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Received Oct 10, 2017; Accepted Dec 26, 2017
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1. Introduction
Sensorineural hearing losses (SNHL) stem from a wide spectrum of diseases affecting the sensory receptors, outer hair cells (OHCs), inner hair cells (IHCs), or auditory neurons (afferent and even efferent fibers) [1–5]. Pure-tone audiometry is the routine clinical audiological test used for measuring hearing sensitivity, and the audiometric classification of hearing impairments is the main basis upon which audiologists determine their rehabilitation choice. Even though it cannot provide any fine-grained reflection of the mechanism of SNHL and may not delineate individual needs, it works in a large majority of cases because it is the damage to OHCs that usually accounts for the hearing impairment, particularly frequency selectivity that, in simple cases, is affected in proportion to the increase in hearing thresholds [6]. However, it is acknowledged that there are “discordant patterns,” that is, subjects with near-normal audiometric thresholds yet difficulties in speech intelligibility, especially in noisy environments [7], Their investigation has led to the discovery of auditory disorders that widely differ from the typical OHC-related SNHL, namely, auditory neuropathies and synaptopathies [8, 9]. Auditory neuropathies are revealed by abnormal auditory-evoked potentials although a recent picture has been substantiated in animal models of short overexposure to intense sounds, which develop noise-induced synaptopathies with no detectable auditory-evoked-potential abnormality [10–12]. In these conditions, pure-tone audiometry is obviously inadequate for predicting suprathreshold auditory perception tasks [13], as discrepancies among metrics of auditory performance are expected when SNHL arises, not from micromechanical stages but from transduction and action-potential generation or conduction. But pure-tone audiometry may also fail to provide a coherent picture of auditory performance in the case of pure OHC dysfunction, as shown by a mutation in the Nherf1 gene expressed only in OHCs at mature stages [14]. The mild ABR hearing threshold elevation of Nherf1−/− mice at high frequencies is contradicted by an absence of high-frequency DPOAEs and by an inordinate sensitivity of mid/high frequencies to low-frequency maskers. This nonconventional functional pattern comes with peculiar OHC hair bundle shape anomalies in the basal part of the...