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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ItemPost mortem study of the intracochlear position of the nucleus standard 22 electrode array( 2000)The final position of an intracochlear cochlear implant electrode array can vary depending on the pathology, the insertion technique used and the type of electrode array used. The distance of the electrodes from the target neural elements and the presence of intracochlear fibrous tissue or new bone formation are believed to affect the performance of the device. A post mortem study was conducted to assess these factors.
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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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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.