Otolaryngology - Theses
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ItemSpeech production skills, vocabulary development, and speech perception abilities in children with hearing loss: intervention and outcomes( 2007)Despite early diagnosis, early fitting of more advanced sensory aids, early intervention, and intensive educational management, many children with severe to profound hearing loss are delayed in their acquisition of spoken language compared with their peers with normal hearing. Some of the greatest challenges facing educators of children with hearing loss include determining where to focus intervention in order to maximise benefit, and establishing the most effective strategies for the development of age-appropriate language. The experimental research in this thesis, conducted across three studies, examined the relationship between hearing, speech production, and vocabulary knowledge, and investigated the contributions of these factors to the overall speech perception performance of children with hearing loss. This research also investigated the areas in which intervention would be most beneficial, and examined the effects of different types of intervention on the development of spoken language and speech perception skills in children with hearing loss. The first study collected and analysed data to validate a simple non-linear mathematical model that describes the effects of hearing, vocabulary knowledge, and speech production on the perception test scores for monosyllabic words by children with hearing loss. Thirty-three primary-school children with hearing loss, fitted with hearing aids and/or cochlear implants, were evaluated, using speech perception, reading-aloud, speech production, and language measures. Results from these measures were analysed using the mathematical model. It was found that performance on an open-set speech perception word test in the auditory-alone mode is strongly dependent on residual hearing levels, lexical knowledge, and speech production abilities. Further applications of the model provided an estimate of the effect of each component on the overall speech perception score for each child. The separation of these components made it possible to ascertain which children would benefit most from specific language intervention, and which children would benefit from more advanced sensory aids. However, further investigation of the effectiveness of different intervention strategies on the development of speech perception skills is required. In the second study, 12 primary school-aged children with hearing loss participated in two types of speech production intervention to determine which was the most effective in improving speech production skills. After an 8-week intensive program, speech production skills improved for all children, with greater improvements evident in the articulation of phonemes trained at a phonological level. Untrained vowels and consonants also improved after intervention. These findings suggest that intensive speech production intervention in the context of words, sentences, and discourse is effective not only in improving the production of those phonemes trained, but may also result in the generalisation of taught speech skills into other aspects of children's spoken language. The final study applied the mathematical model postulated in the first study to the speech perception scores of 21 primary school-aged children with hearing loss. The children participated in intensive speech production and vocabulary intervention programs. The speech production intervention program implemented the strategies that were found to be effective in the second study, while the vocabulary intervention involved learning the meanings of words. The speech production intervention produced a small but significant improvement in the production of consonants in words, while the vocabulary intervention improved knowledge of word meanings substantially. Both types of intervention significantly improved speech perception performance. These findings demonstrate that the relationships between speech perception, speech production, and vocabulary knowledge are causal rather than merely associative. The application of the model also assisted in identifying the most effective methods of improving receptive and expressive spoken language skills for individual children with hearing loss. In summary, the results from this research provided further evidence of the complex relationship between hearing, speech perception, vocabulary knowledge, and speech production. This research highlights the factors requiring consideration in the interpretation of speech perception scores. Separation of the contributions of hearing, lexical knowledge, and speech production to speech perception scores enabled a better understanding of factors contributing to children's performance levels, and facilitated the development of more appropriate intervention. Speech production and vocabulary intervention were shown to be valuable and beneficial in the individual education programs of many children with hearing loss who exhibit delays in spoken language skills. The evaluation, analysis, and intervention methods reported in this thesis provide an experimentally validated program for improving speech perception, speech production, and spoken language skills of school-aged children with hearing loss.
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ItemCochlear implant sound coding with across-frequency delays( 2009)The experiments described in this thesis investigate the temporal relationship between frequency bands in a cochlear implant sound processor. Initial studies were of cochlea-based traveling wave delays for cochlear implant sound processing strategies. These were later broadened into studies of an ensemble of across-frequency delays. Before incorporating cochlear delays into a cochlear implant processor, a set of suitable delays was determined with a psychoacoustic calibration to pitch perception, since normal cochlear delays are a function of frequency. The first experiment assessed the perception of pitch evoked by electrical stimuli from cochlear implant electrodes. Six cochlear implant users with acoustic hearing in their non-implanted ears were recruited for this, since they were able to compare electric stimuli to acoustic tones. Traveling wave delays were then computed for each subject using the frequencies matched to their electrodes. These were similar across subjects, ranging over 0-6 milliseconds along the electrode array. The next experiment applied the calibrated delays to the ACE strategy filter outputs before maxima selection. The effects upon speech perception in noise were assessed with cochlear implant users, and a small but significant improvement was observed. A subsequent sensitivity analysis indicated that accurate calibration of the delays might not be necessary after all; instead, a range of across-frequency delays might be similarly beneficial. A computational investigation was performed next, where a corpus of recorded speech was passed through the ACE cochlear implant sound processing strategy in order to determine how across-frequency delays altered the patterns of stimulation. A range of delay vectors were used in combination with a number of processing parameter sets and noise levels. The results showed that additional stimuli from broadband sounds (such as the glottal pulses of vowels) are selected when frequency bands are desynchronized with across-frequency delays. Background noise contains fewer dominant impulses than a single talker and so is not enhanced in this way. In the following experiment, speech perception with an ensemble of across-frequency delays was assessed with eight cochlear implant users. Reverse cochlear delays (high frequency delays) were equivalent to conventional cochlear delays. Benefit was diminished for larger delays. Speech recognition scores were at baseline with random delay assignments. An information transmission analysis of speech in quiet indicated that the discrimination of voiced cues was most improved with across-frequency delays. For some subjects, this was seen as improved vowel discrimination based on formant locations and improved transmission of the place of articulation of consonants. A final study indicated that benefits to speech perception with across-frequency delays are diminished when the number of maxima selected per frame is increased above 8-out-of-22 frequency bands.