Graeme Clark Collection
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ItemChronic electrical stimulation of the auditory nerve at high rates: I. Effect on residual hearing [Abstract]( 1996)In addition to direct excitation of auditory nerve fibres, cochlear implant patients with small amounts of residual hearing may receive important additional auditory cues via electrophonic activation of hair cells 1. Before incorporating electrophonic hearing into speech processing strategies, the extent of hair cell survival following cochlear implantation must first be determined. We have recently demonstrated widespread survival of hair cells apical to electrode arrays implanted for periods of up to three years, the present report describes the effects of chronic electrical stimulation on hair cell survival.
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ItemChronic electrical stimulation of the auditory nerve at high rates: II. Cochlear pathophysiology [Abstract]( 1996)A major factor in the improved performance of cochlear implant patients has been the use of high stimulus rate speech processing strategies. While these strategies show clear clinical advantage, we know little of their long-term safety. Indeed, recent studies have indicated that high stimulus rates at intensities above clinical limits, can result in neural damage as a result of prolonged neuronal hyperactivity. The present study was designed to evaluate the effects of chronic electrical stimulation of the auditory nerve at high rates, using intensities within clinical limits.
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ItemPhysiological and histopathological effects of chronic monopolar stimulation on the auditory nerve using very high stimulus rates [Abstract]( 1999)Speech-processing strategies using high stimulus rates are used in some cochlear implant systems. While some data suggests that electrical stimulation of the auditory nerve at rates of 2000 pps per channel is safe, there is little data concerning higher rates. The present study was designed to evaluate the safety of a rate of 5000 pps per channel. Under anaesthesia, (ketamine (20 mg/kg. i.v.) and xylazine (3.8 mg/kg. i.v.)), four normal hearing cats were bilaterally implanted with a three channel platinum (Pt) scala tympani electrode array and a return Pt-electrode placed within the temporalis muscle. Each animal was stimulated unilaterally for durations of up to 2700 h using 25μגs per phase charge-balanced biphasic current pulses. The stimuli were delivered at 5000 pps per channel at mid-dynamic range intensities. Acoustically-evoked auditory brainstem responses (ABRs) were recorded during the stimulation regime to monitor the animals' residual hearing. Electrically-evoked auditory brainstem responses (EABRs) were periodically recorded to monitor the status of the auditory nerve and to ensure stimulus intensity remained above threshold. ABRs typically showed poor recovery in the stimulated ear. Longitudinal EABRs recorded from all animals remained relatively stable for the duration of stimulation. Electrode impedances were calculated from daily monitoring of current and voltage waveforms. Two animals that exhibited the highest electrode impedance throughout the duration of stimulation were found to have significant amounts of new bone growth and fibrous tissue in the basal region of the cochlea. However, as one of these animals showed a similar response in the contralateral, unstimulated, implanted cochlea, this response can not be attributed to electrical stimulation per se. There was no statistically significant difference in spiral ganglion cell density in the stimulated cochleae when compared to corresponding regions in controls (p?0.2, Mann-Whitney Rank. Sum Test). These initial results indicate that chronic monopolar stimulation of the cochlea at a rate of 5000 pps per channel does not have an adverse effect on spiral ganglion cell density.
