Contributions of Auditory Nerve Density and Synchrony to Speech Understanding in Older Cochlear Implant Users

The majority of adult cochlear implant (CI) recipients are over the age of 65, and previous research in non-implanted older adults shows that auditory nerve (AN) pathophysiology contributes to senescent declines in speech understanding. However, age-related changes to AN structure and function have not yet been explored as a contributory factor to poorer speech understanding outcomes in older CI users. Here, we explore how estimates of AN disengagement (i.e., AN density) and dyssynchrony in CI users contribute to poorer speech recognition performance observed in older CI users.

We examined electrically evoked compound action potentials (ECAPs) in 47 adult (Male = 25) CI recipients. We measured the interphase gap (IPG) effect for the amplitude-growth function (AGF) slope and the N1-P2 interpeak latency as independent metrics of AN density and dyssynchrony, respectively.

Estimates of AN density and dyssynchrony worsen with increasing age in older CI listeners. These measures were not significantly correlated with one another, but were independently related to speech recognition in noise performance. Lower ECAP IPG effect values (lower density of AN fibers) are observed in older CI users. Longer N1-P2 interpeak latency values (poorer neural synchrony) are also observed in older CI users. When controlling for listener age, poorer AN dyssynchrony contributes to declines in speech-recognition-in-noise performance in CI users.

These results suggest that AN dyssynchrony rather than density is the primary contributing factor to age-related declines in speech understanding in CI users. These results have important implications for better understanding neural contributions to speech understanding in adult CI users.