Ipsilateral M1 transcranial direct current stimulation increases excitability of the contralateral M1 during an active motor task: Implications for stroke rehabilitation

IntroductionAnodal transcranial direct current stimulation (a-tDCS) of the primary motor cortex (M1) elicits an increase in cortical excitability that outlasts the period of stimulation. However, little is known about effects of a-tDCS on the contralateral M1 during and after ipsilateral M1 stimulation. Therefore, we investigated the changes in corticospinal excitability and inhibition of the left M1 during and after 20min of a-tDCS to the right M1.Material and methodsEight healthy participants received real (2mA) and SHAM a-tDCS to the right M1 randomized across 2 testing sessions. Single- and paired-pulse transcranial magnetic stimulation (TMS) was applied to the left M1 to measure changes motor-evoked potential (MEP) amplitude from the right extensor carpi radialis (ECR) at 130% of resting and active motor threshold, cortical silent period (CSP) and short-interval cortical inhibition (SICI). Active motor threshold was measured during a wrist extension contraction that was less than 5% of maximal electromyographic activation of the ECR. TMS measurements were recorded at baseline, every 5min for 20min during and 10min after a-tDCS.ResultsThe results showed a significant (P<0.05) increase in left M1 MEP amplitude and reduction in CSP duration during (10 and 15min) and after (immediately and 10min post) a-tDCS to the right M1, only during the active motor task. A significant reduction (P