Graeme Clark Collection
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ItemVowel perception and hearing impairment( 1981)The perception of vowels by young hearing-impaired and normal-hearing children was investigated in two experimental versions, one which required the child ~o make discriminations among minimal pairs, the other which was based on free choice. Each version focused on three lists which comprised monosyllabic real word-pairs whose initial and final consonants were identical. These lists were constructed around high and low frequency components of vowel spectra in order to explore those acoustic aspects which have been hypothesized (Ling, 1978) as being maximally problematic in high-frequency hearing loss. Results indicated that the normal-hearing children, in contrast to the hearing-impaired, made virtually no errors in either task. The hearing-impaired children made significantly more errors in the free-choice task and in the lists whose vowels contained the highest frequency spectra and among those word-pairs which commenced with nonstops.....
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ItemMultiple-channel cochlear implant( 1981)Receiver-stimulator units and multiple electrode arrays were implanted in the scala tympani of one totally deaf patient on 1 August 1978, and one profoundly deaf patient on 17 July 1979. The first patient, a 46 year old male, lost all hearing following a head injury 18 months prior to surgery. Pure tone and speech audiometry showed no hearing in either ear at the maximum output levels of the audiometer, and no vibro tactile responses were elicited. The second patient, a 63 year old male, had a progressive sensorineural hearing loss extended over 30 years due to bomb blast and chronic infection, and had no help from a hearing aide for 13 years prior to surgery. Pure tone audiometry under headphones showed no hearing in the left or operated ear, and in the right he had the following thresholds: 0.125 kHz � 125 db SPL; 0.25 kHz � 115 db SPL and 0.5 kHz � 117 db SPL. There was no speech discrimination in either ear under headphones or in a monitored sound field.
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ItemFrequency discrimination and the spiral ganglion cell population in cats [Abstract]( 1981)Our previous studies on electrical stimulation of the cochlea and auditory nerve have been performed to provide information about cochlear current distributions and electroanatomy, as well as indicating ways of exciting discrete auditory nerve fibre populations to aid in the design of a multiple channel cochlear implant (Black &Clark, 1980). The present study has been carried out to examine the relationship between the size of the spiral ganglion cell population which remains after experimentally-induced graded degeneration of the auditory nerve has occurred and the ability of an animal to perceive certain electrically induced sounds. This is of special interest for a cochlear hearing prosthesis which requires a residual auditory nerve population large enough to adequately relay the electrically induced activity to higher centres.
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ItemPsychophysical matching of sensations produced by acoustic and electrical stimulation of the auditory nerve [Abstract](Monash University Press, 1983)The aim of this study was to establish an acoustic model of a multiple-channel cochlear implant that could be used in the development of speech coding strategies. Identical psychophysical tests were carried out with electrical stimuli for two cochlear implant patients and with acoustic stimuli for three normally hearing listeners. Each electrical stimulus was a train of biphasic pulses at a constant rate between 50 and 100 pps directed to one of the 10 electrodes spaced at 1.5mm intervals around the basal turn of the cochlea (Clark et al. 1977). The corresponding acoustic stimulus was a train of noise bursts at a rate equal to the electrical pulse rate. The noise bursts were passed through one of 8 bandpass filters with centre frequencies equally spaced on a logarithmic scale from 1140 to 10880 Hz representing 8 different electrodes.
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ItemResults for the Nucleus multiple-electrode cochlear implant in two children [Abstract]( 1988)Two males, 9 years 10 months (CHILD 1) and 5 years 5 months (CHILD 2) at time of surgery, were implanted with the Nucleus multiple-electrode cochlear implant. Both patients were deafened as a result of meningitis in their third year. Assessments of speech perception, speech production and language skills were undertaken at regular intervals, pre and post operatively. For both patients in the audition alone condition, some speech perception post operative scores were significantly higher than pre operative scores and progressive improvements in scores over successive post operative data collection times were seen. Significant differences between the visual alone and auditory-visual condition scores were also observed for CHILD 1 post operatively. Speech production post operative scores were significantly higher than pre operative scores for both patients. The receptive vocabulary scores for both patients improved at a higher rate than that of age-matched normal children. The acquisition of expressive and receptive language skills for CHILD 2 was at a higher rate than that of age-matched children. Differences in the results between the two patients were seen, and this may be related to age and duration of deafness.
