PROJECT SUMMARY/ABSTRACT
Between 25–44 million U.S. adults are estimated to have difficulties hearing in background noise despite having
normal audiometric thresholds. These difficulties can negatively impact health and quality of life. Currently, there
is no consensus on how to diagnose or treat these individuals. The long-term objective of this research program
is to translate knowledge of the auditory mechanisms underlying speech-in-noise (SIN) recognition to improve
the diagnosis and treatment of difficulties hearing in background noise. One potential mechanism is the medial
olivocochlear (MOC) reflex, an ear-brain network in which the brainstem reduces the response of the inner ear
to background noise. Previous studies in humans have yielded conflicting results regarding the association
between the MOC reflex and SIN recognition. These discrepant findings may be due, in part, to lack of inclusion
of individuals with SIN recognition difficulties. It is also possible that the reduction in masking produced by the
MOC reflex reduces listening effort in the presence of background noise, but this area has not been investigated
thoroughly. Finally, the contribution of the afferent (ascending) drive to the measured MOC reflex strength
remains unknown. The proposed studies will address these gaps in knowledge through three aims. Two groups
of age- and sex-matched adults with normal hearing will be recruited (n=104 per group). One group will have
self-reported SIN recognition difficulties and one group will have no self-reported SIN recognition difficulties, as
assessed using a validated questionnaire. Aim 1 will determine the ability of MOC reflex strength to predict self-
reported SIN recognition difficulties. The strength of the MOC reflex will be assessed using contralateral inhibition
of otoacoustic emissions, a non-invasive measure of efferent control of the inner ear. We hypothesize that MOC
reflex strength is a significant predictor of SIN recognition group classification. Aim 2 will determine the
association between MOC reflex strength and listening effort. The same participants from Aim 1 will perform a
word recognition in noise task at two signal-to-noise ratios. Listening effort will be quantified by verbal response
time and self-report. It is hypothesized that MOC reflex strength is a significant predictor of listening effort, and
the strength of the association between MOC reflex strength and listening effort depends on the signal-to-noise
ratio of the speech task. Aim 3 will determine the contribution of afferent drive to MOC reflex strength. The same
participants from Aim 1 will undergo auditory brainstem response testing and MOC reflex testing. Afferent drive
will be quantified as the amplitude of wave I of the auditory brainstem response. Associations between afferent
drive and MOC reflex strength will be assessed using correlational analyses. This aim is exploratory in nature.
The results of these studies will lead to an improved mechanistic understanding of the functional role of the MOC
reflex. Additionally, the results will contribute to the development of better diagnostic tests and interventions for
difficulties with SIN recognition and listening effort.
