Graeme Clark Collection
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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.