Abstract
Neural health is of great interest to determine individual degeneration patterns for improving speech perception in cochlear implant (CI) users. Therefore, in recent years, several studies tried to identify and quantify neural survival in CI users. Among all proposed techniques, polarity sensitivity is a promising way to evaluate the neural status of auditory nerve fibers (ANFs) in CI users. Nevertheless, investigating neural health based on polarity sensitivity is a challenging and complicated task that involves various parameters, and the outcomes of many studies show contradictory results of polarity sensitivity behavior. Our computational study benefits from an accurate three-dimensional finite element model of a human cochlea with realistic human ANFs and determined ANF degeneration pattern of peripheral part with a diminishing of axon diameter and myelination thickness based on degeneration levels. In order to see how different parameters may impact the polarity sensitivity behavior of ANFs, we investigated polarity behavior under the application of symmetric and asymmetric pulse shapes, monopolar and multipolar CI stimulation strategies, and a perimodiolar and lateral CI array system. Our main findings are as follows: (1) action potential (AP) initiation sites occurred mainly in the peripheral site in the lateral system regardless of stimulation strategies, pulse polarities, pulse shapes, cochlear turns, and ANF degeneration levels. However, in the perimodiolar system, AP initiation sites varied between peripheral and central processes, depending on stimulation strategies, pulse shapes, and pulse polarities. (2) In perimodiolar array, clusters formed in threshold values based on cochlear turns and degeneration levels for multipolar strategies only when asymmetric pulses were applied. (3) In the perimodiolar array, a declining trend in polarity (anodic threshold/cathodic threshold) with multipolar strategies was observed between intact or slight degenerated cases and more severe degenerated cases, whereas in the lateral array, cathodic sensitivity was noticed for intact and less degenerated cases and anodic sensitivity for cases with high degrees of degeneration. Our results suggest that a combination of asymmetric pulse shapes, focusing more on multipolar stimulation strategies, as well as considering the distances to the modiolus wall, allows us to distinguish the degeneration patterns of ANFs across the cochlea.
Highlights
The sophisticated structure of the inner ear provides a sense of hearing obtained by transmitting the auditory signals from cochlear sensory hair cells through the myelinated auditory nerve fibers (ANFs) to the cochlear nuclei in the brain stem
Consistent with several clinical studies, which reported that asymmetric pulse shapes are more effective than symmetric biphasic pulse for evaluating polarity sensitivity in cochlear implant (CI) users (Macherey et al, 2006, 2008, 2017; Carlyon et al, 2013; Guérit et al, 2018; Jahn and Arenberg, 2019a), our results showed that when a symmetric biphasic pulse is applied, recognition of peripheral changes based on polarity sensitivity was not feasible
Our findings suggested the following: (i) The asymmetric pulse shape is more suitable for studying polarity sensitivity when other parameters such as stimulation strategy, electrode distance to the modiolus wall, and cochlear turn are considered
Summary
The sophisticated structure of the inner ear provides a sense of hearing obtained by transmitting the auditory signals from cochlear sensory hair cells through the myelinated auditory nerve fibers (ANFs) to the cochlear nuclei in the brain stem. Various clinical studies investigated the cause of the variability of the outcomes through CI users They reported factors such as etiology, using hearing aids, age at implantation, ANF degeneration status, surgical trauma, residual hearing measures, electrode displacement in the cochlea, and duration and age at onset of moderate to profound hearing loss have impacts on CI performances and can be the reason for large variations in hearing performance (Gantz et al, 1993; Summerfield and Marshall, 1995; Waltzman et al, 1995; Albu and Babighian, 1997; Shepherd and Javel, 1997; Friedland et al, 2003, 2010; Gomaa et al, 2003; Sly et al, 2007; Finley and Skinner, 2008; Blamey et al, 2012; Lazard et al, 2012; Holden et al, 2013; Frisch et al, 2015; Cosentino et al, 2016). Over the decades, it has been shown that sensorineural hearing loss changes ANF geometry by decreasing myelination thickness and ANF diameter or loss of the peripheral part, which negatively affects their excitation properties (Bostock et al, 1983; Colombo and Parkins, 1987; Leake and Hradek, 1988; Spoendlin and Schrott, 1989; Nadol, 1997; Smit et al, 2008; Resnick et al, 2018; Heshmat et al, 2020)
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