Graeme Clark Collection
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ItemSpatial representation of the cochlea within the inferior colliculus of neonatally deafened kittens following chronic electrical stimulation of the auditory nerve [Abstract]( 1995)The orderly tonotopic representation of the cochlea is accurately reproduced within the central auditory system of normal hearing animals. Any degradation of this representation as a result of a neonatal hearing loss or chronic electrical stimulation during development could have important implications for the use of multichannel cochlear implants in young children. In the present study we have used 2-deoxyglucose autoradiography (2-00) to examine the topographic representation of the cochlea within the inferior colliculus (IC) of neonatally deafened kittens following periods of chronic intracochlear electrical stimulation.
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ItemThe pitch of amplitude-modulated electrical stimuli in cochlear implantees [Abstract]( 1993)The ability of cochlear implantees to detect amplitude modulation of pulsatile electrical stimulation, suggests that some speech feature information may be conveyed effectively by this means. For example, modulations at the fundamental frequency of speech may provide a voice pitch percept to implantees, particularly in speech processing strategies which generate constant-rate stimulation. The pitch evoked by sinusoidally modulated current pulse trains on a single electrodes has been studied. Modulation frequencies of 100, 150 and 200Hz, and carrier pulse rates varying from 200 to 1200Hz, were used. The results showed that the pitch of the stimulation was related to the modulation frequency, provided that either the carrier rate was a multiple of the modulation frequency, or the carrier rate was sufficiently high (at least four times the modulation frequency for the stimuli studied here). Furthermore, when the modulated stimuli were matched in pitch to non-modulated pulse trains, it was. found that the rate of the matched non-modulated stimuli was close to but somewhat higher than the modulation frequency. This difference depended on the carrier rate and varied among subjects.
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ItemA "Combionic Aid": Combined speech processing for a cochlear implant in one ear and speech processing hearing aid in the other ear [Abstract]( 1993)Independent use of a cochlear implant in one ear and a hearing aid in the other is not acceptable for many implant users with some residual hearing. Psychophysical evidence suggests that there are substantial interactions between acoustic and electrical signals including masking and loudness summation. These effects may contribute to the difficulty in using two independent devices and it is desirable to control the parameters of the electrical and acoustical signals far more accurately than is possible with two independent devices with separate microphones. In order to achieve this control we have developed a Combionic aid incorporating an implant and an 'in1planlcompatible' hearing aid controlled from the same speech processor. The new processor is particularly flexible and can implement a wide variety of speech processing strategies for combined acoustic and electrical stimulation. A benchtop prototype has been tested with five patients using a range of different speech tests. In general, patients do better when they use acoustic and electrical information simultaneously than they do with either alone. Some patients on some tests perform significantly better with the bimodal aid than they do with independent hearing aids and implant processors worn together. Wearable devices have now been built and evaluations of these devices are continuing.
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ItemDecrement in auditory nerve function following acute high rate stimulation in guinea pigs [Abstract]( 1995)Cochlear implants have been shown to successfully provide profoundly deaf patients with auditory cues for speech discrimination. Psychophysical studies suggested that speech processing strategies based on stimulus rates of up to 1000 pulses per second (pps) may lead to an improvement in speech perception, due to a better representation of the rapid variations in the amplitude of speech. However, "neural fatigue" has been known to occur following brief periods of electrical stimulation at rates high enough to ensure that stimuli occur within the neurons relative refractory period, and has been shown to depend on stimulus duration and rate of the evoked neural activity. Prolonged electrical stimulation at these high stimulus rates could, therefore, have an adverse effect on the neurons metabolism and result in cellular energy depletion.
