High-definition transcranial direct-current stimulation of the right M1 further facilitates left M1 excitability during crossed facilitation

The crossed-facilitation (CF) effect refers to when motor-evoked potentials (MEPs) evoked in the relaxed muscles of one arm are facilitated by contraction of the opposite arm. The aim of this study was to determine if high-definition transcranial direct current stimulation (HD-tDCS) applied to the right primary motor cortex (M1) controlling the left contracting arm (50% maximum voluntary isometric contraction-MVIC) would further facilitate CF towards the relaxed right arm. Seventeen healthy right-handed subjects participated in an anodal and cathodal or sham HD-tDCS session of the right M1 (2 mA for 20 min) separated by at least 48 h. Single-pulse transcranial magnetic stimulation (TMS) was used to elicit MEPs and cortical silent periods (CSPs) from the left M1 at baseline, 10 mins into, and after right M1 HD-tDCS. At baseline, compared to resting, CF (i.e., right arm resting, left arm 50% MVIC) increased left M1 MEP amplitudes (+97%) and decreased CSPs (-11%). The main novel finding was that right M1 HD-...