Graeme Clark Collection
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ItemCochlear pathology following chronic electrical stimulation of the auditory nerve. I: Normal hearing kittens( 1992)The present study examines the histopathological effects of long-term intracochlear electrical stimulation in young normal hearing animals. Eight-week old kittens were implanted with scala tympani electrode arrays and stimulated for periods of up to 1500 h using charge balanced biphasic current pulses at charge densities in the range 21-52 µC cm^-2 geom. per phase. Both click and electrically evoked auditory brainstem responses were periodically recorded to monitor the status of the hair cell and spiral ganglion cell populations. In addition, the impedance of the stimulating electrodes was measured daily to monitor their electrical characteristics during chronic implantation. Histopathological examination of the cochleas showed no evidence of stimulus induced damage to cochlear structures when compared with implanted, unstimulated control cochleas. Indeed, there was no statistically significant difference in the ganglion cell density adjacent to the stimulating electrodes when compared with a similar population in implanted control cochleas. In addition, hair cell loss, which was restricted to regions adjacent to the electrode array, was not influenced by the degree of electrical stimulation. These histopathological findings were consistent with the evoked potential recordings. Finally, electrode impedance data correlated well with the degree of tissue growth observed within the scala tympani. The present findings indicate that the young mammalian cochlea is no more susceptible to cochlear pathology following chronic implantation and electrical stimulation than is the adult.
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ItemA physiological investigation of chronic electrical stimulation with scala tympani electrodes in kittens( 1992)A physiological investigation of cochlear electrical stimulation was undertaken in six two-month-old kittens. The scala tympani electrodes were implanted and electrically stimulated using biphasic balanced electrical pulses' for periods of 1000-1500h in four ears. Four ears received implants for same period but without electrical stimulation. The other two ears served as normal control. The results indicated: 1) Chronic electrical stimulation of the cochlea within electrochemically safe limits did not influence the hearing of kittens and the normal delivery of impulses evoked by acoustic and electrical signals on the auditory brainstem pathway. 2) The wave shapes of EABRs were similar to those of ABRs. The aptitudes of EABRs showed a significant increase following chronic electrical stimulation, resulting in a leftward shift in the input/ output function. The absolute latencies and interwave latencies of waves II-III , III -IV and II -IV were significantly shorter than those of ABRs. These results imply that there was no adverse effect of chronic electrical stimulation on the maturing auditory systems of kittens using these electrical parameters and the mechanism of electrical hearing should be further studied.
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ItemA gated differential amplifier for recording physiological responses to electrical stimulation( 1992)Artifact from electrical stimulation imposes a problem for the recording of physiological responses to electrical stimulation. Here we describe a simple, low-cost, gated differential amplifier for the recording of physiological responses to electrical stimulation. The gain of the amplifier is set to 1 during electrical stimulation by setting the gate input to a high logic state to avoid overloading of the amplifier by the artifact. Following electrical stimulation, the gate input is set to a low logic state, resulting in again of 1000 for frequencies between 300 Hz and 25 kHz ( -3 dB points). The gain at low frequencies (0-0.2 Hz) is held constant at 1 to avoid transients in the output signal arising from changes in gain at these frequencies. The gain of the amplifier following stimulation (gate low) was independent of the magnitude of the artifact and was therefore suitable for the measurement of neural field potentials with low impedance electrodes.
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ItemSpeech processing for cochlear implants(JAI Press Ltd, 1992)The cochlear implant is a hearing prosthesis designed to replace the function of the ear. The operation of the prosthesis can be described as a sequence of four functions: the processing of the acoustic signal received by a microphone; the transfer of the processed signal through the skin; the creation of neural activity in the auditory nerve; and the integration of the experience of this neural activity into the perceptual and cognitive processing of the implantee.
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ItemA comparison of speech perception of cochlear implantees using the Spectral Maxima Sound Processor (SMSP) and the MSP (Multipeak) processor( 1992)The Spectral Maxima Sound Processor (SMSP) is a portable speech processor which has recently been developed at the University of Melbourne for use with multiple-electrode cochlear implants. In this processor, the six largest outputs (maxima) of 16 bandpass filters are used to stimulate the cochlea on a place basis at a constant rate. This speech processing strategy has been compared with the MSP(MULTIPEAK) strategy, in which four electrodes are selected for stimulation in every glottal pulse period. The study was undertaken on four postlinguistically deaf adults. The results show that, for this group of subjects, the performance of the SMSP processor was significantly better than that of the MSP(MUL TIPEAK) processor for the recognition of closed-set vowels and consonants, open-set monosyllabic words, and open-set sentences in noise, when using electrical stimulation alone. The SMSP mean scores were: vowels 91.3%, consonants 74.9%, words 57.4%, and sentences in noise 78.7%. The MSP(MULTIPEAK) mean scores were: vowels 76.3%, consonants 59.4%, words 39.9%, and sentences in noise 50.0%.
