cqvip:Objective: To investigate the origin, whether ocular or extraocular, of the short latency frontal potential (N15) reported by Todd et al. (2003) following vestibular stimulation. Methods: Fourteen subjects with low VEMP thresholds (VT) and 9 patients with vestibular or ocular disorders were stimulated at the mastoid with bone-conducted tone bursts (500 Hz, 8 ms) above vestibular threshold, using a B71 bone vibrator. Surface potentials were recorded from Fpz and around the eyes and referred to linked earlobes. Results: The N15 was present at Fpz,but was largest around the eyes (mean amplitude 2.6 μV, peak latency 13.4 ms, with stimulation at +18 dB above threshold) and was generally in phase above and below the eyes. The response was vestibular-dependent and modulated by alteration of gaze direction. The potentials were delayed in a patient with Miller Fisher syndrome and were larger in patients with superior canal dehiscence than in controls. Conclusions: We report a new vestibular-evoked extraocular potential. Its properties are not consistent with an eye movement. It is likely to be produced, mainly or exclusively, by synchronous activity in extraocular muscles (i.e. a myogenic potential). Significance: Vestibularevoked extraocular potentials extend the range of vestibular pathways that can be assessed electrophysiologically, and may be a useful additional test of vestibular function.
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