Abstract
The physiological integrity of spiral ganglion neurons is presumed to influence cochlear implant (CI) outcomes, but it is difficult to measure neural health in CI listeners. Modeling data suggest that, when peripheral processes have degenerated, anodic stimulation may be a more effective neural stimulus than cathodic stimulation. The primary goal of the present study was to evaluate the emerging theory that polarity sensitivity reflects neural health in CI listeners. An ideal in vivo estimate of neural integrity should vary independently of other factors known to influence the CI electrode-neuron interface, such as electrode position and tissue impedances. Thus, the present analyses quantified the relationships between polarity sensitivity and (1) electrode position estimated via computed tomography imaging, (2) intracochlear resistance estimated via electrical field imaging, and (3) focused (steered quadrupolar) behavioral thresholds, which are believed to reflect a combination of local neural health, electrode position, and intracochlear resistance. Eleven adults with Advanced Bionics devices participated. To estimate polarity sensitivity, electrode-specific behavioral thresholds in response to monopolar, triphasic pulses where the central high-amplitude phase was either anodic (CAC) or cathodic (ACA) were measured. The polarity effect was defined as the difference in threshold response to the ACA compared to the CAC stimulus. Results indicated that the polarity effect was not related to electrode-to-modiolus distance, electrode scalar location, or intracochlear resistance. Large, positive polarity effects, which may indicate SGN degeneration, were associated with relatively high focused behavioral thresholds. The polarity effect explained a significant portion of the variation in focused thresholds, even after controlling for electrode position and intracochlear resistance. Overall, these results provide support for the theory that the polarity effect may reflect neural integrity in CI listeners. Evidence from this study supports further investigation into the use of polarity sensitivity for optimizing individual CI programming parameters.
Highlights
Cochlear implants (CIs) stimulate the auditory system by directly depolarizing spiral ganglion neurons (SGNs), and the physiological integrity of the SGNs may contribute to a patient’s success with a cochlear implant (CI)
The present analyses quantified the relationships between polarity sensitivity and (1) electrode position estimated via computed tomography imaging, (2) intracochlear resistance estimated via electrical field imaging, and (3) focused behavioral thresholds, which are believed to reflect a combination of local neural health, electrode position, and intracochlear resistance
The present study evaluated the theory that polarity sensitivity reflects neural status in CI listeners
Summary
Cochlear implants (CIs) stimulate the auditory system by directly depolarizing spiral ganglion neurons (SGNs), and the physiological integrity of the SGNs may contribute to a patient’s success with a CI. Variability in phoneme, word, and sentence perception scores across CI listeners is partially explained by demographic variables that are implicitly related to neural health, such as duration of deafness, age, and hearing loss etiology (Friedland et al 2010; Lazard et al 2012; Holden et al 2013). Some investigations have shown that in vivo estimates of SGN density relate to speech perception performance in CI listeners (e.g., Kim et al 2010; Zhou and Pfingst 2014; DeVries et al 2016; Scheperle 2017; Schvartz-Leyzac and Pfingst 2018). While SGN density constitutes an important aspect of neural health, the status of the peripheral processes may contribute to CI outcomes. Less is understood about how to quantify peripheral process integrity in vivo
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