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Description
Electrical vestibular stimulation (EVS) influences balance by applying weak electrical currents to the scalp. While EVS is conventionally delivered via electrodes placed over the mastoid processes, recent findings show that stimulation using various other electrode montages also induces postural sway. This study aimed to explore which electric field component contributes to the response and compare the vestibular electric field values to human exposure limits established in international guidelines and standards, which currently do not consider vestibular effects.
Counterbalanced sham-controlled double-blind experiments were performed in eight participants standing on a force plate. Alternating currents at 4.6 Hz or 4.8 Hz were applied to four electrode montages, featuring electrodes over the forehead, motor cortex, and cerebellum. All four montages produced a significantly increased body sway compared to sham at the stimulation frequency, with the electric field in the vestibular system correlating with the sway magnitude. Further modelling identified the lateral electric field component as the best predictor of lateral oscillating postural sway.
The in situ electric field magnitudes were at most 100-220 mV/m, depending on the electrode montage, while the lateral component was much weaker, ranging from 40-80 mV/m. These field strengths were well below the occupational exposure limits set by international bodies. The EVS-induced postural sway is subtle and not easily perceived, and it is unclear whether it should be treated as adverse or as an effect that should be avoided. Therefore, the implications of vestibular effects on the human exposure guidelines and standards remain uncertain.
