Graeme Clark Collection
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ItemBrainstem encoding of short voice onset times in natural speech( 2001)An auditory nerve study has shown that short voice onset times (VOTs) in synthetic consonant-vowel syllables are not accurately encoded by the fibres' discharge rate. We have re-examined this issue within the ventral Cochlear nucleus (VCN), using natural speech and a fine-grain analysis of single unit responses. We recorded extracellularly from 93 VCN neurons in rats anaesthetised with urethane (2.5 g/kg ip). After identifying a cell's response type and best frequency (BF), 3 syllables spoken by a male were presented at double rate and 3 intensities (/bεt/, /dεt/, and /gεt/, at 45, 65, and 75 dB SPL). These three syllables differ in their VOTs (the interval between consonant release and the onset of glottal pulses associated with voicing) due to the different points of articulation of the three initial stop consonants. In many neurons (particularly onset cells), these syllables evoked a clear response to consonant release, followed by an interval of inactivity or reduced activity before the periodic response to the vowel's voicing frequency commenced. This interval of reduced or no activity corresponded to a given syllable's VOT. The responses of all cells (BFs: 0.9-19 kHz) to the 9 different syllable-SPL combinations were plotted as Grand Average post-stimulus time histograms. In 8/9 combinations, syllable onset was associated with a statistically significant peak in activity and the next significant peak in discharge rate occurred at the time of voice onset (± I ms). These results indicate that the prominent responses to consonant release and voice onset, produced by the synchronous firing of neurons with a wide range of BFs, accurately encode short VOTs.
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ItemImplantation of the new nucleus C1-3 receiver stimulator and electrode array [Abstract]( 1997)There is an important need to fix the cochlear implant electrode array at a site close to the cochlea, so that the electrode will not slide out, or be subject to differential movement with growth changes. Fixation sites have been in the region of the posterior root of the zygoma and the floor of the antrum. Fixation has been by Dacron mesh ties platinum wire ties, or clips inserted with-special instruments. Biological cements have previously been tried but found to be toxic. The most ideal site is in the region of the cochleostomy.
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ItemSpeech perception in implanted children: effects of preoperative residual hearing and speech processing strategy [Abstract]( 1997)Since the first child was implanted with the Nucleus 22-channel cochlear prosthesis in Melbourne in 1985, the number of implanted children world-wide has rapidly expanded. Over this period, more effective paediatric assessment and management procedures have developed, allowing cochlear implants to be offered to children under the age of 2 years. In addition, a succession of improved speech processing strategies have been implemented in the Nucleus implant system, resulting in increased mean speech perception benefits for implanted adults. Research in the Melbourne and Sydney Cochlear Implant Clinics has also demonstrated that young children can adapt to and benefit from improved speech processing strategies such as the Speak strategy. Reported speech perception results for implanted children show that a considerable proportion (60%) of paediatric patients in the Melbourne and Sydney clinics are able to understand some open set speech using electrical stimulation alone. These results, and the upward trend of speech perception benefits to improve over time with advances in speech processing. have raised questions as to whether severely, or severely-to-profoundly deaf children currently using hearing aids would in fact benefit more from a cochlear implant. To investigate the potential effect of the level of preoperative residual hearing on postoperative speech perception. results for all implanted children in the Melbourne and Sydney cochlear implant programs were analysed. Results showed that as 8 group, children with higher levels of preoperative residual hearing were consistently more likely to achieve open-set speech perception benefits. Potential factors in this finding could be higher levels of ganglion cell survival or greater patterning of the auditory pathways using conventional hearing aids prior to implantation. Conversely, children with the least preoperative residual hearing were less predictable, with some children achieving open-set perception, and others showing more limited closed-set benefits to perception. For these children, it is likely that preoperative residual hearing is of less significance than other factors in outcomes.
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ItemPsychophysics and speech perception with a premodiolar electrode array( 1997)A perimodiolar electrode array, designed to lie closer to the modiolus than the standard Coclear Limited array and developed by the Cooperative Research Centre for Cochlear Implant, Speech and Hearing Research in conjunction with Cochlear Limited, has recently been implanted in an adult patient. While excellent speech results have been obtained using the standard array, it is hypothesised that the improved place coding expected from an array lying closer to the modiolus would result in improved speech perception. Analysis of a modified Stenvers view X-ray revealed that the most apical electrode was inserted to an angle of 311 0 and that the apical half of the array was considerably closer to the modiolus than would have been the case for a standard array. Thresholds and comfortable levels were correlated with the lateral position of the stimulated electrode in the scala tympani, being much reduced for the apical electrodes. Forward masking and electrode discrimination are being measured in order to study the spread of neural excitation and the distinctness of percepts for different electrodes.
