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.