Graeme Clark Collection
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ItemDischarge rate-level functions from dorsal cochlear nucleus single units in response to acoustic and electrical stimulation of the auditory nerve( 1995)Discharge rate-level (I/O) functions possessed by dorsal cochlear nucleus (DCN) units were examined, in response to bipolar electrical stimulation of the cochlea of the barbiturate-anesthetized cat. Spontaneously active units usually possessed nonmonotonic functions with a minimum, and spontaneously inactive units usually possessed monotonic functions or nonmonotonic functions with a maximum (NM+). In response to acoustic high-pass filtered noise, the function relating discharge rate and cut off frequency resembled the same unit's I/O function to electrical stimulation. The I/O functions to acoustic characteristic tones were usually monotonic or NM+. These results suggest that in the DCN, a prerequisite for the generation of acoustic-like responses with an electrical stimulus may be the matching of the cochlear place and spatial extent activated by each stimulus.
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ItemElectrical stimulation of residual hearing in the implanted cochlea( 1995)The average profoundly deaf person using a cochlear implant can now understand more speech than some severely to profoundly deaf people who use a hearing aid. For this reason there will be an increasing need to consider implanting people with residual hearing. In many of these people there could be significant hearing in the operated ear, as a majority of severely to profoundly deaf people are likely to have a symmetrical hearing loss. When three frequency average hearing thresholds were measured on 219 pensioners from the Australian National Acoustic Laboratories (H. Dillon, unpublished findings), 64% had less than a 10-dB difference between thresholds in each ear.
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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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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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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.