Eye Movement Responses to Unilateral and Bilateral Otolithic Stimulation From Bone Conducted Sound

Abstract

Purpose: : Air conducted sound (ACS), delivered through headphones, and bone conducted vibration (BCV) of the head stimulate otolithic receptors in the inner ear and can evoke short latency vestibular induced extraocular muscle potentials (oVEMPs) recorded by electrodes beneath the eyes, that peak at approximately 10 ms. Studies using scleral search coils have confirmed, in a few subjects, the presence of small predictable eye movements following very short duration ACS and BCV. We sought to test whether longer duration BCV stimuli delivered to the mastoids unilaterally and bilaterally induced consistent eye movements and the effect of eye position on the movement.

Methods: : Bone conducted tone bursts at 500Hz with a duration of 150 ms and an inter-stimulus interval of 900ms were delivered at the mastoid unilaterally and bilaterally to healthy subjects using B71 bone conductors that were held in place by a latex cap while the subject’s eyes were directed to various positions. The resulting eye movements were recorded using 250Hz high resolution fire wire cameras.

Results: : A very small response at 10ms post stimulus could be detected in some subjects that was consistent with previous oVEMP responses. However this was followed by a much larger and more sustained response. Eye movements following unilateral stimulation were up to 0.35^o (vertical) and 0.26^o (horizontal), with a latency of around 25ms and with peak amplitudes at approximately 125ms post stimulus. The stimulus caused eye elevation for fixation positions towards the stimulated mastoid and depression for fixation positions opposite the stimulated mastoid. Horizontal responses were towards the stimulated mastoid. With bilateral stimulation the response was effectively a summation of unilateral movements: the vertical component increased (up to 0.57^o) and the horizontal component was reduced or eliminated.

Conclusions: : Bone conducted vibration to the otoliths produces predictable horizontal and vertical conjugate eye movements that are gaze position dependent.

Keywords: eye movements • vestibulo-ocular reflex • neuro-ophthalmology: diagnosis