Graeme Clark Collection
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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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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.
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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. if 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. Recent results with older children have supported the view that early implantation may provide the optimal outcome in most cases. The implantation of very young children raises two areas of concern that do not apply in adults and older children: accurate assessment of degree of hearing loss and auditory potential; and postoperative assessment of outcomes. This paper will describe research results from the University of Melbourne which address these issues and present results for children implanted as young as eighteen months of age.
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ItemSafety studies for a prototype Nucleus 22 channel implant at high stimulation rates [Abstract]( 1995)The safe stimulation of neural tissue requires that the stimulation does not produce any toxic electrochemical by-products and that the nerves are not damaged by the very act of responding to the stimulation. The Nucleus stimulator has been proven safe in several chronic animal studies using pulse rates of up to 500 pps and stimulus levels that produced sensations of moderate loudness, and subsequently in clinical use by more that 10,000 patients. Additional safety studies are necessary before considering the use of the Nucleus stimulator at higher rates. This paper describes in vitro investigations using such high rates while animal studies are currently under way to examine the effects of chronic, high rate stimulation.
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ItemMusical pitch perception by a user of the Nucleus 22-electrode cochlear implant [Abstract]( 1995)The study of how musical sounds are perceived by users of cochlear implants is both interesting and rewarding. In particular, it has the potential to provide detailed information on the perception of pitch with electrical stimulation of the auditory nerve. However, it is difficult to find subjects whose understanding of musical terminology and memory of conventional musical pitch relationships are adequate for this research. In our Melbourne laboratory, we have been fortunate to locate one implant user who received several years' formal training in the tuning of musical instruments before he lost his hearing. This subject is capable of estimating musical intervals in an open-set context, and can also adjust the interval between two "notes" to match a given target. Experiments involving both estimation and production of musical intervals have been carried out. Several parameters of the electrical stimulation have been varied to create notes of different pitches. These include the rate of steady pulse trains delivered to a fixed electrode position; the place of stimulation with a constant pulse rate; selected combinations of rate and place; and the frequency of sinusoidally amplitude-modulated (SAM) pulse trains delivered to a fixed electrode position. The results show that when pulse rate is the variable parameter, the rate ratio for a given musical interval closely approximates that for acoustic signals, provided that the electrode used is in the apical region of the cochlea. When the stimulation is moved to more-basal positions, the rate ratios become larger than the corresponding acoustic frequency ratios. Changing the place of stimulation with a constant pulse rate causes pitch changes that are comparable with the presumed place-to-characteristic frequency mapping of the subject's electrode array. Combinations of place and rate variation result in more complex effects, in which the pitch associated with the place of stimulation generally dominates, but is affected by the rate. The pitch of SAM pulse trains, although probably weaker than that of unmodulated low-rate pulse trains, is related to the modulation frequency. The ratio of modulation frequencies required for a given musical interval is larger than the corresponding ratio of acoustic frequencies in normal hearing.