Between-subject variability and within-subject reliability of the human eye-movement response to bilateral galvanic (DC) vestibular stimulation.

Recent studies have shown that responses to surface galvanic vestibular stimulation (GVS) show substantial interindividual variation. Between-subject variability may be due to individual differences between subjects, or to the poor reliability of the test, or to differences in test details, or to host factors. The aim of the present study was to compare variability between and within subjects in binocular 3-D eye-movement responses to long-duration, maintained, large-amplitude, bilateral, bipolar, surface GVS. Subjects were seated and restrained, and in one condition fixated a small, centrally located visual target; in the other condition, testing was carried out in complete darkness. Surface GVS of 5 mA, with a rectangular waveform was delivered bilaterally for 5 min while eye movements were measured using computerised video-oculography (VTM). In the first experiment, ten subjects participated in both conditions in one session, and in the second experiment, two subjects participated in both conditions for a total of five repeated sessions. The stimulation was well tolerated by all subjects and produced a change in torsional position with the upper pole of both eyes rolling towards the anode and away from the cathode in all subjects in both conditions. Although little vertical nystagmus was evident in either condition, most subjects showed relatively strong horizontal nystagmus (slow phases towards the anode) in darkness. This study confirms previous observations that the torsional response to GVS is highly variable between subjects, whilst also showing for the first time that eye-movement responses to GVS show good within-subject repeatability. This study also demonstrates considerable between-subject variability in the relative ratios of response components (torsional and horizontal nystagmus, torsional position), whereas the relatively small within-subject variability can be characterised more by changes in the overall amplitude of the eye-movement response. Subjects show idiosyncratic oculomotor response patterns to GVS, varying slightly in absolute magnitude between sessions. Thus, GVS may be a more reliable stimulus than may have been anticipated from the literature.