Three-dimensional eye-movement responses to surface galvanic vestibular stimulation in normal subjects and in patients: a comparison.

Vestibular stimulation produces characteristic eye movements or vestibular ocular reflexes (VORs). The spatial components of VORs have been attributed to activation of specific vestibular sensory regions:1 stimulation of a semicircular canal (SCC) mainly produces nystagmus around an axis which is roughly perpendicular to the plane of that canal, whereas stimulation of the otoliths mainly produces changes in ocular torsional position (OTP). Galvanic vestibular stimulation (GVS) of human subjects by currents of 5mA delivered through large-surface-area electrodes on the mastoids is painless and produces a range of vestibular responses,2,3 including characteristic eye movements. By recording human eye movements using 3-D video during prolonged GVS, we have observed that normal subjects show eye-movement response patterns that could be expected from stimulation of a combination of vestibular sensory regions. Reliable characteristic patterns of these eye-movement components suggest that in humans surface GVS acts on otoliths and SCCs in an idiosyncratic fashion. Similarities in the patterns of results between normal subjects provide the basis of a heuristic model that describes eye-movement responses to GVS as the weighted sum of inputs from all vestibular end-organs. In our work, the eye-movement response to GVS is modelled so that stimulation of the otoliths produces mainly OTP changes (upper poles of the eyes rotate toward the anode/away from the cathode). Stimulation of the horizontal canals mainly produces horizontal nystagmus with the slow phases directed towards the anode/away from the cathode. Stimulation of the anterior SCC produces downward eye movement, and stimulation of the posterior SCC produces upward eye movement, with a torsional velocity component. In nor-