The effects of concurrent bilateral anodal tDCS of primary motor cortex and cerebellum on corticospinal excitability: a randomized, double-blind sham-controlled study

Transcranial direct current stimulation (tDCS) applied to the primary motor cortex (M1), and cerebellum (CB) can change the level of M1 corticospinal excitability (CSE). A randomized double-blinded crossover, the sham-controlled study design was used to investigate the effects of concurrent bilateral anodal tDCS of M1 and CB (concurrent bilateral a-tDCS(M1+CB)) on the CSE. Twenty-one healthy participants were recruited in this study. Each participant received anodal-tDCS (a-tDCS) of 2 mA, 20 min in four pseudo-randomized, counterbalanced sessions, separated by at least 7 days (7.11 days +/- 0.65). These sessions were bilateral M1 stimulation (bilateral a-tDCS(M1)), bilateral cerebellar stimulation (bilateral a-tDCS(CB)), concurrent bilateral a-tDCS(M1+CB), and sham stimulation (bilateral a-tDCS(Sham)). Transcranial magnetic stimulation (TMS) was delivered over the left M1, and motor evoked potentials (MEPs) of a contralateral hand muscle were recorded before and immediately after the intervention to measure CSE changes. Short-interval intracortical inhibition (SICI), intracortical facilitation (ICF), and long interval intracortical inhibition (LICI) were assessed with paired-pulse TMS protocols. Anodal-tDCS significantly increased CSE after concurrent bilateral a-tDCS(M1+CB) and bilateral a-tDCS(CB). Interestingly, CSE was decreased after bilateral a-tDCS(M1). Respective alterations in SICI, LICI, and ICF were seen, including increased SICI and decreased ICF, which indicate the involvement of glutamatergic and GABAergic systems in these effects. These results confirm that the concurrent bilateral a-tDCS(M1+CB) have a facilitatory effect on CSE, whereas bilateral a-tDCS(M1) exert some inhibitory effects. Moreover, the effects of the 2 mA, 20 min a-tDCS on the CB were consistent with its effects on the M1.