Associations between physiological indicators of cochlear deafferentation and listening effort in military Veterans with normal audiograms.

Cochlear synaptopathy, a type of cochlear deafferentation that impacts the synapses between the inner hair cells and the afferent auditory nerve fibers, is predicted to result in speech perception in noise difficulty. However, this has been difficult to confirm due to mixed findings in human studies of the relationship between speech perception in noise and physiological indicators of deafferentation (auditory brainstem response [ABR] wave I amplitude, envelope following response [EFR] magnitude, and middle ear muscle reflex [MEMR] magnitude). One possible explanation for the mixed findings is that some listeners with cochlear deafferentation can increase their cognitive effort to compensate for the degraded speech signal so that their speech-in-noise performance is relatively unaffected, obscuring the relationship between deafferentation and speech-in-noise scores. In a population at high risk for noise-induced cochlear deafferentation (military Veterans with normal audiograms), this study evaluated the relationship between physiological indicators of deafferentation (ABR, EFR, and MEMR) and listening effort, as indicated by pupil dilation during a speech-in-noise task. Mean reductions in ABR, EFR, and MEMR magnitude were associated with greater task-related pupil dilation, but not with reduced speech-in-noise performance, although only the ABR was statistically significant after accounting for sex and outer hair cell function. This suggests that cochlear deafferentation may result in increased listening effort during speech-in-noise perception, even if performance on the task is not negatively impacted. The observed relationship between EFR magnitude and pupil dilation was non-linear, suggesting that increased listening effort may only occur after a particular threshold level of deafferentation is reached.