Graeme Clark Collection
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ItemIs deep insertion of the cochlea implant electrode array necessary and possible?( 1995)Hyaluronate (Healon TM) appears to be useful in achieving deeper electrode insertions than are generally achieved at present. However, biosafety studies are still being conducted. If found to be a safe technique, further work to explore different electrical stimulation strategies and speech processing schemes will also need to be undertaken. This work is ongoing.
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ItemSpatial spread of neural excitation in cochlear implants: comparison of measurements made using NRT and forward masking [Abstract]( 1998)Recently developed technology allows intracochlear potentials to be measured in cochlear implant recipients, using telemetry. Neural response telemetry (NRT) enables the measurement of compound action potentials evoked by stimulation of cochlear implant electrodes. These objective measures can now be compared with related psychophysical measures in humans. We will present data, from both NRT and forward masking, on spatial spread of neural excitation due to stimulation of cochlear implant electrodes. The response fields from more apical neurons will spread quite broadly to the sensing electrodes of an implanted array, resulting in misleadingly broad NRT estimates of the spatial spread of neural excitation. Forward masking, which might itself lay claims to some degree of "objectivity", will not suffer from this limitation. Comparison of data from the two measures will help to determine the limitations of NRT as a tool for measuring spatial spread of neural excitation.
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ItemThe development of a precurved cochlear implant electrode array and its preliminary psychophysical evaluation [Abstract]( 1998)A precurved banded electrode array may provide a better interface with the auditory neural pathways for cochlear implants, and provide better speech perception. A prototype arrray has been further evaluated for ease of insertion, siting within the cochlea and induction of any cochlear trauma. The arrays were inserted into the human cochlear under simulated surgery. The bones were embedded in Araldite, X-rayed and sectioned. X-ray reconstruction analyses of the position of the implanted array showed its insertion to be favourable. Cochlear implants with precurved arrays have been implanted in three patients. Psychophysical evaluation and X-ray analyses have shown that as electrode distance from the modiolus decreased: threshold current decreased; dynamic range increased; current spread as measured by forward masking studies, was more focused; electrode discrimination with loudness jitter (being abetter representation of the dynamic speech signal) improved; JNDs for loudness, expressed as a function of dynamic range, decreased.
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ItemPsychophysics and speech perception with a premodiolar electrode array( 1997)A perimodiolar electrode array, designed to lie closer to the modiolus than the standard Coclear Limited array and developed by the Cooperative Research Centre for Cochlear Implant, Speech and Hearing Research in conjunction with Cochlear Limited, has recently been implanted in an adult patient. While excellent speech results have been obtained using the standard array, it is hypothesised that the improved place coding expected from an array lying closer to the modiolus would result in improved speech perception. Analysis of a modified Stenvers view X-ray revealed that the most apical electrode was inserted to an angle of 311 0 and that the apical half of the array was considerably closer to the modiolus than would have been the case for a standard array. Thresholds and comfortable levels were correlated with the lateral position of the stimulated electrode in the scala tympani, being much reduced for the apical electrodes. Forward masking and electrode discrimination are being measured in order to study the spread of neural excitation and the distinctness of percepts for different electrodes.