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Characterizing Vestibular Electrically Evoked Compound Action Potentials in Guinea Pigs


K. Nguyen, J. DiGiovanna, W. Gong, C. Haburcakova, W. Poppendieck, S. Micera, D. Merfeld

ARO Association for Research in Otolaryngology, Baltimore, MD, USA, Accepted poster

Patients with loss of vestibular function suffer from decreased physical and social well-being. Their symptoms may be treated with a vestibular implant that electrically stimulates peripheral afferents in the semicircular canals. Numerous studies with animal models and a few with human subjects demonstrated activation of the vestibular-ocular reflex (VOR) that provides a benchmark of vestibular function. To complement VOR, we measured peripheral neural function through vestibular electrically evoked compound action potentials (VECAP). Four male guinea pigs were instrumented with a scleral search coil and an electrode array with eight sites capable of stimulation and recording inserted in one semicircular canal. A wire electrode placed into the neck muscle served as remote return electrode. A system consisting of PC, Med-El Research Interface Box II, transmitting coil and Pulsar cochlear implant was connected to the array. One electrode site and the remote electrode were used for monopolar stimulation, two other array sites were used for bipolar recording. VECAP was measured in response to single pulses, pulse trains and acute continuous pulses of varying current amplitude (25 s/phase for all cases). VOR responses were measured with the search coil while the subject was alert and head-fixed in a dark room. All experiments were approved by the institutional animal care and use committee. In all subjects, both eye movement and VECAP were evoked. However, few recording sites measured VECAP similar in shape to cochlear implant ECAPs with a negative (N) and a positive (P) wave. Correlation of VOR peak velocity and N-P voltage in response to single pulses revealed a piecewise linear function with a marked increase in peak velocity beyond a certain N-P voltage and could represent a vestibular threshold. In response to pulse trains, N-P voltage was slightly reduced, but not significantly, while VOR responses increased and plateaued with pulse number, suggesting integration. After adaptation to acute continuous stimulation, modulation of current amplitude elicited VOR responses in both directions (e.g., left, right for horizontal canal stimulation); there were analogous changes in N-P voltage.


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