Interplay of age, speech perception in noise, standard and extended high-frequency hearing, and cochlear synaptopathy in individuals with normal audiograms

Older adults often complain about having difficulty understanding speech in noise, despite clinically normal hearing (D-SiN-NH). Two otopathologies known to decline with age and that are not reflected in the conventional audiogram are extended high-frequency (EHF) hearing impairment and cochlear synaptopathy (CS), both hypothesized to be responsible for D-SiN-NH. Nevertheless, the influence of EHF deficits on Auditory Brainstem Response (ABR) wave I amplitude, a commonly used proxy measure for CS, remains equivocal. Furthermore, the combined effects of age-related changes in peripheral auditory function - standard-frequency hearing, EHF hearing, and synaptic integrity - on D-SiN-NH remain poorly understood. Therefore, the current study investigated the relationships among age, standard frequency thresholds, transient-evoked otoacoustic emissions amplitude, EHF thresholds, the slope of the amplitude-intensity function for ABR Wave I, and speech-in-noise recognition performance using structural equation modeling. A cohort of 60 adults spanning a broad age range (18-54 years) with clinically normal audiograms (≤20 dB HL at standard frequencies) participated in the study. The results indicated that elevated EHF thresholds are associated with age-related subclinical outer hair cell (OHC) dysfunction at standard audiometric frequencies (1- 4 kHz) rather than cochlear synaptopathy. Further, the findings were consistent with age-related CS, even when subclinical OHC dysfunction was observed at 1-4 kHz. Importantly, neither EHF hearing loss nor CS showed a significant independent association with D-SiN-NH. In contrast, subtle elevations in standard-frequency thresholds, despite remaining within the clinically normal range, were associated with age-related speech-in-noise difficulties under the present testing conditions.