Study Of Auditory Feedback Regulation Using Swept-tone Otoacoustic Emissions

The central auditory system can not only receives sound information through auditory afferent pathway,but also generates feedback information to regulate the activities of outer hair cells(OHCs)inside the cochlea via medial olivocochlear(MOC)efferents,which was called auditory feedback regulation.Auditory feedback regulation is responsible for the wide dynamic range and exquisite frequency selectivity of the cochlea.It also plays an important role in hearing protection,sound localization and speech perception in noisy enviorments.Otoacoutsic emissions(OAEs)are by-products of activities of OHCs,and any changes in OHC activities induced by the auditory feedback could be truthfully reflected in the changes of OAEs.Therefore,the measurement of OAEs could provide a useful way to study the mechanisms of the auditory feedback.However,there are some limitations in existing methods of OAEs measurements,such as the low efficiency,restricted frequency range and the insufficient resolution.As a result,the results from reported studies regarding the auditory feedback regulation are quite controversial.Therefore,a method of using swept tones as the stimulus to evoke stimulus frequency OAEs(SFOAEs)was proposed to study the auditory feedback regulation in high efficiency and high resolution.In this thesis,the physiological basis of auditory feedback regulation,the significance of using OAEs to study the auditory feedback were introduced in the first chapter.The second chapter describes the key technologies of the swept-tone SFOAE measurement,including the synthesis of swept-tone stimulus in the frequency domain,the calibration of the stimulus levels,the experimental protocols to generate the SFOAEs,and the tracking filter algorithms to separate the SFOAEs from background noises.The hardware implementation of the swept-tone SFOAE measurements were also described.The third chapter demonstrated performance of the swept-tone method(including the feasibility,reliability and validity),illustrating that it is a reliable but more accurate way to measure SFOAEs to study the auditory feedback.The feasibility was illustrated by showing that the SFOAEs were successfully measured in the ear,but not in the tube.The reliability was examined by comparing results from repeated measures and the vadility was verified by the comparison with conventional audiogram test.The verification of the performance of the swept-tone method provided a firm foundation for using it to study the auditory feedback in the next chapter.In the fourth chapter,the swept-tone SFOAEs at the ipsilateral ear with and without the presence of an additional stimulus presented at the contralateral ear were measured.The contralateral acoustic stimulation(CAS)was used to activate the MOC efferents to regulate the activities of the OHCs and how the swept-tone SFOAE would change as a result was studied.The spectra of the swept-tone SFOAEs with and without the contralateral stimulus were compared and the possible amplitude changes across frequencies were thoroughly investigated.The effects of different intensities and different types of CAS to activite auditory feedback regulation on the changes of swept-tone SFOAEs were further investigated in the fifth chapter.The method of swept-tone SFOAEs could overcome the limitation of convetional methods to achieve efficient and reliable measurements of OAEs in very high definition.The swept-tone SFOAE technology could provide an innovative approach to investigate the details of the auditory feedback regulation that is controversial in convetional studies and therefore help us to better understanding the working mechanisms of the auditory system.