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UBC Theses and Dissertations

Studies of the auditory steady-state responses Herdman, Anthony T.


Scalp-recorded auditory steady-state responses (ASSRs) to multiple stimuli may be useful in objectively estimating hearing thresholds of individuals who are unable to reliably respond behaviourally, such as infants. Primary objectives of the present thesis are: (1) to identify the anatomical location of neurons responsible for generating ASSRs; (2) to determine cochlear place specificity of the multiple-ASSR method; (3) to evaluate the benefit of using the multiple- ASSR method in estimating hearing thresholds, as compared to single-stimulus methods; and (4) to determine the accuracy of using multiple-ASSRs to estimate hearing thresholds in subjects with normal or impaired hearing. Results show that the entire auditory system contributes to the generation of ASSRs to modulated stimuli. Cortical neurons are more responsive at lower modulation frequencies (e.g., <40 Hz), whereas brainstem structures primarily generate ASSRs to higher modulation frequencies (e.g., 80 Hz). ASSRs to multiple amplitude-modulated tones (modulated between 77-105 Hz) reflect activation of approximately 1-octave-wide cochlear regions around each carrier frequency. This is similar to results reported for the auditory brainstem response method. As compared to presenting stimuli separately, simultaneously presenting multiple (i.e., at least 4 per ear) amplitude-modulated tones to evoke the ASSR can considerably reduce the time needed to estimate hearing thresholds for octave frequencies between 500 and 4000 Hz. Furthermore, multiple-ASSR thresholds are not different than thresholds for ASSRs to stimuli presented separately. For individuals with normal or impaired hearing, multiple-ASSR thresholds are approximately 5-15 decibels higher than behavioural thresholds, on average. Furthermore, multiple-ASSRs can accurately estimate the behavioural audiogram configurations (threshold by frequency) in subjects with various types of hearing impairments. Collectively, results from the present thesis indicate that the multiple-ASSR method is useful for evaluating hearing thresholds and provides advantages over the conventional objective audiometric methods.

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