To better understand the spread of prosthetic current in the inner ear and to facilitate design of electrode arrays and stimulation protocols for a vestibular implant system intended to restore sensation after loss of vestibular hair cell function, we created a model of the primate labyrinth. Because the geometry of the implanted ear is complex, accurately modeling effects of prosthetic stimuli on vestibular afferent activity required a detailed representation of labyrinthine anatomy. Model geometry was therefore generated from three-dimensional (3D) reconstructions of a normal rhesus temporal bone imaged using micro-MRI and micro-CT. For systematically varied combinations of active and return electrode location, the extracellular potential field during a biphasic current pulse was computed using finite element methods. Potential field values served as inputs to stochastic, nonlinear dynamic models for each of 2415 vestibular afferent axons, each with unique origin on the neuroepithelium and spiking dynamics based on a modified Smith and Goldberg model. We tested the model by comparing predicted and actual 3D vestibulo-ocular reflex (VOR) responses for eye rotation elicited by prosthetic stimuli. The model was individualized for each implanted animal by placing model electrodes in the standard labyrinth geometry based on CT localization of actual implanted electrodes. Eye rotation 3D axes were predicted from relative proportions of model axons excited within each of the three ampullary nerves, and predictions were compared to archival eye movement response data measured in three alert rhesus monkeys using 3D scleral coil oculography. Multiple empirically observed features emerged as properties of the model, including effects of changing active and return electrode position. The model predicts improved prosthesis performance when the reference electrode is in the labyrinth's common crus (CC) rather than outside the temporal bone, especially if the reference electrode is inserted nearly to the junction of the CC with the vestibule. Extension of the model to human anatomy should facilitate optimal design of electrode arrays for clinical application.
Keywords: finite element; inner ear; labyrinth; monkey; rhesus; vestibular; vestibular implant; vestibular prosthesis.