Effects of Lifetime Noise Exposure Viewed Through the Brainstem Reflex Bifocals: Middle Ear Muscle Reflex and Medial Olivocochlear Reflex

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Project Summary Noise-induced hearing loss (NIHL) affects nearly one in four adults in the United States. With recent discov- eries pertaining to cochlear synaptopathy and outer hair cell (OHC) loss in the extended high frequencies, it is clear that much of the early NIHL remains `hidden' under traditional audiological scrutiny. Without early de- tection and intervention, such damage can progress into more severe hearing loss. Thus a critical window for therapeutics/lifestyle changes may go unutilized. However, there are currently no feasible tools that can identify early hearing damage. Prior studies that test the integrity of the auditory afferent pathway have not produced conclusive results. However, short- and long-term noise exposure-related peripheral damage are correlated with changes in the auditory efferent system. Specifically, hyperactivity in the medial olivocochlear reflex (MOCR) and threshold elevation of the middle ear muscle reflex (MEMR) have been reported. Given the protective roles of the MOCR and the MEMR through inhibition of peripheral inputs, and their differential changes with damage, we argue that a combined assay of MOCR and MEMR may serve as a marker for early hearing damage in humans. Using a novel otoacoustic emission (OAE)-based efferent assay, in the proposed studies we aim to (1) eval- uate long-term age-specific changes in efferent and afferent function due to noise exposure and (2) evaluate short-term changes in efferent and afferent function due to noise exposure. We will investigate the concurrent working of the two reflexes across a wide age range (18-50 years) and noise exposure by recruiting individuals from high noise exposure (musicians, veterans, construction workers, farmer) and low noise exposure occupa- tions (students, professors). To evaluate short-term changes due to noise exposure, we will test participants before and after their typical work day. For reliable exposure stratification, noise exposure will be objectively quantified using 5-day sound dosimetry. We will also use the most sensitive afferent measures to allow compari- son with efferent measures. Machine learning approaches will be employed to ascertain relationships among the cochlear, afferent, and efferent function for short- and long-term noise exposures. Findings from project 1 will reveal if the combined MOCR and MEMR metrics can delineate noise exposure effects from aging, and highlight the relationships among cochlear, afferent, and efferent measures. Findings from project 2 will reveal if long-term noise exposure predicts short-term changes following noise exposure and vice- versa. A better understanding of short- and long-term changes in the auditory system following noise exposure will aid in the development of (1) an objective rapid screening test of the auditory efferents capable of detecting noise exposure-related hearing damage and (2) a statistical model to enable predicting impending damage based on efferent function. Together, these tools will contribute to early detection and promote hearing conservation.