Graeme Clark Collection

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    Chronic electrical stimulation of the auditory nerve at high rates: I. Effect on residual hearing [Abstract]
    Xu, J. ; Shepherd, R. K. ; Clark, Graeme M. ( 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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    Chronic electrical stimulation of the auditory nerve at high rates: II. Cochlear pathophysiology [Abstract]
    Shepherd, R. K. ; Xu, J. ; Clark, Graeme M. ( 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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    Spatial representation of the cochlea within the inferior colliculus of neonatally deafened kittens following chronic electrical stimulation of the auditory nerve [Abstract]
    Shepherd, R. K. ; Martin, R. L. ; Brown, M. ; Clark, Graeme M. ( 1995)
    The orderly tonotopic representation of the cochlea is accurately reproduced within the central auditory system of normal hearing animals. Any degradation of this representation as a result of a neonatal hearing loss or chronic electrical stimulation during development could have important implications for the use of multichannel cochlear implants in young children. In the present study we have used 2-deoxyglucose autoradiography (2-00) to examine the topographic representation of the cochlea within the inferior colliculus (IC) of neonatally deafened kittens following periods of chronic intracochlear electrical stimulation.
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    Intracellular responses of anteroventral cochlear nucleus neurones to intracochlear electrical stimulation in the rat [Abstract]
    Paolini, A. G. ; Clark, Graeme M. ( 1996)
    The anterior division of the ventral cochlear nucleus (AVCN) is the first relay station of the auditory pathway. Currently little is known about the intracellular physiological responses of neurones in the AVCN to electrical stimulation of the cochlea. We investigated the effect of cochlear electrical stimulation in the rat AVCN using in vivo intracellular recordings. Male rats were anaesthetised with urethane (1.3g/kg i.p), placed in a stereotaxic frame, the crania and dura removed and the cochlear nucleus exposed.
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    Electrophonically driven single unit responses of the anteroventral cochlear nucleus in cat [Abstract]
    Morrison, N. A. ; Brown, M. ; Clark, Graeme M. ( 1996)
    Electrical stimulation of the cochlea results in both direct and electrophonic excitation of auditory nerve fibres. It has been proposed that electrophonic stimulation results from the creation of a mechanical disturbance on the basilar membrane which has properties similar those resulting from acoustic stimuli. Auditory nerve compound action potential (CAP) forward masking studies1 show the level of frequency specific electrophonic stimulation is highly correlated with the spectral energy of the electrical stimulus waveform. The level of spectral energy in pulsatile biphasic electrical stimuli decreases toward low frequencies suggesting the level of electrophonic stimulation will be diminished in the low frequency region of the cochlea.
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    The pitch of amplitude-modulated electrical stimuli in cochlear implantees [Abstract]
    McKay, Colette M. ; McDermott, Hugh J. ; Clark, Graeme M. ( 1993)
    The ability of cochlear implantees to detect amplitude modulation of pulsatile electrical stimulation, suggests that some speech feature information may be conveyed effectively by this means. For example, modulations at the fundamental frequency of speech may provide a voice pitch percept to implantees, particularly in speech processing strategies which generate constant-rate stimulation. The pitch evoked by sinusoidally modulated current pulse trains on a single electrodes has been studied. Modulation frequencies of 100, 150 and 200Hz, and carrier pulse rates varying from 200 to 1200Hz, were used. The results showed that the pitch of the stimulation was related to the modulation frequency, provided that either the carrier rate was a multiple of the modulation frequency, or the carrier rate was sufficiently high (at least four times the modulation frequency for the stimuli studied here). Furthermore, when the modulated stimuli were matched in pitch to non-modulated pulse trains, it was. found that the rate of the matched non-modulated stimuli was close to but somewhat higher than the modulation frequency. This difference depended on the carrier rate and varied among subjects.
