Graeme Clark Collection
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ItemDevelopment of safe and effective electrodes: a risk management approach [Abstract]( 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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ItemAdvances in relating cochlear implant physiology and psychophysics [Abstract]( 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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ItemInsertion study using new peri-modiolar electrode array designs [Abstract]( 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.
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ItemPsychophysics of a peri-modiolar electrode array( 1999)Psychophysical findings in three adult patients implanted with a CI22 cochlear prosthesis (Cochlear Limited) fitted with a developmental peri-modiolar electrode array have been reported. This array was moulded with a curvature approximating that of the inner wall of scala tympani but, after straightening and insertion into the cochlea, it adopted a position on average approximately half way between the inner and outer walls. However, the considerable variations in electrode trajectories across subjects allowed an analysis which could separate the effects due to longitudinal distance into the cochlea and lateral distance from the modiolus. Radiographic analysis enabled the positions of the individual electrodes to be measured, both longitudinally and laterally. This paper presents additional results and an in-depth analysis of the data, supported by psychophysical findings in subjects with straight arrays. It also introduces a model which describes the effect of distance from the modiolus on the loudness growth function.
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ItemAuditory processing abilities in children using cochlear implants: their relevance to speech perception [Abstract]( 1999)This study aimed to investigate the relationships between some basic auditory processing skills, subject variables and speech perception ability in young children using cochlear implants. A modification of the play audiometry procedure was used to measure electrode discrimination and "rate-of-processing" ability in seventeen 4-10 year old children. In the electrode discrimination task, children responded with a game-like motor response when a repeating stimulation on a reference electrode "changed" to a different electrode. In the "rate-of processing" task, children had to respond to the "change" to a different electrode, when the duration of the stimuli and the time interval between the stimuli were decreased. Normally hearing children were assessed on this task with acoustic stimulation. Nonverbal intelligence, speech feature discrimination and closed-set word recognition were also measured in the children using implants.
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ItemParameter selection and programming recommendations for the ACE and CIS speech processing strategies [Abstract]( 1999)The Nucleus 24 Cochlear Implant system with the SPrint processor provides access to multiple speech processing strategies and a wide range of programming parameters. Strategy comparison studies have suggested that the optimal parameter set and coding strategy varies from individual to individual. It is necessary, however, to establish some default programming parameters and fitting guidelines. Therefore we have investigated the effect of stimulation rate and the number of channels or maxima in the ACE or CIS strategies, as well as the optimal programming strategy for subjects with a limited number of available electrodes. Speech perception was tested using monosyllabic words and sentences in noise, with the evaluation protocol designed to take into account learning effects. Take-home experience was provided with all programs, and subjects were asked to complete a comparative performance questionnaire regarding program preference. Six or eight subjects were enrolled in each study.
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ItemMusical timbre perception investigated using forward-masking( 1999)There is growing general interest in the perception of musical sounds by cochlear implantees. This study was aimed at the perception of one specific aspect of musical timbre, the shape of steady-state frequency spectra. The relationship of the physical and internal spectral shapes was investigated using a forward-masking technique. In addition, the ability of subjects to identify and discriminate selected musical sounds was tested in two related experiments.
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ItemThe efficacy and safety of surface auditory brainstem prosthesis [Abstract]( 1999)The surface auditory brainstem implant (ABI) is undergoing worldwide clinical trials to rehabilitate patients who cannot benefit from cochlear implants. Clinical results showed that neurofibromatosis-II patients implanted with the Multichannel ABI could achieve a beneficial hearing sensation, including open-set speech recognition. Data on the safety of the surface ABIs are limited. The only safety study concerned with their long-term implantation in patients was an analysis on the tissue sheath of an implant removed from a patient.
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ItemPhysiological and histopathological effects of chronic monopolar high rate stimulation of the auditory nerve [Abstract]( 1999)Speech processing strategies based on high rate electrical stimulation have been associated with improvements in speech perception among cochlear implant users. The present study was designed to evaluate the electrophysiological and histopathological effects of long-term intracochlear monopolar stimulation at the maximum stimulus rate of the current Nucleus Cochlear implant system (14493 pulses/s) as part of our ongoing investigations of safety issues associated with cochlear implants.
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ItemGrowth factors from different superfamilies act in synergy when promoting the survival of auditory neurones in vitro [Abstract]( 1999)Several growth factor superfamilies have been implicated in normal inner ear development and in promoting auditory neurone survival. The neuronal promoting ability of neurotrophin-3 (NT-3), brain-derived neurotrophic factor (BDNF), leukemia inhibitory factor (LIF) and transforming growth factor ß-5 (TGF-ß5), alone and in combination, were evaluated in dissociated cell cultures of early postnatal rat auditory neurones. Rat pups were anaesthetised on ice and sacrificed by rapid decapitation, followed by removal of the cochleae under aseptic conditions. Here we provide evidence that synergistic relationships exist between growth factors from diverse superfamilies when promoting auditory neurone survival in vitro. Administered alone, all factors promoted survival of between 10-15% of initially plated neurones, with NT-3 the most potent of the factors. When used in combination, LIF + BDNF, NT-3 + TGF-ß5, and BDNF + TGF-ß5 all promoted auditory neurone survival in a synergistic fashion. In contrast, NT-3 + BDNF and LIP + TGF-ß5 displayed additive survival effects. These results suggest that growth factors may act by interdependent mechanisms when sustaining neuronal integrity, and that a combination of several growth factors may provide a better approach than single factor therapy when developing pharmacological therapies for the treatment of deafness.