Modulation of corticospinal excitability and inhibition of the contralateral M1 during and after ipsilateral anodal tDCS

Anodal transcranial direct current stimulation (a-tDCS) of the primary motor cortex (M1) elicits an increase in cortical excitability that may outlasts the period of stimulation. However, little is known about effects of a-tDCS on the contralateral M1 during ipsilateral stimulation. Therefore, we investigated the changes in corticospinal excitability and inhibition of the contralateral M1 during and after 20mins of a-tDCS at 2mA of the ipsilateral M1. Six participants performed real and sham a-tDCS randomised across 2 testing sessions. Single- and paired-pulsed transcranial magnetic stimulation (TMS) was used to measure changes in motor-evoked potential (MEP) amplitude from the extensor carpi radialis (ECR) ipsilateral to the site of tDCS at 130% of resting and active motor threshold (1.3RMT and 1.3AMT), cortical silent period (CSP) and short-interval cortical inhibition (SICI). AMT was measured using a slight contraction that is less than 5% of maximal voluntary contraction of the ECR during single-pulse TMS. All measures were recorded at baseline, every 5mins for 20mins during and 10mins after tDCS. The results showed a significant increase (P<0.05) in MEP amplitude relative to baseline at the 10th, 15th and 20th min during real a-tDCS and persisted 10mins after cessation only with 1.3AMT. The increase in MEP amplitude was concomitant with a significant reduction (P<0.05) in CSP. SICI was significantly reduced (P<0.05) on the 20th min and 10mins after real a-tDCS of the active task only. No significant changes in MEP amplitude, CSP and SICI were observed in the active or resting task during sham tDCS. The increase in MEP amplitude and reduction in CSP and SICI during and after ipsilateral a-tDCS is most likely to be attributed to a reduction in interhemispheric inhibition that is modulated by atDCS during the performance of an active task.