Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Health and Rehabilitation Sciences

Supervisor

Dr. Susan Scollie, Dr. David Purcell

Abstract

This thesis aimed to explore the applicability of Cortical Auditory Evoked Potentials (CAEPs) and Envelope Following Responses (EFRs) as objective aided outcome measures for use in infants wearing hearing aids. The goals for CAEP-related projects were to evaluate the effect of speech stimulus source on CAEPs, non-linear hearing aid processing on tone-evoked CAEPs, and the effect of inter-stimulus intervals on non-linear hearing aid processing of phonemes. Results illustrated larger amplitude CAEPs with shorter latencies for speech stimuli from word-medial positions than word-initial positions, and no significant effect of the tone burst onset overshoot due to non-linear hearing aid processing. Inter-stimulus intervals in CAEP protocols resulted in significantly lower aided phoneme levels compared to when they occurred in running speech, illustrating potential inaccuracies in representation of relevant hearing aid function during testing.

The major contribution of this thesis includes the proposal and validation of a test paradigm based on speech-evoked EFRs for use as an objective aided outcome measure. The stimulus is a naturally spoken token /susashi/ modified to enable recording of eight EFRs from low, mid and high frequency regions. The projects aimed to evaluate previously recommended response analysis methods of averaging responses to opposite polarities for vowel-evoked EFRs as well as sensitivity of the proposed paradigm to changes in audibility due to level and bandwidth in adults with normal hearing and additionally, due to amplification in adults with hearing loss. Results demonstrated a vowel-specific effect of averaging opposite polarity responses when the first harmonic was present, however the averaging did not affect detection in the majority of participants. The EFR test paradigm illustrated carrier-specific changes in audibility due to level, bandwidth and amplification suggesting that the paradigm may be a useful tool in evaluating unaided and aided audibility, and therefore appropriateness of hearing aid fittings. Further validation is necessary in infants and children wearing hearing aids.

In conclusion, CAEPs and EFRs vary in strengths and limitations, and therefore it is likely that a combination of measures may be necessary to address the variety of hearing disorders seen in a typical audiological caseload.


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