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Amy Nowack, L Ling, S Bierer, C. Kaneko, K. Nie, S. Newlands, T. Oxford, J. T. Rubinstein, J. O. Phillips; Effect of Eye Position on the Response to Electrical Stimulation of the Vestibular End Organ to Control Nystagmus. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4705.
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Our group has developed a vestibular prosthesis to control pathological nystagmus through electrical stimulation. This device is modeled after the cochlear implant, and contains electrodes threaded into each of the semicircular canals. In order to manipulate pathological eye movements effectively, we must understand the relationship between eye position and the eye movements elicited by electrical stimulation with this device.
We aim to characterize behavioral responses generated by electrical signals delivered by the prosthesis when the eye is in different initial positions. For this study, two rhesus monkeys were trained to follow and fixate on a visual target. We applied short duration stimulations (50ms) to the lateral and posterior semicircular canals after head restrained re-fixations. All visual stimuli were removed during stimulation.
Biphasic pulse trains applied to the right lateral and posterior canals produced rapid leftward and oblique (downward and right) deflections of the eye, respectively, at a latency of 10ms. Peak velocities for these elicited eye movements depended on stimulus current and frequency. Stimulation of the lateral canal elicited higher velocities when the eye was deviated contralateral to the implanted canal. For stimulation of the posterior canal, the vertical eye position did not affect the vertical velocity of elicited eye movements, but downward eye positions increased the horizontal velocity component of these movements.We propose that two mechanisms may be responsible for the observed effects. Abducens nucleus activity may explain the lateral position effect on eye velocity. The vertical position effect, on the other hand, may be due to the mechanics of the oculomotor plant.
We must take into account the orbital eye position when calculating the stimulation parameters necessary to drive the controlled eye movements, or to counteract ongoing nystagmus.
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