Abstract

Background Current density is considered an important factor in determining the outcomes of tDCS, and is determined by the current intensity and electrode size. Previous studies examining the effect of these parameters on motor cortical excitability with small sample sizes reported mixed results.

Objective/hypothesis This study examined the effect of current intensity (1 mA, 2 mA) and electrode size (16 cm2, 35 cm2) on motor cortical excitability over single and repeated tDCS sessions.

Methods Data from seven studies in 89 healthy participants were pooled for analysis. Single-session data were analyzed using mixed effects models and repeated-session data were analyzed using mixed design analyses of variance. Computational modeling was used to examine the electric field generated.

Results The magnitude of increases in excitability after anodal tDCS was modest. For single-session tDCS, the 35 cm2 electrodes produced greater increases in cortical excitability compared to the 16 cm2 electrodes. There were no differences in the magnitude of cortical excitation produced by 1 mA and 2 mA tDCS. The repeated-sessions data also showed that there were greater increases in excitability with the 35 cm2 electrodes. Further, repeated sessions of tDCS with the 35 cm2 electrodes resulted in a cumulative increase in cortical excitability. Computational modeling predicted higher electric field at the motor hotspot for the 35 cm2 electrodes.