Introduction: Non-invasive transcranial direct current stimulation (tDCS) can induce polarity-specific changes in cortical excitability. Compared to unilateral anodal tDCS over the non-dominant motor cortex (M1), dual-hemispheric tDCS to the M1 may further accelerate motor reaction of the target hand and alter excitability of the corticospinal tract (CST). However, the effects and individual variability of bi-hemispheric tDCS to the M1 during motor training remain largely unclear in healthy and stroke subjects.
Purpose: We assessed the hypothesis that bi-hemispheric tDCS during single-session motor training alter inter-hemispheric inhibition, CST excitability and bilateral cortical oscillations.
Methods: We enrolled first-time, unilateral ischemic stroke patients between two and four weeks after stroke and matched healthy controls. They were subjected to two 20 min-sessions of dual-hemispheric tDCS (anode over non-dominant or ipsilesional M1, cathode over dominant or contralesional M1; 2mA for 20 mins) and sham tDCS (2mA for 2 mins) in a randomized crossover design during repetitive extension of the non-dominant or paretic extensor carpi radialis muscle. We compared the post-stimulation changes of motor evoked potentials (MEPs), ipsilateral silent period (iSP), short interval intracoritcal inhibition (SICI), as well as resting and unilateral finger lifting-related cortical oscillations by magnetoencephalography (MEG).
Results: Compared to the sham tDCS, the dual-hemispheric tDCS significantly increased MEP amplitudes and reduced SICI at the anodal-stimulated M1, as well as decreased inter-hemispheric inhibition from the cathodal-stimulated M1 with shortened iSP for about 30 mins in healthy controls (n=8). In contrast, reduced MEP amplitudes were observed at the cathodal-stimulated M1. The tDCS effects on cortical oscillations and in stroke patients are currently under investigation.
Conclusions: Task-concurrent dual tDCS may enhance activity-dependent motor plasticity in subacute stroke.