Effects of Age and Noise Exposure on Proxy Measures of Cochlear Synaptopathy

Abstract

Although there is strong histological evidence for age-related synaptopathy in humans, evidence for the existence of noise-induced cochlear synaptopathy in humans is inconclusive. Here, we sought to evaluate the relative contributions of age and noise exposure to cochlear synaptopathy using a series of electrophysiological and behavioral measures. We extended an existing cohort by including 33 adults in the age range 37 to 60, resulting in a total of 156 participants, with the additional older participants resulting in a weakening of the correlation between lifetime noise exposure and age. We used six independent regression models (corrected for multiple comparisons), in which age, lifetime noise exposure, and high-frequency audiometric thresholds were used to predict measures of synaptopathy, with a focus on differential measures. The models for auditory brainstem responses, envelope-following responses, interaural phase discrimination, and the co-ordinate response measure of speech perception were not statistically significant. However, both age and noise exposure were significant predictors of performance on the digit triplet test of speech perception in noise, with greater noise exposure (unexpectedly) predicting better performance in the 80 dB sound pressure level (SPL) condition and greater age predicting better performance in the 40 dB SPL condition. Amplitude modulation detection thresholds were also significantly predicted by age, with older listeners performing better than younger listeners at 80 dB SPL. Overall, the results are inconsistent with the predicted effects of synaptopathy.

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

“…This was true for all the conditions tested (two carrier frequencies, two modulation rates, in quiet and in noise). The absence of age effects on sensitivity to envITDs was also reported by Prendergast et al (2019), despite the fact that Prendergast et al (2019) used a higher modulation rate (255 Hz),…”

“…These agerelated deficits were observed for high (≥ ~100 Hz) but not for lower modulation rates (Grose et al, 2009;Purcell et al, 2004). Measures of temporal envelope processing are often significantly correlated with hearing sensitivity (e.g., Grose et al, 2009;Prendergast et al, 2019;Purcell et al, 2004) suggesting that some of the reported age effects may have been related to deficits in cochlear function rather than aging per se.…”

“…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).…”

“…Direct evidence from human temporal bones shows that loss of auditory-nerve axons becomes increasingly severe with advancing age (Wu et al, 2019), even as some of the observed loss might be due to combined effects of age and noise exposure. Prendergast et al (2019) added a population of older adults to a large group of young normal-hearing individuals tested in their earlier studies (Prendergast et al, 2017a;Prendergast et al, 2017b). Combining younger and older adults resulted in a large total sample of individuals with ages in a range of 18-60 years.…”

“…Combining younger and older adults resulted in a large total sample of individuals with ages in a range of 18-60 years. Prendergast et al (2019) used a few measures of temporal processing, including amplitudemodulation (AM) detection thresholds, interaural envelope-phase-discrimination (envIPD) thresholds, and the magnitude of electroencephalographic (EEG) envelope following responses (EFRs). None of the measures was significantly correlated with age.…”