Graeme Clark Collection
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ItemA speech processing strategy for an electro-tactile vocoder [Abstract]( 1980)Past attempts at using the skin for recognition of tactile patterns derived from acoustic speech signals have largely been unsuccessful for perception of running speech. Problems facing researchers in this field include: frequency discrimination, especially for electrical stimulation, temporal and spatial resolution, real time speech processing and tactile pattern configuration strategies. It is considered that recent developments in speech processing which allow real time estimation of formant frequencies and vocal tract area functions will enable a successful speech aid to be developed. Based on results of the Tadoma (or Hofgaard) Method, in which speech is perceived by the deaf-blind using tactile and kinesthetic senses to determine movements of a speaker's articulators, a model is evaluated which enables a tactile display of articulatry information derived from parameters extracted from the speech signal by real time speech processing. Psychophysical measurements of percepts of computer derived patterns were carried out concentrating in particular on patterns more likely to be important for phonemic and speech discrimination. In this way it is hoped to validate the model as a useful speech aid for the profoundly and partially deaf.
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ItemIntracellular responses of anteroventral cochlear nucleus neurones to intracochlear electrical stimulation in the rat [Abstract]( 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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ItemElectrophonically driven single unit responses of the anteroventral cochlear nucleus in cat [Abstract]( 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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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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ItemTemporal response properties of primary-like units in the anteroventral cochlear nucleus to acoustic and electrical stimulation [Abstract]( 1998)Auditory information is encoded in the central auditory pathway in both the spatial and temporal domains. The magnitude of the contributing role of each domain remains unresolved. Scala tympani electrical stimulation of the auditory pathway may provide insights into the relative importance of these different coding domains. This study investigates the temporal response of acoustically-characterised primary-like units in the anteroventral cochlear nucleus (AVCN) to acoustic and electrical stimulation. Response synchrony to acoustic stimulation at and below the units characteristic frequency (CF) and to electrical stimulation at rates up to 1000 Hz has been analysed with respect to the phase of the stimulus and the intervals between successive spikes. Normal hearing adult cats were anaesthetised with pentobarbitone sodium (Nembutal; 45 mg/k.g l.V.) and the AVCN exposed via a dorsal approach. Micropipette electrodes (4 -30 M?) were advanced dorsoventrally through the AVCN and the response timing of isolated units recorded on computer for off-line analysis. To compare the variance in the timing of the response with respect to the phase of the stimulus and with respect to the intervals between successive responses, cross correlation of spike timing and cross-correlation of spike intervals were performed. For acoustic stimulation these results show that the variance in the timing of the response with respect to the phase of the stimulus is lower (i.e. higher correlation) than that seen in the interval variance. A similar result was not however found following electrical stimulation. Cross-correlations on different order intervals, (i.e. 1st, 2nd, 3rd etc.), show that variance across the different order intervals does not systematically vary. These data suggest that the temporal response to acoustic stimuli at this level of the auditory pathway more precisely codes stimulus phase than stimulus interval.
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ItemElectrical stimulus induced changes in excitability of the auditory nerve( 1997)High rate electrical stimulation of the auditory nerve using stimulus intensities well above the clinical limits can induce a significant reduction in the excitability of the auditory nerve as measured by a decrement in the amplitude of the electrically evoked auditory brainstem response (EABR). Two potential mechanisms may be associated with this stimulus induced reduction in activity: 1) stimulus induced prolonged neuronal hyperactivity; and 2) the generation of adverse electrochemical productions from the electrode surface. The purpose of the present study was to assess the extent to which adverse electrochemical damage contributes to the stimulus induced reduction in auditory nerve excitability. Twenty-six adult guinea pigs anaesthetized with ketamine (40 mg/kg i.p.) and xylazine (4 mglkg i.p.), were bilaterally implanted and unilaterally stimulated for two hours using a stimulus intensity of two or four times EABR threshold. Stimulus rates of 200, 400, or 1000 pulses/s (pps) were delivered via a standard platinum scala tympani electrode or large surface area ("high Q") platinum electrode.
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ItemAdvances in cochlear implant speech processing [Abstract]( 1997)Our early research emphasized there was a restriction on the amount of speech and other acoustic information that could be transmitted to the nervous system by electrical stimulation of the auditory nerve. It also showed the need to use multiple-channel stimulation, and minimize channel interaction. As a result our research over the last 30 years has been directed towards optimizing the information presented to the auditory nervous system. This has involved extracting the energy of the first and second formants (FO/F2-WSP II; FO/FI/F2-WSP III; Multipeak-MSP) as well as the outputs of high band pass fixed filters (Multipeak - MSP) and coding these outputs as cochlear place of stimulation. The voicing frequency was coded as rate of stimulation. Our most recent speech processing strategy (SPEAK) extracts a specified number of .maximal outputs from a series of band pass filters, rather than selecting the peaks of energy which was the case with the other strategies. The voltages from the maximal outputs are used to stimulate appropriate electrodes on a place coding basis. The stimuli are presented at a constant stimulus rate to reduce channel interaction. Voicing is conveyed as amplitude variations.
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ItemA stimulation of spatio-temporal firing across auditory nerve fibres( 1997)Present cochlear implant speech processing strategies give recipients a perception of sound inferior to that of the normal hearing population. Since it is beyond current technology to achieve an electrically evoked auditory-nerve output identical to that of normal hearing, stimulation strategies are limited to approximating certain features of the neural firing patterns. The importance of the spatio-temporal firing patterns of an ensemble of auditory nerve fibres to speech perception has been stated in previous studies (1,2). This paper utilises a composite model of the cochlea and hair-cell/auditory nerve transduction using artificial and speech signals as input to produce a spatio-temporal excitation pattern which represents the fluctuating firing probability of the auditory neurons. A model of electrical stimulation of the auditory nerve is then used to show how stimulation strategies currently used produce neural firing patterns qualitatively different to those produced by normal hearing. Our investigations indicate that it is possible to generate electrical stimulation parameters that cause the spatio-temporal responses of the neural population to better approximate normal hearing. These responses enable us to identify stimulation parameters required to obtain the chosen neural firing patterns. A number of examples illustrate the utility of this method, revealing the spatio-temporal responses for varying numbers of neurons and electrode displacements.
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ItemA stochastic model of the electrically stimulated nerve designed for the analysis of large-scale population [Abstract]( 1997)Accurate models of Auditory Nerve (AN) response to electrical stimulation may aid in the development of speech processing strategies for cochlear implants. Most models of AN response to electrical stimulation utilize deterministic (non-random) description in spite of strong evidence for stochastic (random) activity in physiological data. Inclusion of stochastic activity in complex models of neural response such as the Hodgkin-Huxley equations has proven to be computationally expensive. They are therefore unsuitable at this time for the calculation of large-scale population responses which could be required for the investigation of sound coding in ensembles of nerve fibers, for the explanation or prediction of psychophysical results, or for the development of speech processing strategies for cochlear implants. It is therefore necessary to develop a simpler model of single-fiber response to electrical stimulation which includes stochastic activity.