Selective effects of head posture on ocular vestibular-evoked myogenic potential (oVEMP) by bone-conducted vibration.

OBJECTIVE: By altering head postures from sitting, supine to head hanging, this study investigated the effects of gravitational force on ocular vestibular-evoked myogenic potential (oVEMP) via either air-conducted sound (ACS) or bone-conducted vibration (BCV) stimuli. METHODS: Twenty healthy volunteers underwent the oVEMP test via ACS or BCV stimuli with the sitting, supine, and head hanging positions on the same day in a randomized order. RESULTS: All subjects had clear BCV oVEMPs in the three head postures. No significant differences existed in terms of mean nI and pI latencies, the nI-pI interval, and asymmetry ratio regardless of various positions. However, the mean nI-pI amplitude with the head hanging position (15.9 ± 6.4 µV) was significantly larger than that with the sitting position (13.8 ± 6.0 µV), but not significantly larger than that with the supine position (14.7 ± 6.1 µV). Nevertheless, such a difference in reflex amplitude does not exist in oVEMPs elicited by ACS stimuli. With the sitting position, mean linear acceleration at the mastoids in response to BCV stimuli was -0.06 ± 0.02, 0.20 ± 0.04 and -0.04 ± 0.02 g along the x-, y-, and z-axis, respectively, which did not differ significantly from those with the head hanging position. CONCLUSION: By altering head postures from sitting to head hanging, gravitational force can exert a selective effect on the reflex amplitude of oVEMPs elicited by BCV stimuli, but not by ACS stimuli. SIGNIFICANCE: Compared to ACS mode, BCV mode can provoke higher response rate, generate earlier and larger waveforms, and be influenced by both dynamic shearing force and static gravitational force to enlarge the reflex amplitude of oVEMPs.