Linearity, Symmetry and Additivity of the Human Eye-movement Response to Maintained Unilateral and Bilateral Surface Galvanic (DC) Vestibular Stimulation

Recent studies have shown that, although responses to long-duration, constant-current surface galvanic vestibular stimulation (GVS) show substantial interindividual variability, individual subjects show a reliable, repeatable, idiosyncratic oculomotor response pattern to GVS. It follows that GVS may be a more reliable stimulus than may have been anticipated from the literature. The aim of the present study was to examine the metrics of 3D eye-movement responses to maintained (120 s), unilateral and bilateral surface GVS. Eye movements were measured using computerised video-oculography. Two experiments were conducted: Experiment 1 examined whether the normal response is linear over increasing levels of current; and Experiment 2 examined (1) whether the normal response to surface GVS is symmetrical when comparing stimulated sides, (2) whether the normal response to surface GVS is symmetrical when the polarity of the stimulating current was reversed, and (3) whether there is additivity in the normal response to combinations of unilateral/bilateral surface GVS. Five subjects participated in Experiment 1 and eight subjects participated in Experiment 2. In both experiments, the onset of stimulation produced characteristic eye-movement responses: changes in torsional position with the upper pole of both eyes rolling towards the anode and away from the cathode; together with horizontal and torsional nystagmus with slow phases towards the anode and away from the cathode; and negligible vertical nystagmus. These responses reversed direction at stimulus offset. In the fixation condition of Experiment 1, the magnitude of ocular torsional position (OTP) and torsional nystagmus responses showed a linear relationship over conditions of increasing current strength, as did OTP, torsional and horizontal nystagmus responses in darkness. The results of Experiment 2 showed that responses to unilateral stimulation are symmetrical between stimulated sides, symmetrical between stimulating polarities, and additive (with respect to responses to bilateral stimulation). The principles derived from these findings, as well as those of recent studies, provide a foundation for future work investigating eye-movement responses to surface GVS in patients with known types of vestibular dysfunction.