Adaptation to steady-state electrical stimulation of the vestibular system in humans.

OBJECTIVES: Efforts are being made toward the development of a vestibular implant. If such a device is to mimic the physiology of the vestibular system, it must first be capable of restoring a baseline or "rest" activity in the vestibular pathways and then modulating it according to the direction and velocity of head movements. The aim of this study was to assess whether a human subject could adapt to continuous electrical stimulation of the vestibular system, and whether it was possible to elicit artificial smooth oscillatory eye movements via modulation of the stimulation.
METHODS: One bilaterally deaf patient with bilateral vestibular loss received a custom-modified Med-E1 cochlear implant in which one electrode was implanted in the vicinity of the left posterior ampullary nerve. This electrode was activated with biphasic pulse trains of 400-micros phase duration delivered at a repetition rate of 200 pulses per second. The resulting eye movements were recorded with 2-dimensional binocular video-oculography.
RESULTS: Successive "on-off" cycles of continuous electrical stimulation resulted in a progressively shorter duration of the nystagmic response. Once the adapted state was reached upon constant stimulation, amplitude or frequency modulations of electrical stimulation produced smooth oscillatory conjugated eye movements.
CONCLUSIONS: Although this is a case study of one patient, the results suggest that humans can adapt to electrical stimulation of the vestibular system without too much discomfort. Once the subject is in the adapted state, the electrical stimulation can be modulated to artificially elicit smooth eye movements. Therefore, the major prerequisites for the feasibility of a vestibular implant for human use are fulfilled.