Effect of a tDCS electrode montage on implicit motor sequence learning in healthy subjects

BACKGROUND: This study was undertaken to test the hypothesis that a combination of excitatory anodal transcranial direct current stimulation (tDCS) to the contralateral motor cortex and inhibitory cathodal tDCS to the ipsilateral motor cortex of the motor performing hand (Bi-tDCS) would elicit more implicit motor sequence learning than anodal tDCS applied to the contralateral motor cortex alone (Uni-tDCS). METHODS: Eleven healthy right-handed adults underwent a randomized crossover experiment of Uni-tDCS, Bi-tDCS, or sham stimulation. Subjects performed a 12-digit finger sequence serial reaction time task with the right hand at baseline (Pre), at immediately (Post 1), and 24 hours after stimulation (Post 2). The ratios of reaction times of predetermined repeating sequence versus random sequence were subjected to statistical analysis. RESULTS: The paired t test showed that reaction time ratios were significant decreased by all stimulation types at Post 1 versus Pre (P < 0.01). However, mean reaction time ratios showed a significant decrease after Uni-tDCS (P < 0.01) and Bi-tDCS (P < 0.01), but only a marginal decreased after Sham (P = 0.05) at Post 2, which suggests that motor sequence learning is consolidated by Uni-tDCS and Bi-tDCS, but only partially consolidated by sham stimulation. No significant differences were observed between Uni-tDCS and Bi-tDCS in terms of in reaction time ratios at Post 1 or 2. CONCLUSIONS: No significant difference was found between Uni-tDCS and Bi-tDCS in terms of induced implicit motor sequence learning, but tDCS led to greater consolidation of the learned motor sequence than sham stimulation. These findings need to be tested in the context of stroke hand motor rehabilitation.