The current behavioural dead region (DR) diagnosis methods such as psychophysical tuning curves and the threshold-equalising noise test require extensive subject co-operation. These present methods cannot be applied to infants. The work presented in the thesis aimed to develop a fast objective DR diagnosis method that could be applied to sleeping hearing-impaired infants. A novel fast objective electrophysiological method of recording response amplitude curves (RACs) which could enable objective DR diagnosis was developed.RACs were derived by recording auditory steady-state response amplitudes using modulated signals in the presence of narrow-band maskers. Two RAC methods were investigated. In the swept method, RACs were recorded in a single test run by recording the response amplitudes across the frequency range of a continuously swept-frequency narrow-band masker. In the fixed method, response amplitudes of eight separate test runs, each in the presence of differing fixed-frequency narrow-band maskers, were recorded.RACs were recorded in normally hearing adult subjects. The results showed that for normally hearing subjects in condition 1 (swept masker), the mean recorded RAC tip for a 2-kHz signal was 2250 Hz and the repeatability coefficient of two repeated recordings in each subject was 389 Hz; in condition 2 (fixed masker), the respective values were 2251 Hz and 342 Hz. These results indicated that the swept masking method is a viable and fast way to record RACs in normally hearing adults.RACs and psychophysical tuning curves (PTCs) were recorded in hearing-impaired adult subjects in order to asses the tip-frequency agreement between the tests. In some cases there were difficulties in using the required signal and masker levels due to maximum sound level limits. The RACs were poorly shaped and had poor repeatability. These findings indicate that the RAC method that was successfully applied to normally hearing subjects requires further development for use with the hearing impaired. The possible causes for the differences in the accuracy of the method between normally hearing and hearing-impaired subjects are discussed. The work presented in this thesis provides the basis upon which further research can be taken forward. It is envisaged that this work, together with further research, will lead to a clinically-effective objective DR diagnosis method.

Cochlear dead regions (DRs) are areas in the cochlea where inner hair cells and/or neurones are functioning so poorly that a sound that causes peak basilar membrane motion in that region is more efficiently detected via off-frequency listening. The Threshold Equalising Noise (TEN) test is a clinical test procedure for detecting DRs. Psychophysical Tuning Curves (PTCs) can be used to identify the boundary frequency of the DR although the clinical importance of doing this has yet to be determined. Some studies have suggested that the reduction of amplification well inside the DR may be beneficial; however, other studies have been unable to replicate these findings in a more typical clinical population. Three studies were completed in order to:1. determine the prevalence of DRs in a clinical sample of the UK adult population,2. investigate repeatability, agreement and clinical feasibility of the TEN-test and fast PTCs in a clinical setting, and 3. determine the benefit of high-frequency amplification in ears with and without DRs, when listening to nonsense syllable speech material in quiet and babble. In the first study, 343 hearing-impaired adults were tested for DRs using the TEN-test. In total, 36% (95% confidence interval 31-41) of these adults had a DR in at least one ear, but frequently at 4 kHz only. Only 3% (1-5) of participants had a DR spanning more than three consecutive frequencies. These findings suggest that DRs usually only span 1 or 2 clinically-relevant frequencies. In the second study, the TEN-test was completed on 70 ears at frequencies between 0.5 and 4 kHz. Fast PTCs were measured on 20 ears at ≥ 2 frequencies. The TEN-test and fast PTCs were highly repeatable on retest (97% and 100%, respectively). There was 87% agreement between the two procedures in terms of the presence of off-frequency listening, with the TEN-test less likely to detect a DR than fast PTCs. Compared to the TEN-test, fast PTCs had a 10% lower ‘conclusive finding’ rate and the test duration was typically 40 minutes longer. Therefore, the TEN-test is more clinically acceptable, but it may underestimate the extent of a DR because of its inability to precisely identify the boundary frequency. In the third study, 18 ears with a high-frequency DR and 18 matched ears without a DR were tested. Vowel-Consonant-Vowel (VCV) stimuli were presented in quiet and babble when listening with an unfiltered and three low-pass filtered hearing aid settings. Best performance was obtained in the unfiltered condition; however the DR group performed significantly poorer than the controls in babble. There was no evidence to support reducing amplification in ears with a DR. However, participants with DRs may benefit from counseling about the limitations of listening in noise. In summary, DRs are relatively prevalent in hearing-impaired adults and can be diagnosed most efficiently in a clinical setting using the TEN-test. However, DRs are often restricted to a narrow frequency range and, in the typical adult clinical population, there is no evidence to support deviating from prescription targets.

Objective: To date, the main focus in frequency lowering hearing aid studies has been in relation to speech perception abilities. With improvements in hearing aid technology, there is a growing interest in musical perception as a dimension that could improve hearing aid users’ quality of life. The purpose of this study was two-fold: Firstly, to develop a test of music perception for adult hearing aid users and secondly, to evaluate the influence of non-linear frequency compression (NFC) on music perception with the use of the Music Perception Test (MPT) compiled by the researcher. Research design and research sample: Phase 1 entailed the compilation of the MPT and can be described as design-based. A quasi-experimental research design was selected to establish the structure of the method employed in Phase 2, which involved the fitting of participants (n=40) with NFC hearing aids. Objective data was obtained with the hearing aids with NFC active and inactive. Phase 3 was characterized by a survey design which elicited subjective impressions of the participants’ musical experiences with NFC active and inactive. Results: Results proved that normal hearing adults as well as adults using hearing aids were able to complete all the sub-tests of the MPT. Furthermore, the use of NFC resulted in a statistically significant improvement in hearing aid users’ perception of timbre and melody, but not of pitch. Overall, no statistically significant improvement in their perception of rhythm was observed, although their performance on some rhythm sub-tests improved significantly. The use of NFC also brought about a statistically significant improvement in hearing aid users’ perception of the music qualities of overall fidelity, tinniness and reverberance. Although participants experienced the loudness, fullness, crispness, naturalness and pleasantness of music more positively with NFC, these benefits were not statistically significant. Conclusion: The MPT can be used successfully for assessing music perception in hearing aid users within the South African context and may therefore result in more accountable hearing aid fittings. The use of NFC may increase hearing aid users’ appreciation of music whilst not influencing music perception negatively. Given that a large percentage of hearing aid users express a loss in enjoyment of music, audiologists should not ignore the possible benefits of NFC, especially if one takes into account that previous research indicated speech perception benefits with this technology. / Thesis (DPhil)--University of Pretoria, 2012. / Speech-Language Pathology and Audiology / Unrestricted