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J Physiol Volume 577, Number 3, 795-803, December 15, 2006 DOI: 10.1113/jphysiol.2006.116939
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Neuroscience

Transcranial direct current stimulation of the primary motor cortex affects cortical drive to human musculature as assessed by intermuscular coherence

Hollie A. Power1, Jonathan A. Norton1,2, Cheryl L. Porter1, Zoe Doyle1, Isaiah Hui1 and K. Ming Chan1,3

1 Centre for Neuroscience
2 Department of Biomedical Engineering
3 Division of Physical Medicine and Rehabilitation, University of Alberta, Edmonton, Alberta, Canada

Intermuscular coherence analysis can be used to assess the common drive to muscles. Coherence in the ß-frequency band (15–35 Hz) is thought to arise from common cortical sources. Intermuscular coherence analysis is a potentially attractive tool for the investigation of motor cortical excitability changes because it is non-invasive and can be done relatively quickly. We carried out this study to test the hypothesis that intermuscular coherence analysis was able to detect cortical excitability changes in healthy subjects following transcranial direct current stimulation (tDCS). tDCS has been shown to increase (anodal stimulation) or decrease (cathodal stimulation) the size of the muscle potential evoked by TMS. We found that anodal tDCS caused an increase in motor evoked potential (MEP) size that was paralleled by an increase in ß-band intermuscular coherence. Similarly, the reduction in MEP size produced by cathodal tDCS was paralleled by a reduction in ß-band intermuscular coherence, while sham stimulation did not result in any change in either MEP amplitude or ß-band intermuscular coherence. The similar pattern of change observed for MEP and intermuscular coherence may indicate similar mechanisms of action, although this cannot be assumed without further investigation. These changes do suggest that at least some of the action of tDCS is on cortical networks, and that combined tDCS and intermuscular coherence analysis may be useful in the diagnosis of pathologies affecting motor cortical excitability.

(Received 11 July 2006; accepted after revision 3 October 2006; first published online 5 October 2006)
Corresponding author K. M. Chan: 513 Heritage Medical Research Centre, University of Alberta, Edmonton, AB, Canada T6G 2S2. Email: ming.chan{at}ualberta.ca






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