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ItemThe development of auditory comprehension in children after receiving a cochlear multiple-channel implant( 1992)Since late 1989, half the cochlear implant patients at the Royal Victorian Eye and Ear Hospital have been young children. There is a gradual improvement of auditory comprehension in most cases using the criteria of environmental sound detection, as well as, prosody, high frequency phoneme and word discrimination. The rate of improvement and final result depend on the duration of deafness, presence of residual hearing, and quality of auditory-oral habilitation. Younger children usually progress more quickly than older children. Some adolescents who use Total Communication and who have no residual hearing, achieve assistance with lipreading. Children with Usher's Syndrome should be actively encouraged to participate in auditory-oral habilitation should they become totally deaf or blind.
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ItemRadiological evaluation of multiple-channel intracochlear implant insertion depth [Abstract]( 1993)Post-operative plain film x-rays are necessary in all multiple-channel cochlear implant patients to confirm intracochlear position of active electrodes, detect possible electrode kinking and provide a reference if post-operative slippage occurs. In addition precise documentation of multiple-channel intracochlear electrode insertion depths is necessary for comparison of speech recognition results between patients and may be of use for future speech processing strategies. In the present study a method has been devised using a modified Stenver's view to more accurately document insertion depths of the electrode array and location of individual electrodes on 50 multiple-channel cochlear implant patients. Surgical estimates of insertion depth are shown to have great variability in regard to distance along the basilar membrane when compared with x-ray documentation. The technique shows promise to facilitate mapping of binaural implant patients, young children, difficult adult cases, and possible new speech processing strategies as well as research activities requiring localization of individual electrodes.
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ItemPerformance of postlinguistically deaf adults using multipeak and MPEAK+A0 speech coding strategies with the nucleus cochlear implant system [Abstract]( 1993)Two independent evaluations of a new speech coding strategy for the Nucleus cochlear implant system, MPEAK+AO, were conducted. The MPEAK+AO strategy adds to the standard Multipeak strategy an extra pulse per each FO-based stimulus period. This additional stimulation is delivered to the most apical electrode in the user's array and represents the amplitude envelope of the low frequency portion of the acoustic signal associated with the voice fundamental.
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ItemCochlear implant skin flap design: the vascular pattern of the postauricular region( 1993)Cochlear implantation proved to be a safe surgical procedure. Complications are rare, but the most significant of them are skin flap related problems, occasionally resulting in removal of the device. To evaluate the vascular pattern of the scalp and its implications for cochlear implant skin flap design we performed a dye injection study on cadavers. Our results on ten specimens indicate, that the blood supply for the skin in the postauricular region is provided inferiorly by indirect musculocutaneous perforators, posteriorly by the occipital artery, superiorly by the superficial temporal artery and anteriorly by the network around the base of the auricle and by cutaneous branches of the postauricular artery. A flap for cochlear implantation raised in this region cannot be based on one single axial source artery and has to rely on a variable number of different arterial contributors, resulting in a combination of random, axial and/or musculocutaneous flap. Inferiorly-based flaps as the inverted U shaped or the extended enaural (Hannover) are considered to be superior to the C-shaped anteriorly-based flap.
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ItemResponses from single units in the dorsal cochlear nucleus to electrical stimulation of the cochlea( 1992)To help improve our understanding of how the brain responds to electrical stimulation of the auditory nerve we have examined the responses of dorsal cochlear nucleus (DCN) units to both acoustic stimulation and electrical stimulation of the cochlea. This work extended our previous studies which have compared the responses to electrical and acoustic stimulation In the auditory nerve (Javel et al 1987, Ann. Otol. Rhinol. laryngeal. Suppl. 128, 96:2630) and the ventral cochlear nucleus (Shepherd et al 1988, NIH Contract NO1-NS-72342, 5th Quarterly Progress Report).