studies showing external bias of freely chosen movement by transcranial magnetic stimulation. (3,4) With recent technical advances in this type of stimulation, however, we felt that there were some limitations in the experimental setting and the interpretation of results in the earlier studies.

Brasil-Neto et al showed that their normal subjects more often chose the hand contralateral to the site stimulated when the reaction time was less than 200 ms. (4) This hand preference was only observed when transcranial magnetic stimulation was delivered to Ml, but not to the prefrontal regions. They used a figure of eight shaped coil that was moved to different areas (Fz, sites 5 cm lateral to Fz, Ml bilaterally, and in the air) before giving stimulation. In this setting, subjects knew the stimulation site before the selection of movement and this might have biased their selection. In addition, their arbitrary classification of response time (200 ins) could have limited the …