'I-wave' Recruitment Determines Response to tDCS in the Upper Limb, but Only So Far.

Brain Stimul

Movement Neuroscience Laboratory, Department of Sport and Exercise Science, and Centre for Brain Research, The University of Auckland, Auckland, New Zealand. Electronic address:

Published: July 2016

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Background: Anodal transcranial direct current stimulation (a-tDCS) can facilitate primary motor cortex (M1), but the modulation of motor evoked potentials (MEPs) by a-tDCS varies between participants, and may depend on the balance between early versus late I-wave recruitment, as assessed by the difference in MEP latency between latero-medial and anterior-posterior cortical currents induced by transcranial magnetic stimulation (TMS).

Objective: To date, the dependence of tDCS after-effects on I-wave recruitment has only been investigated in intrinsic hand muscles. In order to better understand the effects of tDCS across the upper limb, the present study examined I-wave recruitment and MEP modulation by a-tDCS or dual-hemisphere tDCS in muscles of the forearm (Extensor Carpi Radialis; ECR) and proximal upper limb (Biceps Brachii; BB).

Methods: We conducted a randomized double-blind study with 18 healthy adults. Each received anodal, dual-hemisphere, or sham tDCS over M1 in separate sessions (tDCS, 1 mA for 15 min).

Results: Linear regression analyzes showed a-tDCS modulated MEP size dependent on the latency difference in the ECR (P = 0.01) but not BB (P = 0.28). Individuals with small MEP latency differences showed the expected facilitation of ECR MEPs after a-tDCS, whereas those with large MEP latency differences had suppressed ECR MEPs after a-tDCS. This relationship was not present after dual-hemisphere or sham tDCS in either muscle (all P > 0.32).

Conclusions: I-wave recruitment can predict the after-effects of a-tDCS in the distal but not proximal upper limb. These findings provide further insight into the variability of tDCS after-effects, and the relationship between I-wave recruitment and putative mechanisms of tDCS.

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http://dx.doi.org/10.1016/j.brs.2015.07.027DOI Listing

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