Combined ionic direct current and pulse frequency modulation improves the dynamic range of vestibular canal stimulation.

BACKGROUND: Vestibular prostheses emulate normal vestibular function by electrically stimulating the semicircular canals using pulse frequency modulation (PFM). Spontaneous activity at the vestibular nerve may limit the dynamic range elicited by PFM. One proposed solution is the co-application of ionic direct current (iDC) to inhibit this spontaneous activity. OBJECTIVE: We aimed to test the hypothesis that a tonic iDC baseline delivered in conjunction with PFM to the vestibular semicircular canals could improve the dynamic range of evoked eye responses. METHODS: Gentamicin-treated chinchillas were implanted with microcatheter electrodes in the vestibular semicircular canals through which pulsatile and iDC current was delivered. PFM was used to modulate vestibulo-ocular reflex (VOR) once it was adapted to a preset iDC and pulse-frequency baseline. Responses to stimulation were assessed by recording the evoked VOR eye direction and velocity. RESULTS: PFM produced VOR responses aligned to the stimulated canal. Introduction of an iDC baseline lead to a small but statistically significant increase in eye response velocity, without influencing the direction of eye rotation. CONCLUSIONS: Tonic iDC baselines increase the dynamic range of encoding head velocity evoked by pulsatile stimulation, potentially via the inhibition of spontaneous activity in the vestibular nerve.