Co-modulation of stimulus rate and current from elevated baselines expands head motion encoding range of the vestibular prosthesis

An implantable prosthesis that stimulates vestibular nerve branches to restore sensation of head rotation and vision-stabilizing reflexes could benefit individuals disabled by bilateral loss of vestibular sensation. The normal vestibular system encodes head movement by increasing or decreasing firing rate of the vestibular afferents about a baseline firing rate in proportion to head rotation velocity. Our multichannel vestibular prosthesis emulates this encoding scheme by modulating pulse rate and pulse current amplitude above and below a baseline stimulation rate (BSR) and a baseline stimulation current. Unilateral baseline prosthetic stimulation that mimics normal vestibular afferent baseline firing results in vestibulo-ocular reflex (VOR) eye responses with a wider range of eye velocity in response to stimuli modulated above baseline (excitatory) than below baseline (inhibitory). Stimulus modulation about higher than normal baselines resulted in increased range of inhibitory eye velocity, but decreased range of excitatory eye velocity. Simultaneous modulation of rate and current (co-modulation) above all tested baselines elicited a significantly wider range of excitatory eye velocity than rate or current modulation alone. Time constants associated with the recovery of VOR excitability following adaptation to elevated BSRs implicate synaptic vesicle depletion as a possible mechanism for the small range of excitatory eye velocity elicited by rate modulation alone. These findings can be used toward selecting optimal baseline levels for vestibular stimulation that would result in large inhibitory eye responses while maintaining a wide range of excitatory eye velocity via co-modulation.