Simulation of apically grounded cochlear implant stimuli using neural stimulation models

PURPOSE: Cochlear implant (CI) electrode arrays typically do not extend along the entire cochlear duct, often resulting in poor stimulation of the most apical nerves. To better stimulate apical auditory nerve fibers (ANFs), previous studies have examined placing an isolated ground contact within the helicotrema, providing the ability to direct current apically. In this study, we utilize finite difference method (FDM) simulations and computational ANF models to study the effect of apical ground placement on apical ANF stimulation. METHODS: High-resolution resistivity maps based on postoperative CT scans for 3 CI patients were used in FDM-based generation of cochlear voltage maps. Models corresponding to the use of a typical extracochlear ground and a helicotrema ground were used to drive ANF models, allowing for generation of activation thresholds at each of 75 modeled ANF bundles throughout the cochlea. Thresholds between the two models were compared to assess the effects of an apical grounding contact on improving apical ANF stimulation. RESULTS: In all 3 patients, we observe a relative reduction in stimulation thresholds for apical ANFs when an apical ground is present as opposed to an extracochlear ground. Additionally, when an apical ground is used, we observe an increase in stimulation thresholds for more basal ANFs. CONCLUSIONS: Apical ground contacts can increase activation of typically under-stimulated low frequency ANFs without the need for longer contact arrays, providing a solution for improving low frequency sound perception and potentially leading to better hearing quality.