Electrode-distance dependent after-effects of transcranial direct and random noise stimulation with extracephalic reference electrodes

Objective: To evaluate the importance of the distance between stimulation electrodes, in various montages, on the ability to induce sustained cortical excitability changes using transcranial direct and random noise stimulation. Methods: Twelve healthy subjects participated in four different experimental conditions. The stimulation electrode was always placed over the primary motor cortex; the reference electrode was placed at the contralateral orbit or at the ipsilateral/contralateral arm. MEPs were recorded in order to measure changes in cortical excitability over time. Results: The distance between the two electrodes correlates negatively with the duration and magnitude of induced after-effects. Conclusions: In particular when using extracephalic reference electrodes with transcranial electric stimulation techniques, the stimulation intensity has to be adapted to account for interelectrode distance. Significance: Electrode distance plays a critical role in the induction for stimulation after effects in tDCS and tRNS studies, and must be taken into account in future studies and also when making comparisons with the published literature. (C) 2010 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.