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    A "Combionic Aid": Combined speech processing for a cochlear implant in one ear and speech processing hearing aid in the other ear [Abstract]
    Dooley, Gary J. ; Blamey, Peter J. ; Seligman, Peter M. ; Clark, Graeme M. ( 1993)
    Independent use of a cochlear implant in one ear and a hearing aid in the other is not acceptable for many implant users with some residual hearing. Psychophysical evidence suggests that there are substantial interactions between acoustic and electrical signals including masking and loudness summation. These effects may contribute to the difficulty in using two independent devices and it is desirable to control the parameters of the electrical and acoustical signals far more accurately than is possible with two independent devices with separate microphones. In order to achieve this control we have developed a Combionic aid incorporating an implant and an 'in1planlcompatible' hearing aid controlled from the same speech processor. The new processor is particularly flexible and can implement a wide variety of speech processing strategies for combined acoustic and electrical stimulation. A benchtop prototype has been tested with five patients using a range of different speech tests. In general, patients do better when they use acoustic and electrical information simultaneously than they do with either alone. Some patients on some tests perform significantly better with the bimodal aid than they do with independent hearing aids and implant processors worn together. Wearable devices have now been built and evaluations of these devices are continuing.
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    Development of safe and effective electrodes: a risk management approach [Abstract]
    Clark, Graeme M. ; COWAN, ROBERT ; Saunders, Elaine ; TYKOCINSKI, MICHAEL ; Cohen, Lawrence ; Treaba, Claudiu ; Briggs, Robert S. ; GIBSON, PETER ( 1999)
    Results from studies on experimental animals, computer modelling and preliminary psychophysical studies with three patients, have confirmed the potential for subjective improvement with electrode arrays which lie closer to the modiolus than does the Nucleus straight array. Results of psychophysical studies with three cochlear implant patients, using developmental pre-curved arrays, confirm the feasibility of improving patients' performance through improvements in electrode design. In particular, it was found in psychophysical tests, with patients using a developmental pre-curved electrode array, that both maximum comfortable level and threshold reduced with decreasing distance of a stimulated electrode from the modiolus, and that the dynamic range increased. More intense neural excitation patterns were obtained with the closer electrodes. From this it is inferred that the development of more sophisticated electrode arrays, positioned closer to the modiolus than is currently the case with the Nucleus standard array, will enable the development of improved speech processing strategies. There are technical constraints in the design of a peri-modiolar array, and currently a number of approaches to this problem have been investigated. Whilst the goal of the design is that it be effective for sophisticated and variable manners of stimulus delivery, a primary constraint is safety.
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    Advances in relating cochlear implant physiology and psychophysics [Abstract]
    Bruce, Ian C. ; White, Mark W. ; Irlicht, Laurence S. ; O'Leary, Stephen J. ; Clark, Graeme M. ( 1999)
    More than a decade has passed since apparent discrepancies between physiological and psychophysical thresholds in cochlear implant users were first pointed out. This incongruity has been largely ignored in the intervening time. In a recent series of studies we have undertaken to determine if the definition of threshold in physiological studies is the cause of these differences. Analysis of auditory nerve physiology indicates that fluctuations in the membrane potential are a significant source of stochastic activity (noise) in electrical stimulation, such that responses are best described by discharge probability as a function of stimulus intensity, rather than just a simple deterministic (zero-noise) threshold. We hypothesize that quite low discharge probabilities in individual fibers may be sufficient to account for psychophysical thresholds, if responses in a population of fibers are used in this task by higher auditory pathways.
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    Insertion study using new peri-modiolar electrode array designs [Abstract]
    Treaba, Claudiu ; Clark, Graeme M. ; Cowan, Robert S. ; Tykocinski, Michael J. ; Cohen, Lawrence T. ; Saunders, Elaine ; Pyman, Brian C. ; Briggs, Robert S. ; Dahm, Markus C. ( 1999)
    Intracochlear multi-channel cochlear implants have been shown to successfully provide auditory information for profoundly deaf patients by electrically stimulating discrete populations of auditory nerve fibers via a scala tympani (ST) electrode array. Histological and radiological examination of implanted human temporal bones showed that the current straight Nucleus® array is usually positioned against the outer wall of the ST. An electrode array close to the modiolus could be expected to reduce stimulation thresholds and result in a more localized neural excitation pattern.