Otoacoustic emission (OAE)-based measurement of the functioning of the human cochlea and the efferent auditory system

The discovery of otoacoustic emissions (OAE) has advanced our understanding of cochlear mechanics and the efferent auditory system. OAE are sounds generated within normal cochlea either spontaneously or in response to stimulation. The ability to measure OAE non-invasively, objectively and quickly makes a powerful tool to probe cochlear mechanics. Stimulation of the efferent auditory system causes changes in cochlear amplification processes and hence changes characteristics of OAE. Contralateral acoustic stimulation, commonly called OAE suppression, provides an index of the efferent auditory system (specifically, medial olivocochlear bundle) functioning. OAE is also a sensitive tool to demonstrate subtle changes in cochlear functioning caused by various pathological (e.g., noise exposure, aspirin toxicity, etc.) and non-pathological (e.g.,posture, efferent stimulation) factors. Although OAE are frequently used in both clinic and laboratory, their generation mechanism was not clearly understood until recently. It is currently accepted that distortion product otoacoustic emissions (DPOAE) are composed of two separate components, named wave- and place-fixed emissions. They not only arise from two different cochlear locations but also from two fundamentally different processes. Wave-fixed components arise from distortion sources and manifest a phase that is almost independent of frequency, where as, place-fixed components arise from reflection sources and have a phase that increases systematically with frequency. The overall aim of the work presented in this thesis was to use various OAE methods to examine cochlear function and the efferent auditory system. A related objective was to substantiate the functional relevance of the efferent auditory system in speech-in-noise perception, in order to address the clinical significance of measuring OAE suppression. Cochlear functioning was potentially manipulated by three treatments separately: one extrinsic (electromagnetic radiation exposure from mobile phone) and two intrinsic (posture and efferent activation). Potential changes in auditory function due to mobile phone exposure were evaluated in a within-subject study in a double-blind design (n=35). A comprehensive examination of the auditory system was conducted using audiometry,OAE and auditory event related potentials (ERP). The second experiment used mechanism-based DPOAE to investigate posture-induced changes in cochlear functioning (n=15). Similar DPOAE measurements were performed to evaluate the effect of contralateral acoustic stimulation on cochlear functioning (n=14). The last experiment examined the relationship between contralateral suppression of transient evoked otoacoustic emissions (TEOAE) and recognition of speech in noise (n=13). Results indicate that (i) acute exposure to mobile phone radiation does not cause any significant changes in auditory functions measured by TEOAE suppression, DPOAE or ERP (however, there were changes in auditory thresholds at 6 and 8 kHz), (ii) posture induced cochlear changes and contralateral acoustic stimulation cause significantly greater reduction in place-fixed components than wave-fixed components, and (iii) the efferent auditory system plays an anti-masking role in speech-in-noise recognition. It appears that wave- and place-fixed components are differentiallysensitive to changes in cochlear functioning. Collectively, the present results provide emerging empirical support for the need to separate the wave- and place-fixed components in DPOAE measurements. Because of inherent differences in the generation of wave- and place-fixed components, it is suggested that the separation of the components may improve the efficiency of DPOAE-based measures of cochlear dysfunction and also, of the efferent auditory system function.