Graeme Clark Collection
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ItemReduction in excitability of the auditory nerve in guinea pigs following acute high rate electrical stimulation [Abstract]( 1996)Electrical stimulation of neural tissue involves the transfer of charge to tissue via electrodes. Safe charge transfer can be achieved using biphasic current pulses designed to reduce the generation of direct current (DC) or the production of electrochemical products. However, neural stimulators must also use capacitors in series with electrodes, or electrode shorting between current pulses, to further minimize DC due to electrode polarization.
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ItemEvaluation of a new Spectral Peak coding strategy for the Nucleus 22 channel cochlear implant system( 1994)Sixty-three postlinguistically deaf adults from four English-speaking countries participated in a 17-week field study of performance with a new speech coding strategy, Spectral Peak (SPEAK), and the most widely used strategy, Multipeak (MPEAK), both of which are implemented on wearable speech processors of the Nucleus 22 Channel Cochlear Implant System; MPEAK is a feature-extraction strategy, whereas SPEAK is a filterbank strategy. Subjects' performance was evaluated with an experimental design in which use of each strategy was reversed and replicated (ABAB). Average scores for speech tests presented sound-only at 70 dB SPL were higher with the SPEAK strategy than with the MPEAK strategy. For tests in quiet, mean scores for medial vowels were 74.8 percent versus 70.1 percent; for medial consonants, 68.6 percent versus 56.6 percent; for monosyllabic words, 33.8 percent versus 24.6 percent; and for sentences, 77.5 percent versus 67.4 percent. For tests in noise, mean scores for Four-Choice Spondees at +10 and +5 dB signal-to-noise ratio (S/N) were 88.5 percent versus 73.6 percent and 80.1 percent versus 62.3 percent, respectively; and for sentences at +15 dB, +10, and +5 dB S/N, 66.5 percent versus 43.4 percent, 61.5 percent versus 37.1 percent, and 60.4 percent versus 31.7 percent, respectively. Subjects showed marked improvement in recognition of sentences in noise with the new SPEAK filterbank strategy. These results agree closely with subjects' responses to a questionnaire on which approximately 80 percent reported they heard best with the SPEAK strategy for everyday listening situations.
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ItemMultichannel cochlear implantation in children: a summary of current work at The University of Melbourne( 1991)This paper summarizes research work relating to multichannel cochlear implantation in children at the University of Melbourne. Ongoing safety studies relating to the implantation of young children are discussed. Results of these studies suggest that special design considerations are necessary for a prosthesis to be implanted in children under the age of 2 years. Results of clinical assessment of implanted children and adolescents are also discussed in terms of speech perception, speech production, and language development, and some possible predictive factors are suggested. Preliminary data suggests that a high proportion of young children can achieve open-set speech perception with the cochlear implant given appropriate training and support. Initial results with adults using new speech processing hardware and a new coding scheme are also presented. These suggest that improved speech perception in quiet and competing noise is possible with the new system.
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ItemDirect current measurements in cochlear implants: an in vivo and in vitro study( 1998)Direct current (DC) was measured both in vivo and in vitro in cochlear implant electrodes with stimulation at moderate to high pulse rates in monopolar and bipolar modes. In vivo DC was approximately 2-3 times higher than that measured in vitro. In vivo DC levels were <100 nA even at very high rates, although DC levels increased as a function of stimulus rate and charge intensity. DC levels were lower: in the monopolar than in the bipolar stimulation condition. Stimulation with a monopolar capacitively coupled extracochlear electrode showed even lower DC levels in the intracochlear .electrodes. Our results indicated that the Nucleus electrode shorting system is able to maintain a low level of DC during very high rate stimulation for both monopolar and bipolar modes.
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ItemChanges in excitability of the auditory nerve following electrical stimulation using large surface area electrodes [Abstract]( 1998)High rate intracochlear electrical stimulation at intensities well above clinical limits can induce significant reductions in the excitability of the auditory nerve. Such changes are primarily associated with stimulus induced neuronal activity, although direct current (DC) can also contribute. In the present study we examined the extent of stimulus induced change in auditory nerve excitability using large surface area platinum (Pt) electrodes (high-Q). These electrodes have an effective surface area 10-20 times larger than standard Pt electrodes, resulting in lower DC and charge density for a common stimulus. Twenty-three guinea pigs anaesthetized with ketamine (40 mg/kg i.p.) and xylazine (4 mg/kg i.p.), were bilaterally implanted with either high-Q or standard Pt electrodes, and unilaterally stimulated for two hours using a stimulus intensity of 0.34 μC/phase at stimulus rates of 200,400, or 1000 pulses/s (pps). Electrically evoked auditory brainstem responses (EABRs) were recorded before and periodically following the acute stimulation. No reduction in EABR amplitude was observed at 200 pps for both stimulating electrodes. However, EABRs were reduced significantly at 400 and 1000 pps. At 200 pps there was no significant difference (p>0.05 ANOVA) in the post-stimulus recovery of EABR amplitudes following stimulation with either high-Q or standard Pt electrodes. There was, however, significantly greater EABR recovery following stimulation with the high-Q electrode compared with the standard Pt electrode at 400 (p<0.05) and 1000 pps (p<0.05). These data indicate that large surface area high-Q electrodes can significantly reduce stimulus induced changes in auditory nerve excitability, and may therefore have important clinical application.
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ItemElectrical stimulus induced changes in excitability of the auditory nerve( 1997)High rate electrical stimulation of the auditory nerve using stimulus intensities well above the clinical limits can induce a significant reduction in the excitability of the auditory nerve as measured by a decrement in the amplitude of the electrically evoked auditory brainstem response (EABR). Two potential mechanisms may be associated with this stimulus induced reduction in activity: 1) stimulus induced prolonged neuronal hyperactivity; and 2) the generation of adverse electrochemical productions from the electrode surface. The purpose of the present study was to assess the extent to which adverse electrochemical damage contributes to the stimulus induced reduction in auditory nerve excitability. Twenty-six adult guinea pigs anaesthetized with ketamine (40 mg/kg i.p.) and xylazine (4 mglkg i.p.), were bilaterally implanted and unilaterally stimulated for two hours using a stimulus intensity of two or four times EABR threshold. Stimulus rates of 200, 400, or 1000 pulses/s (pps) were delivered via a standard platinum scala tympani electrode or large surface area ("high Q") platinum electrode.
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ItemCochlear implants: high rate stimulation studies and the effect of electrode position [Abstract]( 1996)This paper summarizes our recent findings investigating the safety of high rate electrical stimulation, and reviews the effects of electrode position on auditory excitability. These studies used charge balanced biphasic pulses and electrode shorting between stimuli to minimize any residual charge or direct current. High rate (400-1000 pulses/s) electrical stimulation of the auditory nerve can result in significant stimulus induced reductions in auditory nerve excitability at stimulus levels well above those used clinically (1). The extent of this reduction was dependent on stimulus rate, intensity and duty cycle, implying that such changes were related to the degree of evoked activity.