Motion sensations, postural sway, and side effects for copolar galvanic vestibular stimulation

Galvanic vestibular stimulation (GVS), traditionally applied with oppositely polarized electrodes on the two mastoids (behind the ears), produces self-tilt/rotation perceptions and postural reflexes primarily about the head-centered roll axis (i.e., out the nose). However, there are reports of non-vestibular side effects, such as skin tingling at the electrodes, metallic taste, and visual flashes, which have not been systematically characterized. Previous work suggests pitch sensations may be produced by introducing a distal electrode(s) and co-polarizing the mastoid electrodes, but copolar montages have not been compared. In this series of experiments investigating GVS for creating pitch sensations, we first quantified the relative prevalence, strength, and directionality of sensations induced by 2 previously used copolar electrode montages, along with the typical binaural bipolar montage. Subjects were blinded to montage in all studies. Across current amplitude and waveform (DC and various sine waves), we found both copolar montages produce noticeable motion sensations in roughly half of participants, predominantly in the anterior-posterior (pitch) direction and corresponding postural sway, but substantial side effects, particularly for skin tingling. To compare four copolar montages, we performed force-choice experiments, with pairings of different GVS montage and stimuli. Copolar montages all produced similar effects, including anterior-posterior sway, but the distal electrode on the Forehead tended to induce greater motion sensations, and the Shoulder had less side effects, for equal current applied at the mastoids. In sum, this work systematically characterizes motion sensations, postural sway responses, and the side effects associated with various copolar montages and stimulus parameters.