Increased listening effort and cochlear neural degeneration underlie speech-in-noise deficits in normal hearing middle-aged adults

Abstract

Middle age represents a critical period of accelerated brain changes and provides a window for early detection and intervention in age-related neurological decline. Hearing loss is a key early marker of such decline and is linked to numerous comorbidities in older adults. Yet, ~10% of middle-aged individuals who report hearing difficulties show normal audiograms. Cochlear neural degeneration (CND) could contribute to these hidden hearing deficits, though its role remains unclear due to a lack of objective diagnostics and uncertainty regarding its perceptual outcomes. Here, we employed a cross-species design to examine neural and behavioral signatures of CND. We measured envelope following responses (EFRs) - neural ensemble responses to sound originating from the peripheral auditory pathway - in young and middle-aged adults with normal audiograms and compared these responses to young and middle-aged Mongolian gerbils, where CND was histologically confirmed. We observed near identical changes in EFRs across species that were associated with CND. Behavioral assessments revealed age-related speech-in-noise deficits under challenging conditions, while pupil-indexed listening effort increased with age even when behavioral performance was matched. Together, these results demonstrate that CND contributes to speech perception difficulties and elevated listening effort in midlife, which may ultimately lead to listening fatigue and social withdrawal.

Figure 1.. Age-related CND occurs prior to overt changes in hearing thresholds and can be assessed non-invasively by measuring phase-locked neural envelope following responses.

(A) Thirty middle-aged (MA, 40–55 yrs, mean = 46.1±4.6 yrs) and 36 young adults (YA, 18–25 years, mean = 21.17± 1.8yrs) participated in this study. (B) All participants had clinically normal hearing thresholds with some evidence of threshold losses at extended high frequencies above 8 kHz typically not tested in the clinic. Hearing thresholds in dB HL are shown on the Y axis and frequency in kHz is plotted on the X axis. (C) Outer hair cell function assessed using DPOAEs is comparable between YA and MA up to 4kHz and showed age-related decreases at higher frequencies. Both cohorts show no evidence of self-reported tinnitus (D) or hyperacusis measured as LDLs (E), have comparable self-reported noise exposure levels (F), and comparable working memory scores assessed using OSPAN (G). (H) EFRs to modulation frequencies of 1024Hz can be reliably recorded in young and middle-aged adults using ‘tiptrodes’. The panel shows grand-averaged FFT traces for YA and MA. (I) Middle-aged adults showed significant declines in EFR amplitudes at 1024Hz AM, with putative neural generators in the auditory nerve. (J) Signal-to-noise ratios were 8dB on average for YA and 4dB for MA. (K) Statistically significant decreases in EFR amplitudes were selective for 1024Hz AM, the modulation frequency with putative generators in the auditory nerve. All panels: Error bars and shading represent standard error of the mean (SEM). Asterisks represent p<0.05, ANOVA.

Figure 2.. Cross-species experiments in a rodent model show that EFRs are a sensitive biomarker for histologically confirmed CND.

(A) Cross-species comparisons were made with young (22± 0.86 weeks, n = 14) and middle-aged (80± 0.76 weeks, n = 13) Mongolian gerbils, with identical stimuli, recording, and analysis parameters. (B) Middle-aged gerbils did not show any age-related decreases in hearing thresholds. (C) Age-related decreases in EFR amplitudes were isolated to the 1024Hz modulation frequency, similar to middle-aged humans in Fig1K. (D) CND was quantified for a subset of these gerbils (n = 10 young and 10 middle-aged) using immunostained organ of Corti whole mounts, where afferent excitatory synapses were quantified using 3D reconstructed images. (E) Cochlear synapse counts at the 3kHz cochlear region corresponding to the carrier frequency for the EFRs was significantly decreased in middle-aged gerbils, despite matched auditory thresholds. (F) EFR amplitudes at 1024Hz AM were significantly correlated with the number of remaining cochlear synapses, suggesting that these EFRs are a sensitive metric for CND with age. All panels: Error bars and shading represent standard error of the mean (SEM). Asterisks represent p<0.05, ANOVA.

Figure 3.. Increased listening effort precedes behavioral deficits in speech in noise perception in middle-aged adults.

(A) Speech perception in noise was assessed using the QuickSIN test, which presents moderate context sentences in varying levels of multi-talker babble. Pupillary measures were analyzed in two time-windows – 1. during stimulus presentation, and 2. after target sentence offset and prior to response initiation (B) No significant age-related differences were observed in clinical QuickSIN scores presented as dB SNR loss. (C) QuickSIN performance is matched between middle-aged (MA) and younger adults (YA) until the most difficult noise condition (SNR 0). The x-axis shows the SNR condition that the target sentences were presented in, with 25dB being the easiest noise condition, and 0dB being the most difficult noise condition. The y-axis shows participant accuracy in repeating key words from the target sentences as percent correct. (D) Grand-averaged pupillary responses measured during task listening as an index of effort exhibit modulation with task difficulty, with greater pupillary dilations observed in harder conditions for both groups. (E) Middle-aged adults show consistently higher pupillary responses during performance on the QuickSIN task and at SNR levels prior to when overt behavioral deficits are observed. (F) Grand-averaged pupillary responses measured after target sentence offset as an index of effort exhibit greater modulation with task difficulty, compared to changes in the listening window. (G) Trends seen in the listening window were amplified in this integration window, with middle-aged adults showing even greater effort, especially at moderate SNRs where behavior was matched.

Figure 4.. Listening effort and CND provide complementary contributions to speech in noise intelligibility.

(A) Behavioral performance at the most challenging SNR was significantly correlated with the EFR measures of CND, with lower EFR amplitudes being associated with poorer behavioral performance. (B) Pupillary responses at 10 dB SNR from the integration window were significantly correlated with behavioral performance at 0dB SNR, (B) These correlations between pupillary responses at 10 dB SNR and behavioral performance at 0dB SNR was also found in the listening window, even though there were no group differences in age, further strengthening the link between listening effort at moderate SNRs and behavioral performance at challenging SNRs. (D) an elastic net regression model with 10-fold cross validation (cv) was fit to the QuickSIN scores at 0dB SNR. The tuning parameter Lambda controls the extent to which coefficients contributing least to predictive accuracy are suppressed. (E) A lollipop plot displaying the coefficients (β) contributing to explaining variance on QuickSIN performance suggests that CND, listening effort and subclinical changes in hearing thresholds all contribute to QuickSIN performance. (F) QuickSIN scores predicted by the elastic net regression are corelated with actual participant QuickSIN scores.