Graeme Clark Collection
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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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ItemAdvances in cochlear implant speech processing [Abstract]( 1997)Our early research emphasized there was a restriction on the amount of speech and other acoustic information that could be transmitted to the nervous system by electrical stimulation of the auditory nerve. It also showed the need to use multiple-channel stimulation, and minimize channel interaction. As a result our research over the last 30 years has been directed towards optimizing the information presented to the auditory nervous system. This has involved extracting the energy of the first and second formants (FO/F2-WSP II; FO/FI/F2-WSP III; Multipeak-MSP) as well as the outputs of high band pass fixed filters (Multipeak - MSP) and coding these outputs as cochlear place of stimulation. The voicing frequency was coded as rate of stimulation. Our most recent speech processing strategy (SPEAK) extracts a specified number of .maximal outputs from a series of band pass filters, rather than selecting the peaks of energy which was the case with the other strategies. The voltages from the maximal outputs are used to stimulate appropriate electrodes on a place coding basis. The stimuli are presented at a constant stimulus rate to reduce channel interaction. Voicing is conveyed as amplitude variations.
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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.
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ItemTemporal coding in auditory neurons to electrical stimulation [Abstract]( 1997)The temporal response of the auditory pathway following intracochlear electrical stimulation will reflect the level of encoded temporal information, which is important for the further developmentof cochlear implant speech processing strategies, and in tum lead to a better understanding of temporal coding of acoustic stimuli Temporal coding of sound frequencies is based on the phase or time locked neural response seen to low frequency acoustic stimuli. The ability of neurons to respond in a time locked manner may determine the degree of encoded temporal frequency information. Electrophysiological studies have shown that the degree of response synchrony to charge balanced biphasic electrical stimuli is far greater than that seen to acoustic stimuli. We have investigated the temporal response properties of single units in the anteroventral cochlear nucleus (AVCN) in the cat to rates of electrical stimulation up to 800 pulses/s.
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ItemHigh rate electrical stimulation of the auditory nerve: physiological and pathological results [Abstract]( 1995)Previous experimental studies have shown that chronic electrical stimulation of the auditory nerve using charge balanced biphasic current pulses at rates of up to 500 pulses per second (pps) do not adversely affect the adjacent spiral ganglion population. More recently, a number of clinical trials have indicated that speech processing strategies based on high pulse rates (1000 pps and more), can further improve speech perception. In this paper we summarize our results following acute and chronic electrical stimulation of the auditory nerve using high pulse rates.
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ItemLatest results and future directions in speech processing for the Nucleus multichannel cochlear prosthesis [Abstract]( 1995)The past two years has seen the introduction of the Speak speech encoding scheme for most patients using the Nucleus 22-channel cochlear prosthesis. This scheme, based on the Spectral Maxima Speech Processor (SMSP) developed at the University of Melbourne, uses a bank of 20 band-pass filters to present detailed spectral information to the intracochlear electrode array. Clinical trials of this speech processor have shown highly significant improvements over the previous Multipeak scheme in English, German, French and Japanese speaking patients. The largest improvements were evident for open-set testing in background noise, which represents a more realistic measure of everyday benefit than testing in quiet. The latest results for adults who have changed from Multipeak to Speak will be presented, along with results over time for newly-implanted patients using the Speak scheme. New research aimed at improving the speech processing in both the spectral and temporal domains will also be discussed.
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ItemTemporal integration of pulsatile stimuli distributed across multiple electrodes [Abstract]( 1995)Several speech processing strategies in current use present pulsatile stimulation nonsimultaneously on multiple intracochlear electrodes. In most cases the rate of stimulation is constant, and variations in the pulse amplitudes are presumed to contribute to the overall delivery of information to the implant user. However, it is not known whether such amplitude modulations are combined across nearby electrode positions, or whether they produce separable percepts when presented on multiple electrodes. This question, addressing the issue of channel interactions in multiple-electrode implants, is important to the development of advanced sound processing strategies. It was investigated in a psychophysical experiment using amplitude-modulated pulse trains presented at an overall carrier rate of 1000 Hz, with pulses being delivered alternately to two electrodes. The modulation had a period of 10 ms, within which two pulses (one on each electrode) had a high amplitude, while the remaining eight had a low amplitude. The time was varied between the two high pulses in each period (which can be regarded as a phase shift). The smallest time delay (1 ms) produced an overall 100 Hz modulation pattern, whereas the largest delay (5 ms) produced a 200 Hz modulation pattern. All of the stimuli produced identical 100 Hz patterns on each of the component electrodes, and so would be distinguishable only if the combined pattern were being perceived. The hypothesis was that this combined pattern would be perceived only for small electrode separations. This hypothesis was confirmed experimentally in five subjects by using a four-interval forced-choice task where the subject was asked to identify the one stimulus which was different. Phase differences of 1 - 2 ms could be detected provided that the electrode separation was within about 3 - 4 mm. A subsequent experiment using single-interval pitch estimation tasks showed that the differences in the temporal patterns were perceived in the same way as rate pitch differences. In conclusion, our experiments showed that, when modulated pulsatile stimuli are delivered to nearby electrode positions, implantees may perceive the modulations of the combined stimuli and not just the modulations on the separate electrodes.