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ItemThe auditory cortex and auditory deprivation: experience with cochlear implants in the congenitally deaf [Abstract]( 1995)The primary auditory cortex (AI) exhibits a topographic representation of the organ of Corti in normal hearing animals. Plasticity studies have shown that this orderly representation of frequency can be modified following a restricted hearing loss or by behavioural trainingl,2. Little is known, however, of the effects of a profound hearing loss on AI, although a number of early studies have suggested an enhancement of activity from other modalities3. Knowledge of the functional status of the central auditory pathway in the profoundly deaf, and the ability of these structures to undergo reorganization particularly following long periods of auditory deprivation - are important issues for the clinical management of cochlear implant patients. In this paper we review our recent clinical and experimental experience with cochlear implants in the congenitally deaf.
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ItemResults of multichannel cochlear implantation in very young children [Abstract]( 1995)Most researchers and clinicians working in the cochlear implant field have assumed that profoundly deaf children will have a better prognosis in terms of speech perception, speech production and language development, implanted at as young an age as possible. However, it has been difficult to gather direct evidence for this hypothesis due to the problems in assessing children under the age of five years with formal tests.
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ItemSpeech perception benefits for children using the Speak speech processing strategy in quiet and noise [Abstract]( 1995)The Speak speech processing strategy, based on the Spectral Maxima Speech Processor (SMSP) developed at the University of Melbourne, has now been implemented in the Spectra 22 speech processor developed by Cochlear Pty Limited, and clinical trials of both patients changing from the previous Multipeak strategy to Speak and patients starting up with Speak have been conducted. Results in adult patients changing to Speak have shown significant improvements in speech perception in quiet and particularly in background noise as compared with Multipeak.
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ItemSpeech perception benefits for implanted children with preoperative residual hearing [Abstract]( 1995)Since the implantation of the first children with the Nucleus 22-channel cochlear prosthesis in Melbourne in 1985, there has been rapid expansion in the number of implanted children world-wide. Improved surgical technique and experience in paediatric assessment and management have contributed to a trend to implant very young children. At the same time there has also been continuing development of improved speech processing strategies resulting in greater speech perception benefits.
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ItemSpeech perception benefits for implanted children with preoperative residual hearing [Abstract]( 1995)Since the implantation of the first children with the Nucleus 22-channel cochlear prosthesis in Melbourne in 1985, there has been rapid expansion in the number of implanted children world-wide. Improved surgical technique and experience in paediatric assessment and management have contributed to a trend to implant very young children. At the same time there has also been continuing development of improved speech processing strategies resulting in greater speech perception benefits. In the Melbourne program, over 60% of children obtain significant scores on open-set word and sentence tests using their cochlear implant alone without the aid of lipreading. As parents and professionals have become aware of these improved benefits to speech perception benefits in profoundly deaf children, there have been requests to consider implanting severely-to-profoundly deaf children. In these children with higher levels of residual hearing, only those children with poorer-than-expected performance on speech perception tests using hearing aids have been considered for surgery. A number of such cases have now been implanted in the Melbourne program. The speech perception benefits for this group are reported and are being compared with benefits for the profoundly deaf group of children.
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ItemSpeech perception benefits for children using the Speak speech processing strategy in quiet and noise [Abstract]( 1995)The Speak speech processing strategy, based on the Spectral Maxima Speech Processor (SMSP) developed at the University of Melbourne, has now been implemented in the Spectra 22 speech processor developed by Cochlear Pty Limited, and clinical trials of both patients changing from the previous Multipeak strategy to Speak and patients starting up with. Speak have been conducted. Results in adult patients changing to Speak have shown significant improvements in speech perception in quiet and particularly in background noise as compared with Multipeak. Preliminary studies with children changing from Multipeak to Speak strategy, measured over a 10 month period, have also shown significant benefits from use of the Speak scheme in both quiet and noisy test situations. Results of follow up studies of these children after more than one year experience with the Speak processing strategy are presented. Statistical analysis of performance over time suggests that an increase in benefit is observed in children after additional experience with the Speak processing strategy. In addition, results for children who have used only the Speak processing strategy from the time of implantation are also presented. The results confirm that the Speak processing strategy provides significant benefits in quiet, and particularly in the presence of background noise for both groups of patients.