Peripheral phoneme encoding and discrimination in aging and hearing impairment

Speech perception difficulties in noise are common among older adults and individuals with hearing impairment, but also occur among younger adults whose audiometric thresholds are within the clinically-normal range. We examined how aging, cochlear synaptopathy (CS), and outer hair cell (OHC) damage affect speech encoding and phoneme discrimination. Envelope-following responses (EFRs) to rectangular amplitude-modulated (RAM) tones and speech-like phoneme pairs were recorded in quiet using EEG, and behavioral discrimination was assessed in quiet, and with ipsilateral and contralateral noise. Stimuli were designed to target temporal envelope (TENV) or temporal fine structure (TFS) encoding. Results showed that RAM-EFR amplitudes decreased gradually with increasing age, consistent with emerging CS, while magnitudes of high-frequency TENV-based EFRs in quiet were most reduced in older hearing-impaired subjects with combined CS and OHC damage. In contrast, EFRs targeting low-frequency TENV encoding in quiet remained preserved. Behaviorally, discrimination of predominantly TFS-based phoneme contrasts worsened with OHC loss and age in quiet and contralateral noise, respectively, whereas discrimination of predominantly TENV-based contrasts showed no significant age effect. Considering that high-frequency contrasts are discriminated via place-based spectral cues, low-frequency contrasts rely on TFS, and the EFR reflects primarily TENV, this framework explains why EFRs decline for high-frequency cues without perceptual loss, while EFRs remain stable for low-frequency cues even as TFS-based discrimination deteriorates. These findings highlight the need to investigate how neural coding deficits relate to perception. Combining electrophysiological and behavioral measures might provide a framework for detecting subclinical auditory deficits to earlier diagnose age-related and hidden hearing loss.