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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ItemA comparative study of phase-contrast and conventional x-ray imaging in human temporal bone samples( 2001)This study compared a new x-ray modality, phase-contrast radiography, with conventional radiography for imaging in human temporal bones and also investigated its potential application in the development of electrode arrays for advanced cochlear implants. Nucleus standard electrode arrays and peri-modiolar Contourn.4 electrode arrays were implanted into the cochleae of 10 human temporal bones. Both conventional and phase-contrast radiographs were taken of ~ach temporal bon~. The phase-contrast radiographs showed significant improvements over conventional radiographs in the detail of temporal bone images. These improvements included enhanced contrast at the edge of canal type features, inherent image magnification, higher spatial resolution, and ability to use detectors such as Imaging Plates. The results demonstrate that phase-contrast imaging can have important advantages in visualisation of anatomical details of both the inner ear structures and the microelectrode. It can provide a clearer definition of electrode location in relation to cochlear walls. This study demonstrates the feasibility of applying phase-contrast radiography to studies of the human temporal bone. However, its usefulness in the imaging of larger objects or perhaps even with patients in a clinical setting will require further investigation.
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ItemApplication of advanced radiographic technology in cochlear implant research( 2001)The effective development of peri-modiolar or other advanced electrode arrays for cochlear implants requires detailed analysis of the insertion procedure and electrode positioning in the cochlea. Routine x-ray techniques cannot provide sufficient detail to meet this need. A new micro-focus x-ray imaging system has been built for our research. The system consists of a x-ray tube with a sub 10-micron focal spot mounted below an adjustable work surface and an image intensifier placed approximately 100 cm above the x-ray aperture. A variety of intracochlear electrode arrays and human temporal bones were studied using this system. The micro-focus x-ray imaging system allows for micro-fluoroscopy to visualise the real time implantation procedure. It also enables capturing of images onto reusable phosphor imaging plates or films for subsequent viewing or analysis. Images are produced at up to 95 times magnification with superior resolution and enhanced contrast. This new radiographic technology plays an important role in development of safe and effective advanced intracochlear electrode arrays.
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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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ItemThe role phase-contrast imagining in intra-cochlear electrode development( 2000)In order to improve the design of intracochlear multichannel electrode arrays, it is fundamental that we have knowledge of the exact anatomical , position of the electrode within the scala of the cochlea. Currently, conventional skull radiography is still the mainstay of post-operative radiological assessment of electrode positioning. The present work investigates the use of phase-contrast radiography, a new x-ray modality, to provide improved imaging of the inner ear and the intracochlear electrode array in the human temporal bone (TB).
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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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ItemHigh rate electrical stimulation of the auditory nerve: physiological and pathological results [Abstract]( 1995)Previous experimental studies have shown that chronic electrical stimulation of the auditory nerve using charge balanced biphasic current pulses at rates of up to 500 pulses per second (pps) do not adversely affect the adjacent spiral ganglion population. More recently, a number of clinical trials have indicated that speech processing strategies based on high pulse rates (1000 pps and more), can further improve speech perception. In this paper we summarize our results following acute and chronic electrical stimulation of the auditory nerve using high pulse rates.