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Relationships between envelope-following responses and speech intelligibility in noise: early markers of impaired hearing?
Despite having normal audiometric thresholds, many people have difficulties understanding speech in challenging listening environments. The origin of these suprathreshold hearing deficits are not well understood, even though human temporal bone studies provided evidence that a substantial amount of synapses and cochlear nerve terminals innervating the inner hair cells (IHC) can deteriorate due to noise overexposure or aging before audiometric hearing loss occurs (i.e., cochlear synaptopathy). This hearing deficit results in a reduced neural information transfer along the ascending auditory pathway and is believed to play an important role in our ability to understand speech in noise. To quantify synaptopathy in humans, non-invasive but indirect electrophysiological measures of peripheral hearing such as the envelope following response (EFR) have been proposed. EFRs are sensitive to synaptopathy in animal models, but their diagnostic sensitivity in humans as well as their relationship to speech intelligibility are still unclear.
This study aims to clarify how the EFR relates to different aspects of speech encoding in listeners with normal or impaired hearing. We consider young normal-hearing (yNH), elderly normal-hearing (oNH) and elderly hearing-impaired (oHI) participants as groups reflecting different degrees of age-related synaptopathy (yNH vs oNH) or outer-hair-cell deficits (oNH vs oHI). We focus on broadband speech in noise (OLSA) as well as low-pass (< 1.5 kHz) and high-pass (> 1.65 kHz) filtered speech stimuli to study how different cochlear frequency regions (and associated coding mechanisms) contribute to the EFR-vs-speech intelligibility relation in the different subgroups. The stimuli for the EFR were 4-kHz centered amplitude-modulated tones, narrow-band noise and sharp-envelope modulated tones. The results of this study shed light on the relationship between speech intelligibility and the underlying neural coding mechanisms and its impairments; an important first step towards better understanding and characterizing suprathreshold hearing deficits.