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ItemChronic electrical stimulation of the auditory nerve using non-charge balanced stimuli [Abstract]( 1998)Cochear implants use charge balanced biphasic current pulses and electrode shorting between current pulses to minimise potentially damaging direct current (DC). In the present study we evaluated the effectiveness of the electrode shorting technique using a non-charge balanced stimulus regime. Under general anaesthesia (ketamine (20 mg/kg. i.m.) and xylazine (3.8 mg/kg. i.m)), eight normal hearing cats were bilaterally implanted with two channel platinum scala tympani electrodes. Each animal was stimulated unilaterally for 500 to 2200 h using 50 μs monophasic current pulses. The stimuli were delivered at rates of 500 or 2000 pulses per channel continuously at mid-dynamic range intensities. Electrically-evoked auditory brainstem responses (EABR) were periodically recorded to monitor the status of the auditory nerve and to ensure stimulus intensity remained above threshold. At a stimulus rate of 500 pulses/s, electrode shorting effectively reduced DC levels to ≤ 0.3µA. Longitudinal EABR's recorded from these animals , remained relatively stable over the stimulus duration. These cochleae showed minimal tissue response and there was no statistically significant difference in spiral ganglion cell density when compared with controls (p=0.21, Mann-Whitney U-test). Chronic stimulation at 2000 pulses/s resulted in increased DC levels (0.6-2.8µA). These cochleae exhibited a highly significant reduction in spiral ganglion cell density when compared with controls (p<0.0001), and their EABR's typically displayed an elevation in threshold as a function of stimulus duration. The present findings indicate that continuous non-charge balanced stimuli at rates of 2000 pulses/s can result in significant loss of spiral ganglion cells, presumably as a result of increased DC levels.
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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.
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ItemInvestigation of curved intracochlear electrode arrays [Abstract]( 1992)It has been demonstrated that the Melbourne/Cochlear multi-channel cochlear implant is safe and effective for use in profoundly-totally deaf patients. Recent studies have highlighted the importance of deaf insertion and placing the electrodes closer to the spiral ganglion neurons. In order to improve the electrode insertion depth and proximity to the modiolus, we have investigated curved electrode arrays. Prototypes of such arrays and their accessory inserter have been made. Trial insertions were performed on skeletonized cochleae of human temporal bones. The preliminary results showed that, when compared with conventional straight electrode arrays, the curved arrays could be inserted deeper and located closer to the modiolus. These findings indicate that the curved --.~ electrodes currently under investigation should result in a reduction in stimulus threshold and improve pitch perception and may also result in the use of more channels of stimulation.
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ItemElectrical stimulation of the auditory nerve: comparison of half-band with full-band scala tympani bipolar electrodes( 1993)The Melbourne/Cochlear auditory prosthesis uses an intracochlear electrode array containing 22 circumferential full-band electrodes mounted on a Silastic carrier. It could be hypothesized that half-band electrodes, oriented towards the modiolus, would produce lower stimulus thresholds than conventional full-band electrodes. This hypothesis is based on the assumption that, compared with full-band electrodes, half-band electrodes would produce an electrical field in which a greater proportion of the current would excite a defined group of neurons. In order to verify this hypothesis we recorded electrically evoked auditory brainstem responses (EABRs) for both full- and half-band electrodes inserted in the scala tympani of deafened cats. EABR thresholds for half-band electrodes oriented towards the modiolus were not significantly different from thresholds evoked using full-band electrodes (p>0.05, paired t-test), whereas thresholds evoked using half-band electrodes oriented towards the outer scala wall were significantly higher (p<0.01) than either the modiolar half-band or the full-band electrodes. These physiological results suggest that the electrical field generated within the auditory nerve by modiolar oriented half-band electrodes does not differ significantly from that produced by full-band electrodes. On the basis of these results, together with the fact that half-band electrodes would have higher current densities and electrode impedances, and would require careful orientation during implantation, we consider that there is no benefit in incorporating half-band electrodes in the design of scala tympani electrode arrays.
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ItemCochlear implantation in young children: studies on head growth, leadwire design and electrode fixation in the monkey model [Abstract]( 1992)For the safety of cochlear implantation in children under two, the implant assembly must not adversely effect the tissue of compromise head growth. Furthermore, growth changes and tissue responses should not impair functioning of the device. Dummy receiver-stimulators, interconnect plugs and leadwire-lengthening systems have been implanted for periods of 40 months in the young monkey to most effectively model the implantation of the young human child. The results show that implanting a receiver-stimulator package has no effect on skull growth or brain tissue under the package. The system for fixing the electrode at the fossa includes proved effective. There was marked osteoneogenesis in the mastoid cavity and this also resulted in fixation of the leadwire outside the cochlea. This study provides evidence for the safety of cochlear implantation in young children.