Vestibular involvement in transcranial electrical stimulation: body sway as a marker of unintended stimulation

AbstractBackgroundTranscranial electrical stimulation (tES) techniques are widely used to modulate brain excitability, though their mechanisms remain unclear.ObjectiveWe aimed to demonstrate that an alternating current applied to several commonly used tES electrode montages can influence the vestibular system.MethodsBody-sway responses during alternating current stimulation at 4.6 or 4.8 Hz were measured in eight participants standing on a force platform. Sham and five active conditions, including common motor cortical tES electrode montages, were applied in a double-blind experiment.ResultsAll active conditions increased lateral body sway compared with sham at the stimulus frequency, with very large to huge effect sizes. The sway magnitude was proportional to the montage-specific electric field within the vestibular system, obtained from a computational model. The frequency and phase of the body sway were locked to the stimulus frequency and phase. An additional experiment in three participants showed that the sway response was the largest in the lower legs, similarly for both tES with a motor cortical electrode montage and an electrical vestibular stimulation montage. Together, the results suggest a direct effect of tES on the vestibular system.ConclusionUnwanted vestibular effects may interfere with the interpretation of tES studies across a wide range of typical tES frequencies and current intensities.HighlightsAlternating current stimulation produces a lateral body sway at the stimulation frequency.Larger lateral electric field in the vestibular system produces a larger sway.Several common tES montages can directly influence the vestibular system.Unwanted vestibular effects may complicate the interpretation of tES studies.