MEASUREMENTS OF COCHLEAR DEVELOPMENT IN INFANTS

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It has become well accepted that measurements of otoacoustic acoustic emissions (OAEs) provide an assessment of gross cochlear functioning, and as such are becoming an accepted tool for screening infant hearing. They also have provided important information for, and constraints on, the modeling of cochlear dynamics. Increasingly, age-related and stimulus- related changes or differences in OAEs are being interpreted as indicators of changes or differences in cochlear functioning. For example, changes in OAEs with contralateral acoustic stimulation have been proposed as a tool for studying the functioning of the cochlear- efferent system. However some of our previous results, along with results from other laboratories, suggest that changes in middle/external ear functioning can produce changes in OAEs similar to those being interpreted as changes in cochlear functioning. It is important, therefore, to carefully assess changes/differences in OAEs caused by changes/differences in the middle ear. We propose to accomplish this by two methods: one is a continuation of longitudinal measurements of SOAEs in the infants whom we have been following from birth; the other is the concurrent measurement of spontaneous and evoked emissions, along with broadband middle ear input impedance. The second major focus of this project is to determine the extent to which unstable SOAEs may be a factor in the surprisingly high prevalence of tinnitus in normal-hearing children (on the order of 10%). this will be addressed in a study in which SOAEs will be measured in children who are determined to have tinnitus based on their responses to a questionnaire. The evidence suggests that the primary reason that some SOAEs are heard as tinnitus whereas most are not lies in adaptation. Certain frequency and amplitude instabilities in SOAEs lead to a release from adaptation. In a companion study, psychophysical studies of adaptation for low level tones with amplitude and/or frequency variations similar to those we have measured in SOAEs will be performed in order to determine which types of instabilities are most likely to be heard as tinnitus.