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This study was designed to measure ocular movements evoked by galvanic (DC) stimulation using computerised video-oculography. Long duration (>30 s) galvanic vestibular stimulation at currents of up to 5 mA through large-area surface electrodes over the mastoid processes causes maintained changes in the ocular torsional position of both eyes in healthy human subjects. With the subject seated and the head held firmly, torsion was measured by a computer-based image-processing system (VTM). Torsion was recorded in darkness, with or without a single fixation point. With bilateral stimulation, the upper poles of both eyes always torted away from the side of cathode placement and toward the anode. For unilateral stimulation, torsion was directed away from the cathode or toward the anode. The magnitude of ocular torsion was dependent on current strength: with bilateral stimulation the peak torsion was on average 2.88 degrees for 5-mA current intensity compared with 1.58 degrees for 3 mA. A smaller amplitude of torsion was obtained for unilateral stimulation. The average peak torsion was the same for both eyes for all forms of stimulation. Our findings indicate that low-intensity galvanic stimulation evokes ocular torsion in normal subjects, an effect which is consistent with an action on otolith afferents.

Responses of single neurons in the vestibular nerve to high-intensity clicks were studied by extracellular recording in anaesthetised guinea pigs. One hundred and two neurons in the posterior division of the superior branch or in the inferior branch of the vestibular nerve were activated at short latency by intense clicks. The latency of activation was short (median 0.9 ms) and the threshold was high: the click intensity for evoking the response of these cells was around 60 dB above the auditory brainstem response threshold. Animals were tilted and rotated to identify physiologically the sensory region of the labyrinth from which the activated neurons originated. Seventeen neurons responded to static tilt as well as clicks. These results show that vestibular receptors, probably the otoliths, respond to clicks at intensities corresponding to those used in a new clinical test of the vestibulo-collic pathway.