The main purpose of this work is to explore whether and how much multichannel signal processing strategies can be beneficial for improving the detection procedure for auditory late response (ALR) in clinical applications in comparison with single channel recording. To achieve this target, four multichannel noise reduction methods based on independent component analysis (ICA) were proposed for noise reduction for multichannel recording of ALR. The four alternative component selection strategies introduced in this work are: Magnitude Squared Coherence (MSC) [based on coherency of the ICs with an evoking stimulus], the maximum Signal to Noise Ratio (Max-SNR) of ICs over a particular interval, the kurtosis (maximum non-Gaussianity of the ICs), and minimum entropy of the ICs. The proposed methods are applied for the noise reduction of auditory late response (ALR) data captured using 63 channel EEG from 10 normal hearing participants. The performances of the proposed methods for improving signal quality were compared with each other and also with the single channel alternatives. All automated component selection approaches produced high SNR for multichannel ALR data. MSC-ICs produced significantly higher SNR than Max-Kurt-ICs or Min-Entropy-ICs. However the performance of MSC-ICs and Max-Fmp-ICs were not significantly different. Therefore, the MSC-ICs approach was selected for further work. MSC-ICs were used for three different clinical applications: Finding hearing threshold level, exploring the effect of attention and exploring inter- and intra- subject variability. The results for MSC-ICs were compared to the single channel signal processing alternative of weighted averaging. The results confirm that the multichannel signal processing can significantly improve the detection procedure for threshold measurement and for measuring the effects of attention. However, no significant enhancement was found for detecting inter- and intra- subject variability with multichannel processing over single channel alternative. The MSC-ICs method was also used in an application for removing cardiac artifact from the ALR recordings and the results was compared with an existing artifact rejection platform based on constraint ICA (cICA). The results of this comparison show that the proposed method can significantly improve the quality of cardiac artifact rejection from ALR data. Finally, the use of MSC-ICs was explored for reducing the required time for recording ALR. Time reduction was investigated in two senses: 1. reducing the number of stimulus repetitions. 2. Optimizing the position and the number of the recording electrodes in multichannel recordings (potentially saving the time required to place many electrodes on the scalp). The results show that using multichannel processing can significantly reduce the number of stimulus repetitions and consequently the time of recording in comparison with the single channel alternative. Minimum required number of stimulus repetition (average over10 subjects) for having SNR equal to single channel processing at Cz was found to be 74 for un-weighted averaging and 85 for weighted averaging. Moreover, the results of optimal electrode placement procedure confirm that, the ALR can be recorded form the vertex (with the same SNR as when ALR is recorded using 63 channels) by using fewer numbers of electrodes. For the data set of this study (10 normal hearing adults) the same SNR as with 63 channels was achieved by using 40 channels. Placing 40 electrodes (instead of 63) on the scalp decreases the required time for recording ALR considerably, i.e. 53% improved.

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.