Hidden Hearing Loss: Perception, Neural Signature and Clinical Implications

Abstract

Clinically, the gold standard for diagnosing hearing loss is pure tone audiometry. About 1%-10% of patients undergoing testing in the audiology clinic report having hearing difficulties, without displaying detectable audiometric threshold shifts. This phenomenon is often referred to as hidden hearing loss (HHL). Recent studies discovered that auditory deafferentation – the loss of inner hair cells (IHC) or the loss of synapses between IHC and auditory nerve fibers (ANF) – is not necessarily accompanied by a detectable audiometric threshold shift and might thus be a potential cause of HHL. The relationship between aging or noise exposure and IHC-ANF synapse loss has been established in multiple animal species, and so has the relationship between IHC-ANF synapse loss and HHL-like behavior. However, the perceptual cost of auditory deafferentation, e.g., the loss of afferents via the loss of IHC-ANF synapses, on speech comprehension in humans remains unquantified. Since the loss of IHC-ANF synapses is faster than the loss of outer hair cells (OHC), and HHL is thought to emerge as IHC-ANF synapses are lost, HHL would be expected to emerge earlier than clinical hearing loss. As such, HHL could be used to predict later, more severe hearing loss, which would make early intervention possible. However, while this hypothesis is tempting, to date there is no solid evidence in humans to suggest that people with HHL are more likely to develop clinical hearing loss later in life. This dissertation seeks to investigate the perceptual effects of auditory deafferentation, the neural signatures related to hearing difficulties in noise, and the prognostic significance of speech perception in noise difficulties for clinical hearing loss. Chapter 1 lays out the physiological understanding of auditory deafferentation and highlights gaps in knowledge in the field. Chapter 2 uses a computational encoding-decoding model to quantify the perceptual cost of auditory deafferentation for speech perception and shows that speech perception is relatively robust with severe auditory deafferentation. Chapter 3 investigates whether EEG could be used to indicate the perceptual difficulties in noise and whether EEG based measures are correlated with other indicators of HHL. Chapter 4 examines whether hearing difficulties in noise earlier in life can predict clinical hearing loss later in life with the Baltimore Longitudinal Study of Aging (BLSA) data set. Collectively, these studies demonstrate that (1) hearing difficulties in noise are multifold, (2) peripheral loss can only partially explain hearing difficulties in noise, (3) EEG related measures can be used to indicate subjective hearing quality, and (4) speech perception difficulty in noise predicts the subsequent hearing threshold decline.