Graeme Clark Collection
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ItemContributing factors to improved speech perception in children using the nucleus 22-channel cochlear prosthesis( 1997)It has been established that use of multiple-channel intracochlear implants can significantly improve speech perception for postlinguistically deafened adults. In the development of the Nucleus 22-channel cochlear implant, there have been significant developments in speech processing strategies, providing additional benefits to speech perception for users. This has recently culminated in the release of the Speak speech processing strategy, developed from research at the University of Melbourne. The Speak strategy employs 20 programmable bandpass filters which are scanned at an adaptive rate, with the largest outputs of these filters presented to up to ten stimulation channels along the electrode array. Comparative studies of the Speak processing strategy (in the Nucleus Spectra-22 speech processor), with the previously-used Multipeak (Multipeak) speech processing strategy (in the Minisystem-22 speech processor), with profoundly deaf adult cochlear implant users have shown that the Speak processing strategy provides a significant benefit to adult users both in quiet situations and particularly in the presence of background noise. Since the first implantation of the Nucleus device in a profoundly hearing-impaired child in Melbourne in 1985, there has been a rapid growth in the number of children using this device. Studies of cochlear implant benefits for children using the Nucleus 22-channel cochlear implant have also shown that children can obtain significant benefits to speech perception, speech production and language, including open-set understanding of words and sentences using the cochlear implant alone. In evaluating contributing factors to speech perception benefits available for children, four specific factors are important to investigate: (1) earlier implantation -resulting from earlier detection of deafness; (2) improved hardware and surgical techniques -allowing implantation in infants; (3) improved speech processing, and (4) improved habilitation techniques. Results reported previously have been recorded primarily for children using the Multipeak strategy implemented in the MSP speech processor. While it is important to evaluate the factors which might contribute to improvements in speech perception benefits, an important question is the effect of improved speech processing strategy, since this will determine what is perceived through the device. Given that adult patients changing to the Spectra speech processor had also shown improved perception in noisy situations, and the fact that children are in general in noisy environments in the classroom setting for a large proportion of their day, it was of obvious interest to evaluate the potential for benefit in poor signal-to-noise ratios from use of the Speak processing strategy and from specific training in the ability to perceive in background noise. The study was aimed at evaluating whether children who were experienced in use of the Multipeak speech processing strategy would be able to changeover to the new Speak processing strategy, which provides a subjectively different output. Secondly, the study aimed to evaluate the benefits which might accrue to children from use of controlled habilitation in background noise.
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ItemInvestigations on a curved intracochlear array( 1995)The electrode array of a multiple-channel cochlear implant lies against the outer wall of the scala tympani. From this position electrical current spreads to excite residual neural elements, particularly spiral ganglion cells within the modiolus. It is not clear whether the spread of current from the outer wall is optimal for multiple-channel speech processing, but placement closer to the target nerves could result in lower thresholds. This could have benefits through the use of shorter pulse durations and extended battery life. Computer modeling studies and animal experiments have suggested that for localized current the optimal electrode position is adjacent to the modiolus. At the University of Melbourne it was felt that an electrode with a curve matching the internal cochlear spiral would remain close to the modiolus after insertion. A curved electrode was developed and an inserting tool was designed and produced (Treaba et al, this suppl, this section). Preliminary studies suggested that the electrode array did indeed remain close to the modiolus. Before further development of this type of electrode design, it was necessary to determine whether modifications to the surgical technique for its insertion were required. It was also important to ensure that the curved electrode fabricated for clinical trial would lie closer to the modiolus than to the outer wall of the scala tympani. This study was undertaken to examine these issues.
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ItemSpeech perception in children using the advanced Speak speech-processing strategy( 1995)The Speak speech-processing strategy, developed by the University of Melbourne and commercialized by Cochlear Pty Limited for use in the new Spectra 22 speech processor, has been shown to provide improved speech perception for adults in both quiet and noisy situations. The present study evaluated the ability of children experienced in the use of the Multipeak (Mpeak) speech-processing strategy (implemented in the Nucleus Minisystem-22 cochlear implant) to adapt to and benefit from the advanced Speak speech-processing strategy (implemented in the Nucleus Spectra 22 speech processor). Twelve children were assessed using Mpeak and Speak over a period of 8 months. All of the children had over 1 year's previous experience with Mpeak, and all were able to score significantly on open-set word and sentence tests using the cochlear implant alone. Children were assessed with both live-voice and recorded speech materials, including Consonant-Nucleus-Consonant monosyllabic words and Speech Intelligibility Test sentences. Assessments were made in both quiet and in noise. Assessments were made at 3-week intervals to investigate the ability of the children to adapt to the new speech-processing strategy. For most of the children, a significant advantage was evident when using the Speak strategy as compared with Mpeak. For 4 of the children, there was no decrement in speech perception scores immediately following fitting with Speak. Eight of the children showed a small (10% to 20%) decrement in speech perception scores for between 3 and 6 weeks following the changeover to Speak. After 24 weeks' experience with Speak, 11 of the children had shown a steady increase in speech perception scores, with final Speak scores higher than for Mpeak. Only 1 child showed a significant decrement in speech perception with Speak, which did not recover to original Mpeak levels.
