The vestibular system, a complex system involving the inner ear, is responsible for maintaining eye stability and posture during head movements.  Rotational and linear accelerations of the head are sensed by the end organs of the vestibular labyrinth, and are used to initiate and modulate reflexes.  Dysfunction of the vestibular system can produce symptoms ranging from mild dizziness to debilitating attacks of vertigo.  Our group has developed a vestibular prosthesis to replace vestibular function through the use of electrical stimulation. The device has three electrodes which are surgically implanted into the three semicircular canals of vestibular labyrinth to allow for direct electrical stimulation of the end organ. Our ultimate goal is to artificially restore vestibular function in patients, in part to accurately control eye movements.  To do this, we need to understand the relationship between eye position and eye movements during stimulation with the prosthesis. Previous studies in our laboratory have shown that transient eye velocities elicited by short stimulation are linearly related to the initial position of the eye in the orbit.  We need to understand if during prolonged stimulation eye position also has a significant effect on eye velocity. In this study, we characterize the effects of eye position on the maintained velocities elicited by a long stimulus train. These observations will allow us to more accurately control eye movements to stabilize the eye with electrical stimulation of varying durations, or at different frequencies of head rotation. We have trained one rhesus monkey to track a moving visual target.  A long duration stimulus was administered to each of the three semicircular canals in that animal.  All visual stimuli were removed during the stimulations.  Preliminary results suggest that there is a relationship between eye position and eye velocity elicited by long duration stimuli.

 The vestibular system is composed of the semicircular canal system, which detects rotational movements, and the otolith organ system, which senses linear accelerations and tilt with respect to gravity. The two systems can interact in a complex way during movement. The vestibular-ocular reflex (VOR) stabilizes a retinal image during head movement by commanding the eyes to move in a direction opposite to that of the head movement. Under some conditions, both the otolith organs and the semicircular canals contribute to the VOR. Unilateral or bilateral loss of vestibular function often results in vertigo and oscillopsia, which are debilitating and could be life-threatening because they result in falls or injury. In principle, a vestibular prosthesis could treat a functional loss of inner ear vestibular function by sending electrical stimulation into the three semicircular canals via electrodes to help maintain balance and stable vision. However, if the otolith organs are still functional, the signals from the prosthesis might be interpreted incorrectly in different head orientations, limiting the efficacy of the treatment. The interaction between the output from the otolith organs and artificial electrical stimulations from the implant is not known. In this experiment, we oriented a head-fixed rhesus monkey in different static positions along either of the roll and pitch axes prior to electrically inducing a VOR using the implanted vestibular prosthesis. Results showed that stimulation in the posterior canal induced a higher eye velocity in backward-tilted position than in the upright position, while stimulation in the anterior canal induced a slower eye velocity in the forward position than in the upright position. This project demonstrates that the performance of the implant changes under different body orientations. The prosthesis provides a unique experimental tool allowing us to disambiguate canal and otolith inputs in the intact organism.