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Effects of noise exposure on auditory function

Bakay, WMH; (2017) Effects of noise exposure on auditory function. Doctoral thesis , UCL (University College London).

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Abstract

Hidden Hearing Loss (HHL) is a form of cochlear damage not detected by standard clinical audiometry. Recent work has shown that despite normal hearing thresholds there can be a substantial degree of cochlear neuropathy at the inner hair cell synapse. HHL could explain why patients with normal audiograms suffer from hearing problems like tinnitus, hyperacusis, and difficulty understanding speech-in-noise. The precise mechanism of how HHL could lead to these symptoms is not known. This thesis investigates, using a mouse model, how noise-induced HHL affects neuronal processing in the central auditory system. My results demonstrate for the first time that HHL causes the development of a putative neuronal correlate of tinnitus as well as profound impairment of auditory adaptive coding. I show that spontaneous firing rates in the inferior colliculus (IC) were significantly elevated at and above the frequency region of the noise exposure. This increase was greater than that seen with stimuli causing permanent threshold shifts. Moreover, I show that HHL compromises the ability of neurons to shift the dynamic range of their response following changes in the acoustic environment. This deficit was most pronounced for the loudest environment tested (80dB SPL), suggesting that it could originate from selective damage to high threshold AN fibres. Furthermore, I have discovered that adaption in the IC does not depend solely on the current stimulus history on a time-scale of hundreds of milliseconds, it also reacts to variations in the acoustic environments on a longer time scale. This meta-adaptation is also severely compromised by HHL for acoustic environments involving high sound intensities. My findings thus indicate how tinnitus could develop even without hearing threshold increase, and the deficits in adaptive coding and meta-adaptation caused by HHL could explain why some people with normal hearing thresholds struggle to understand speech in high-level background noise.

Type: Thesis (Doctoral)
Title: Effects of noise exposure on auditory function
Event: UCL (University College London)
Language: English
UCL classification: UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences > Faculty of Brain Sciences
URI: http://discovery.ucl.ac.uk/id/eprint/1553166
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