Middle Ear Muscle Reflex and Word Recognition in “Normal-Hearing” Adults: Evidence for Cochlear Synaptopathy?

Abstract

Objectives: Permanent threshold elevation after noise exposure, ototoxic drugs or aging is caused by loss of sensory cells; however, animal studies show that hair cell loss is often preceded by degeneration of synapses between sensory cells and auditory nerve fibers. The silencing of these neurons, especially those with high thresholds and low spontaneous rates, degrades auditory processing and may contribute to difficulties understanding speech in noise. Although cochlear synaptopathy can be diagnosed in animals by measuring suprathreshold auditory brainstem responses, its diagnosis in humans remains a challenge. In mice, cochlear synaptopathy is also correlated with measures of middle-ear muscle (MEM) reflex strength, possibly because the missing high-threshold neurons are important drivers of this reflex. We hypothesized that measures of the MEM reflex might be better than other assays of peripheral function in predicting difficulties hearing in difficult listening environments in human subjects. Design:We recruited 165 normal-hearing healthy subjects, between the ages of 18 and 63, with no history of ear or hearing problems, no history of neurologic disorders and unremarkable otoscopic examinations. Word recognition in quiet and in difficult listening situations was measured in four ways: using isolated words from the NU-6 corpus with either a) 0 dB signal-to noise, b) 45% time compression with reverberation, or c) 65% time compression with reverberation and d) with a modified version of the QuickSIN. Audiometric thresholds were assessed at standard and extended high frequencies (EHFs). Outer hair cell function was assessed by distortion product otoacoustic emissions (DPOAEs). Middle-ear function and reflexes were assessed using three methods: the acoustic reflex threshold as measured clinically, wideband tympanometry as measured clinically and a custom wideband method that uses a pair of click probes flanking an ipsilateral noise elicitor. Other aspects of peripheral auditory function were

Effects of age and hearing loss on perceptual and physiological measures of temporal envelope processing and spatial release from speech-on-speech masking

“…Despite extensive efforts, evidence consistent with the hypothesized adverse effects of cochlear synaptopathy on auditory perception in living humans remains elusive (Grose et al, 2017;Guest et al, 2017a;Johannesen et al, 2019;but cf. Liberman et al, 2016;Mepani et al, 2020;Prendergast et al, 2019;Prendergast et al, 2017a;Prendergast et al, 2018;Shehorn et al, 2020;e.g., Spankovich et al, 2014). Early studies of cochlear synaptopathy tested mainly young normal-hearing listeners trying to avoid confounding effects of hearing loss that could explain the observed effects or offset them (Garrett et al, 2019).…”

“…A smaller number of studies have included older normal or near-normal hearing individuals when exploring different proxy measures of cochlear synaptopathy (Mepani et al, 2020;Prendergast et al, 2019). The main reason for limiting the study samples to young adults is that older individuals often have higher audiometric thresholds even when the thresholds are within the limits of "normal hearing" as defined by ANSI standards (ANSI, 1996).…”

Dextromethorphan Attenuates Sensorineural Hearing Loss in an Animal Model and Population-Based Cohort Study

“…In recent years, a hidden hearing loss theory has advocated that synaptic loss is the primary pathology even with only temporary threshold shifts, and that this synaptic loss is independent from both IHC and SGN loss [ 6 , 30 ]. Cochlear synaptopathy may contribute to hearing impairment in millions of people [ 8 , 31 ]. If these damaged synaptic connections can be maintained or restored by any potential drugs or other therapeutic management, it could undoubtedly improve the hearing function [ 32 ].…”

An exploratory investigation of speech recognition thresholds in noise with auralisations of two reverberant rooms

“…Research on auralisations can offer the means to study HA performance in more realistic sound environments (Compton-Conley et al 2004;Minnaar, Favrot, and Buchholz 2010;Mueller et al 2012;Minnaar et al 2013;Grimm, Kollmeier, and Hohmann 2016;Cubick and Dau 2016). Various sound materials for speech recognition tests that include variations in reverberation have also been developed (Noffsinger, Wilson, and Musiek 1994) and are becoming more widely used, for instance, in research on cochlear synaptopathy (Liberman et al 2016;Mepani et al 2020).…”