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ItemTemporal coding of frequency: neuron firing probabilities for acoustic and electric stimulation of the auditory nerve( 1995)A better understanding of the temporal coding of frequency, and its application to electrical stimulation of auditory nerve fibers, should lead to advances in cochlear implant speech processing. Past research studies have suggested that the intervals between nerve action potentials are important in the temporal coding of frequency. For sound frequencies up to approximately 500 Hz, the shortest or predominant intervals between the nerve action potentials are usually the same as the periods of the sound waves. The intervals between each nerve action potential can be plotted as an interval histogram. Although there is evidence that the intervals between spikes are important in the temporal coding of frequency, it is not known up to what frequency this applies. It is also not known whether the information transmitted along individual fibers or an ensemble of fibers is important, to what extent the coding of frequency is interrelated with the coding of intensity, the relative importance of temporal and place coding for different frequencies, and finally, how well electrical stimulation can simulate the temporal coding of sound.
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ItemCross-fiber interspike interval probability distribution in acoustic stimulation: a computer modeling study( 1995)Electrical stimulation strategies for cochlear implants may be improved by studying temporal frequency coding in single auditory fibers and across fibers in acoustic stimulation (Clark et al, this suppl, section 5). In single nerve fibers, phase locking between action potentials and the acoustic stimulus can only be maintained at frequencies below about 600 Hz. At these frequencies, the time interval between successive action potentials, called the interspike interval (lSI), is distributed around the period of the stimulus, and it can therefore be used to code frequency within single fibers. At higher frequencies, the phase locking of individual nerve fibers diminishes, but it may still be possible to retain phase-locking properties by combining the action potentials in an ensemble of nerve fibers. In an ensemble of fibers, the lSI in each nerve is affected by factors such as the spectral shape of the stimulus, the characteristic frequency, and the firing characteristics of the nerve. The lSI between the fibers, however, is further affected by the propagation or phase delay of the traveling wave. It is therefore uncertain how these factors would affect frequency coding across fibers. It is possible that the propagation delay between the fibers may lower the phase locking in an ensemble of nerves -because the probability that the majority of nerves in an ensemble will fire simultaneously may be low. It is also possible that the combined firing statistics of the fibers in an ensemble may result in a higher degree of synchrony such that the predominant intervals in an ensemble are preserved over a wider frequency range than in a single fiber. Are these accurate postulations of the physical system? In a future electrical stimulation strategy that incorporates temporal frequency coding, is it necessary to mimic the spatial-temporal delay in the firing patterns caused by the traveling wave? These are important questions that need to be studied and answered. (From Introduction)
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ItemTemporal coding of speech information for cochlear implant patients( 1987)A modified speech-processing strategy incorporating the temporal coding of information strongly correlated with the first formant of speech was evaluated in a long-term clinical experiment with a single patient. The aim was to assess whether the patient could learn to extract information from the time domain in addition to the time domain cues for voice excitation frequency already received from the initial strategy. It was found that the patient gained no significant advantage from the modified strategy, but there was no disadvantage either, and the patient expressed a preference for the modified strategy for everyday use.
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ItemSpeech perception in implanted children: effects of speech processing strategy and residual hearing(Mendoza Editor, 1997)The ability of implanted children to adapt to different speech processing strategies has been demonstrated for the Nucleus implant system. Children previously experienced with the Multipeak speech processing strategy. were able to gain significant improvements in consonant, word and sentence perception using the Speak speech processing strategy. suggesting some degree of neural plasticity in neural-auditory coding. Of 192 implanted children with different degrees of preoperative residual hearing, 65% were found to obtain significant scores on open-set speech materials using electrical stimulation alone. Those children with more residual hearing had a greater probability of achieving open-set understanding and at a minimum level, perceived high frequency consonant information which would not have been available through conventional hearing aids.
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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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ItemA stimulation of spatio-temporal firing across auditory nerve fibres( 1997)Present cochlear implant speech processing strategies give recipients a perception of sound inferior to that of the normal hearing population. Since it is beyond current technology to achieve an electrically evoked auditory-nerve output identical to that of normal hearing, stimulation strategies are limited to approximating certain features of the neural firing patterns. The importance of the spatio-temporal firing patterns of an ensemble of auditory nerve fibres to speech perception has been stated in previous studies (1,2). This paper utilises a composite model of the cochlea and hair-cell/auditory nerve transduction using artificial and speech signals as input to produce a spatio-temporal excitation pattern which represents the fluctuating firing probability of the auditory neurons. A model of electrical stimulation of the auditory nerve is then used to show how stimulation strategies currently used produce neural firing patterns qualitatively different to those produced by normal hearing. Our investigations indicate that it is possible to generate electrical stimulation parameters that cause the spatio-temporal responses of the neural population to better approximate normal hearing. These responses enable us to identify stimulation parameters required to obtain the chosen neural firing patterns. A number of examples illustrate the utility of this method, revealing the spatio-temporal responses for varying numbers of neurons and electrode displacements.