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ItemVentral cochlear nucleus and auditory nerve fibre responses to electrical stimulation of the cat cochlea [Abstract]( 1987)Discharge patterns of cat auditory nerve fibres and ventral cochlear nucleus neurones to constant-current biphasic pulses have been recorded. The study was carried out to determine the input-output characteristics of the neurones, for variations in the rate and intensity of electrical stimulation. Neural discharges were highly synchronized with the current pulses at suprathreshold stimulus intensities, and saturation discharge rates usually equalled stimulus pulse rates up to 800 pulses/s. The electrically-driven discharge patterns of ventral cochlear nucleus neurones exhibiting "primary-like" responses were similar to those recorded from electrically-stimulated auditory nerve fibres. Comparison of the neural responses to electrical and acoustic stimulation may help in the design of improved speech processors for cochlear implants. Field potential responses were depressed at high stimulus rates and charge densities, suggesting stimulus-induced reduction in neural excitability. The effects of stimulus parameter variations on field potentials may help in determining safety limits of the electrical stimulus.
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ItemElectrical stimulation of the auditory nerve: effects of high stimulus rates [Abstract](Australian Physiological and Pharmacological Society, 1986)We have previously described non-damaging stimulus levels for chronic intracochlear electrical stimulation (Shepherd et al, 1983). However, maximum safe stimulus levels have yet to be clearly defined. Moreover, the importance of various stimulus parameters and their effects on the auditory nerve is not well understood. In the present study we have examined the effects of stimulus repetition rate on the auditory nerve by monitoring the Electrically-evoked Auditory Brainstem Response (EABR). Such information is necessary if speech processing strategies incorporating high pulse rates (i.e. > 300 pps) are to be made available to cochlear implant patients.
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ItemTraining and assessment of children using a multi-electrode cochlear implant [Abstract]( 1986)A training and assessment protocol has been developed for a research study on deaf children using multi-electrode cochlear implants. The areas of assessment and training include speech perception and production, language and communication skills. Material which is appropriate for language-and developmental-age is selected from a large battery of: tests to ensure that the individual abilities of children are addressed. A single-subject, time-series design has been adopted for regular assessment of the child's performance. The participating child receives a minimum of six months' pre-operative training and assessment using high gain hearing aids or a tactile device. Also included are audiological and medical evaluations to determine the child's suitability for inclusion in the study. Approximately two months are allocated for pre-operative, operative and postoperative surgical management as well as fitting and setting the external speech processor. Post-operatively, the same training and assessment procedures continue for a number of years. Extensive psychophysical studies are also undertaken to measure the child's abilities to discriminate simple stimuli which differ in electrical stimulus parameter values.
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ItemPercepts produced by electrical stimulation of the human cochlea [Abstract]( 1982)Electrical stimulation of the residual auditory nerve fibres in a postlingually deaf patient was effected by ten electrodes implanted 1.5 mm apart in the scala tympani. Biphasic current pulses with each phase fixed at 180 µs were used. Psychophysical results obtained by activating one electrode at a time showed the following characteristics: (a) loudness was found to increase with both current level and repetition rate; (b) pitch increased with repetition rate; (c) pitch and sharpness increased in the apical to basal direction in accordance with the tonotopic organisation of the cochlea; (d) dissimilarity measures obtained by triadic comparisons provided evidence that the sensations produced by repetition rate and electrode position are perceptually separable; (e) for short-duration stimuli the discrimination performance for electrode trajectories was much better than for repetition rate trajectories. For simultaneous activation of two electrodes, triadic comparisons showed that two perceptual components, one related to the more basal electrode and the other to the more apical one, could be discerned.
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ItemChronic electrical stimulation of the auditory nerve in cats( 1982)One requirement for the success of a cochlear hearing prosthesis is that long-term electrical stimulation must not have adverse effects on the residual spiral ganglion cell population. Electrochemically 'safe' stimulation regimes have been defined for the cortex (Brummer &Turner, 1977). However, few investigators have examined the effects of long-term intracochlear electrical stimulation. Walsh et al (1980), stimulating with current densities greater than the 'safe' limits defined by Brummer &Turner (1977), for periods of up to 800 hours at current levels of 4.0-8.0 mA, recorded slight local neural degeneration adjacent to the electrodes.