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ItemThe development of speech processing strategies for the University of Melbourne/cochlear multiple channel implantable hearing prosthesis.( 1992)The speech processing strategies that have been used with the University of Melbourne/Cochlear multiple channel implantable hearing prosthesis have been developed systematically from the inaugural one that extracted the second formant and presented this on a place coding basis and the voicing frequency which determined the rate of stimulation. Speech processing has also depended heavily on biological research to ensure that the stimulus parameters used or the operative approach did not damage the spiral ganglion cells it was hoped to stimulate. The advances in speech processing from Melbourne primarily have been to extract more features and spectral information and present this on a place coding basis. This has led to a progressive improvement in speech perception, and a small number of patients can achieve nearly 100% correct scores for open sets of phonetically-balanced words using electrical stimulation alone.
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ItemCochleotopic selectivity of a multichannel scala tympani electrode array using the 2-deoxyglucose technique( 1992)The 2-deoxyglucose (2-DG) technique was used to study the cochleotopic selectivity of a multichannel scala tympani electrode array in four cats with another acting as an unstimulated control. Each animal was unilaterally deafened and a multichannel electrode array inserted 6 mm into the scala tympani. Thresholds to electrical stimulation were determined by recording electrically evoked auditory brainstem responses (EABRs). Each animal was injected with 2-DG, and electrically stimulated using bipolar electrodes located either distal or proximal to the round window. The contralateral- ear was stimulated with acoustic tone pips at frequencies that matched the electrode place. Stimulation of both distal and proximal bipolar electrodes at 3X EABR threshold, evoked localized 2-DG labelling in both ipsilateral cochlear nucleus (CN) and the contralateral inferior colliculus (IC), which was very similar in orientation and breadth to labelling evoked by the contralateral tone pips. The cochleotopic position of labelling to proximal stimulation was located in the 24-26 kHz region of each structure, whereas the distal labelling was located around 12 kHz. Distal stimulation at 10 X EABR threshold produced very broad 2-DG labelling in IC centered around the 12 kHz place. The present 2-DG results clearly illustrate cochleotopic selectivity using multichannel bipolar scala tympani electrodes. The extent of this selectivity is dependent on electrical stimulus levels. The 2-DG technique has great potential in evaluating the efficacy of new electrode array designs.
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ItemA comparison of burst and amplitude modulated electrical stimulation of the cochlear( 1992)On average, the maximum firing rates of cells in the inferior colliculus, when stimulated with either bursts or ramps (amplitude modulated bursts) of biphasic pulsatile electrical stimuli, increased as the pulse rate was increased from 125 to 4000 pulses per second (pps). The fact that this firing rate has increased, on average, up to 4000 pps is evidence that a mechanism for high pulse rate discriminability exists. This firing rate increase was not on a 1:1 basis with the stimulus, but rather a time-averaged firing rate determination. Ramp stimuli generate a wider dynamic range of firing rates than those of burst stimuli, suggesting the potential for a higher rate of information transfer for cochlear implant patients. The finding of temporal information in transient “onset” responses (a response seen only in the first 10 ms post-stimulus onset) of ramp-evoked responses-more than burst-evoked responses-support high pulse rate discriminability and the use of ramp stimuli for encoding high pulse rate information to implant patients.
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ItemCochlear pathology following chronic electrical stimulation using non charge balanced stimuli( 1991)During the course of a chronic intracochlear electrical stimulation study using charge balanced biphasic current pulses, one animal inadvertently received a short period of direct current (DC) stimulation at a level of approximately 1 µA. Subsequent, the animal was chronically stimulated using a poorly charge balanced waveform that produced a DC level of approximately 2 µA. Extensive pathological changes were observed within the cochlea. These changes included widespread spiral ganglion cell loss and new bone growth that extended throughout all turns of the cochlea. Significant changes in the morphology of the electrically evoked auditory brainstem response (EABR) were associated with these pathological changes. EABRs recorded prior to the DC stimulation exhibited a normal waveform morphology. However, responses recorded during the course of the DC stimulation were dominated by a short latency response believed to be vestibular in origin. The response thresholds were also significantly higher than levels recorded before the DC stimulation. In contrast, the contralateral cochlea, stimulated using charge balanced stimuli, showed no evidence of adverse pathological changes. Furthermore, EABRs evoked from this cochlea remained stable throughout the chronic stimulation period. Although preliminary, the present results illustrate the adverse nature of poorly charge balanced electrical stimuli. These results have important implications for both the design of neural prostheses and the use of DC stimuli to suppress tinnitus in patients.
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ItemElectrical stimulation of the auditory nerve in deaf kittens: effects on cochlear nucleus morphology( 1991)The present study examines the effects of long-term electrical stimulation of the auditory nerve on the morphology of neurons in the cochlear nucleus in young, sensorineural deaf animals. Kittens, systemically deafened using kanamycin and ethacrynic acid, received bilateral cochlear implants and were stimulated unilaterally for periods of up to four months. After sacrifice, cross-sectional areas of neuron somata were measured with an image-analysis system and compared using nonparametric statistics. The areas of cell somata within the anteroventral cochlear nucleus (AVCN) on the stimulated side were significantly larger than those of corresponding somata on the control, unstimulated side (P < 0.001). However, there was no statistically significant difference among dorsal cochlear nucleus (DCN) neurons. These results indicate that long-term electrical stimulation of the auditory nerve can at least partially negate some effects of early postnatal auditory deprivation at the level of the cochlear nucleus.