| Summary: | 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. === Medicine, Faculty of === Audiology and Speech Sciences, School of === Graduate
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