Event:

For many people, the first indication they might have a hearing problem comes from difficulties they experience following conversations in background noise. Often, however, their audiogram – the ’gold-standard’ test of hearing function – indicates normal or near-normal hearing thresholds. Recently, hearing problems not evident in the audiogram have been linked to cochlear synaptopathy, a form of damage to the synaptic contact between the hair cells in the cochlea and the auditory nerve fibres (ANFs).

Synaptopathy through noise exposure and ageing effectively disconnects a substantial fraction of ANFs from the hair cells, and high-threshold ANFs – fibres activated only by relatively loud sounds – appear particularly vulnerable. Referred to colloquially as ’hidden hearing loss’ (HHL), the detrimental effects of peripheral synaptopathy could contribute to tinnitus and impact on complex listening tasks such as processing speech in background noise.

Here, we demonstrate in the midbrain of gerbils exposed to a single, controlled noise insult, and in human listeners, evidence of increased neural gain in the central auditory pathways, indicative of damage to high-threshold ANFs. In noise-exposed gerbils, we found that neural responses recorded from the inferior colliculus were higher than in control animals for speech stimuli at 60, but not at 75 dB SPL. Moreover, in the noise-exposed animals, discrimination performance for 60-dB SPL ’vowel-consonant-vowel’ stimuli (VCVs) in background noise (speech-shaped, +12 to -12 dB signal-to-noise ratio) was higher than for 75 dB SPL VCVs, whereas in control animals the opposite relation was observed, with better performance at 75 dB SPL.

A similar pattern was evident in human listeners, where a marker for HHL in the auditory brainstem response (ratio of wave V (generated by the midbrain) to wave I (generated by the auditory nerve)) showed a significant negative correlation to the change in listening performance when the intensity of the background noise was increased from 60 to 80 dB SPL. The data are consistent with the hypothesis that reduced neural output from the cochlea at higher sound levels increases central gain through homeostatic mechanisms that seek to normalize neural activity, which could explain differences in listening performance in different levels of background noise.