Vertical and horizontal eye movement responses to unilateral and bilateral bone conducted vibration to the mastoid

Bone-conducted vibration (BCV) delivered to the skull causes linear acceleration of the mastoids and results in small potentials (oVEMPs) from the extraocular muscles in humans. Very small eye movements to both air- and bone-conducted sound with latencies of around 9 ms and amplitudes up to 16.5 mdeg have also been reported (where mdeg = millidegrees =0.001 degree). To clarify the otolith-ocular responses we investigated the effect of unilateral and bilateral BCV (500 Hz of the mastoid) on eye movements in human subjects, and the effect of gaze position on these movements using a video-based eye-movement recording system. We found very small, short-latency eye-movement responses similar to those previously reported, but these were followed by larger (up to 0.57 degrees) prolonged responses peaking at around 120 ms post-stimulus. For unilateral stimulation the eyes moved downwards when directed away from the stimulated mastoid and upwards when directed towards stimulation. Horizontal movements were towards the stimulated mastoid with little effect of gaze position. Bilateral stimulation enhanced the downward responses, suggesting that the depressor muscles are preferentially activated by BCV. Bilateral stimulation either eliminated the horizontal response or produced convergence of the eyes: the convergence suggests that the eye-movement response is probably primarily driven by the contralateral mastoid.