Vestibular Prosthesis Stimulus Optimization

A multichannel vestibular prosthesis that measures 3D head rotation and encodes it via selective electrical stimulation of vestibular nerve branches could improve quality of life for individuals disabled by diseases of the inner ear that cause bilateral loss of vestibular sensation. Spread of current from a stimulating electrode to nerve branches other than its intended target manifest as a misalignment of the perceived and actual head rotation axes. The perceived head rotation axis and velocity can be assayed by monitoring the axis and velocity of eye movement caused by the angular vestibulo-ocular reflex (aVOR).

We investigated the effects of varying pulse duration (PD), interphase gap (IPG), and current amplitude of biphasic pulses on aVOR responses evoked by pulse rate modulated stimuli delivered to electrodes implanted in the three semicircular canals of 5 chinchillas rendered bilaterally vestibular deficient. We found that increasing the stimulation pulse rate resulted in a linear increase of the aVOR-mediated eye movement response velocity while maintaining a relatively constant axis of rotation. Increasing current amplitude increased eye velocity but also increased the misalignment between the intended and actual axes of rotation. Stimulation with shorter PD over the range of 28 – 340 μs required less charge per phase to elicit a given peak aVOR response velocity, and also evoked responses with less aVOR axis misalignment. Varying IPG from 25 – 175 μs had no significant effect on aVOR responses. We conclude that while pulse rate modulation is the most appropriate basic code for prosthetic stimulation of the vestibular nerve, responses also depend on current and pulse duration, which can be optimized to better encode high velocity head rotations.

Various parameters of a typical stimulus pulse:

Shorter pulse durations lead to better alignment of eye movement responses to desired axis (i.e., the axis of head rotation being encoded by the prosthesis). When electrode-nerve coupling is strong and selective (lower 4 lines below), misalignment remains good for all pulse widths we use. When coupling is not very selective (upper 4 lines), shortening the pulse duration has the biggest beneficial effect.