Response variability following transcranial direct current stimulation (tDCS) highlights need for exploring different tDCS electrode montages. Corticospinal excitability (CSE), cortico-cortical excitability and intra-individual variability was compared following conventional and high-definition (HD) anodal (a-tDCS) and cathodal (c-tDCS) tDCS. Fifteen healthy males attended four sessions at-least one-week apart: conventional a-tDCS, conventional c-tDCS, HD-a-tDCS, HD-c-tDCS. TDCS was administered (1 mA, 10-minutes) over primary motor cortex (M1), via 6 × 4 cm active and 7 × 5 cm return electrodes (conventional tDCS) and 4 × 1 ring-electrodes 3.5 cm apart over M1 (HD-tDCS). For CSE, twenty-five single-pulse transcranial magnetic stimulation (TMS) peak-to-peak motor evoked potentials (MEP) were recorded at baseline, 0-minutes and 30-minutes post-tDCS. Twenty-five paired-pulse MEPs with 3-millisecond (ms) inter-pulse interval (IPI) and twenty-five at 10 ms assessed short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF). MEP standardised z-values standard deviations represented intra-individual variability. No significant changes in CSE from baseline were reported for all four interventions. No significant differences were reported in CSE between conventional and HD a-tDCS, but significant differences between conventional and HD c-tDCS 0-minutes post-tDCS. Conventional tDCS significantly reduced intra-individual variability compared to HD-tDCS for a-tDCS (0-minutes) and c-tDCS (30-minutes). No changes were reported for SICI/ICF. These novel findings of increased intra-individual variability following HD-tDCS, at the current stimulus parameters, highlight need for further nuanced research and refinement to optimise the HD-tDCS dosage-response relationship.
