Graeme Clark Collection
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ItemA speech processing strategy for multiple-electrode cochlear implant prostheses(Monash University Press, 1983)Speech studies in a number of research centres have shown that useful speech information could be presented to deaf patients using single or multiple electrode cochlear implant prostheses (Parking & Anderson, 1983). In our laboratory, speech processing strategies were formulated on the basis of psychophysical results. This paper examines the psychophysical characteristics of the hearing sensations produced by electrical stimulation using scala tympani electrodes in postlingually deaf patients; a speech processing strategy is then discussed on the basis of these characteristics.
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ItemPhysiological and histopathological effects of chronic intracochlear electrical stimulation(Monash University Press, 1983)Direct and r.f. currents are known to result in destruction of neural tissue. However, it is now apparent that non-destructive electrical stimulation can be achieved by the use of biphasic pulsatile stimuli (Lilly, 1960; Mortimer et al., 1970; Hughes et al., 1980). Although maximum biologically safe stimulation regimes have yet to be clearly defined, the evidence of a number of investigators suggests that charge density per phase and charge injection per phase are important parameters when establishing biologically safe levels of electrical stimulation (Pudenz et al., 1975; Pudenz et al., 1977; Brown et al., 1977; Babb et al., 1977). Furthermore, considerable attention has been given to ensure that the stimulus is not producing adverse electrochemical reactions that could result in physical or toxic injury to the biological environment. Brummer et al. (1977) have defined the upper limit of electrochemically safe electrical stimulation for platinum electrodes as charge balanced biphasic pulses at a maximum charge density of 300 ?C/cm2 geom./phase.
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ItemSelection of speech processing for cochlear implant prostheses(Monash University Press, 1983)In this paper we consider a framework against which to discuss strategies for the design of speech processors for cochlear implant prostheses. We hope to encourage discussion of the bases for such a framework even though it may seem a distant objective owing to the large gaps in our understanding of several component parts of cochlear implant systems. The existence of such a framework would provide a background against which to view the current diverse cochlear implant systems and to evaluate their performance.
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ItemThe implanted round window membrane in the cat [Abstract](Monash University Press, 1983)In cochlear implants the round window is convenient for the electrode insertion into the scala tympani because the surgical approach is reasonably easy and the inserted electrode lies close to systematically organised nerve fibres in the spiral lamina. However, complications might occur when a poor seal, extensive tissue damage or surgical asepsis are present that lead to a reduction in the nerve fibre population which is needed for electrical stimulation. Published articles available do not describe the role of the window membrane in cochlear implants. Probably this can be referred to the finidng of abundant scar tissue in the window niche and around the electrode giving the impression of a safely implanted electrode. This study performed on seven cats over 5 months was concerned with morphological properties of the implanted window membrane at different stages after implantation. In addition, horseradishperoxydase was used as a tracersubstance to give data concerning the sealing properties of the implanted round window membrane.
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ItemInitial results for six patients with a multiple-channel cochlear prosthesis(Monash University Press, 1983)A total of eight patients have been assessed with the multi-channel cochlear prosthesis at the University of Melbourne. The first two patients were implanted with a prototype device in 1978 and 1979, and their results with various speech evaluation procedures have been reported and summarized in detail elsewhere (Clark & Tong, 1982). Briefly, these results indicated that some very significant benefit could be obtained for these patients when using the cochlear prosthesis with external speech processing, particularly when using the device in conjunction with lipreading. It was also shown that some significant understanding of speech was possible without lipreading (open-set) for both patients, although this was fairly limited.
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ItemThe auditory brainstem response in hearing and deaf cats evoked by intracochlear electrical stimulation(Monash University Press, 1983)This study was performed to investigate in detail the auditory brainstem response (ABR) for intracochlear electrical stimulation. Brainstem response audiometry is a simple, noninvasive procedure with the responses under many stimulus conditions being readily understood in terms of single auditory nerve discharge properties. The amplitude and latency behaviour of the Nl brainstem response correlates well with that recorded directly from the auditory nerve (Huang & Buchwald, 1978). In addition, the brainstem response can be divided into frequency-specific components corresponding to tonotopical locations in the cochlea, as exhibited in the method of derived responses (e.g. Parker &Thornton, 1978). It is therefore well suited to both physiological and clinical investigation of auditory function and therefore should be useful in evaluating auditory function under conditions of electrical stimulation of the cochlea.
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ItemElectrical stimulation of the human cochlea: psychophysical and speech studies(Plenum Publishing Corporation, 1981)This report describes psychophysical and speech studies conducted on two of our post-lingually deaf patients implanted with the nature of the hearing sensations produced by the individual electrodes, and to investigate the feasibility of the transmission of speech information to higher centres by means of cadences of stimulation using on electrode at a time. Two totally deaf patients (MC1 and MC2) participated in these studies.
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ItemCombining tactile, auditory and visual information for speech perception( 1988)Four normally hearing subjects were trained and tested with all combinations of a highly degraded auditory input, a visual input via lipreading, and a tactile input using a multichannel electrotactile speech processor. When the visual input was added to any combination of other inputs, a significant improvement occurred for every test. Similarly, the auditory input produced a significant improvement for all tests except closed-set vowel recognition. The tactile input produced scores that were significantly greater than chance in isolation, but combined less effectively with the other modalities. The less effective combination might be due to lack of training with the tactile input, or to more fundamental limitations in the processing of multimodal stimuli.
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ItemThe influence of electrode geometry on the electrically evoked auditory brain stem response( 1988)The electrically-evoked auditory brainstem response (EABR) consists of a series of far-field potentials that reflect synchronous neural activity within the auditory brainstem in response to a transient electrical stimulus. The EABR appears relatively simply organized in terms of its amplitude and latency behaviour. The growth in amplitude of wave IV of the EABR, for example, reflects changes in the amplitude of the electrically-evoked VIII nerve compound action potential as a function of stimulus intensity. In addition, single unit population studies have shown a monotonic relationship between the growth in EABR amplitude and the number of nerve fibres being stimulated (Merzenich and White, 1977). The EABR can therefore, provide an insight into the response of the auditory nerve to electrical stimulation. We have used this technique to investigate the efficacy of electrical stimulation of the auditory nerve using a variety of stimulating electrode geometries.
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ItemAdvances with the 22-channel cochlear implant(Kugler & Ghedini, 1988)The Nucleus 22 Channel Cochlear Implant was developed on the basis of work at the University of Melbourne. Between 1967 and 1978, there was extensive research with animals and human temporal bones, especially regarding safety, psychophysics, histopathology and surgical approaches. As a result of this work, it was decided to develop a multi-channel intracochlear implant. The 22 channel implant has been used in more than 500 patients world-wide but there were many steps on the road to this success. Today, over one third of the post-lingually profound, deaf adults implanted with this device have significant speech understanding without lipreading.