Graeme Clark Collection
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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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ItemClinical results for profoundly deaf patients using the 22-elctrode cochlear prosthesis [Abstract]( 1987)The 22-electrode cochlear prosthesis developed in Australia by the University of Melbourne and Cochlear Pty. Limited has been in clinical use in Melbourne for five years. Results for 40 postlingually deaf adults have shown significant communication benefit for 90% of patients. Twenty patients (50%) have demonstrated the ability to understand conversational speech without lipreading or visual cues. No electronic or mechanical failures have been observed in any of the implanted devices. There have been no serious medical complications, but psychological disturbance has occurred in two cases. Experience with prelingually deaf adults has indicated that initial hearing responses for this group are not as good as for the postlingually deaf patients. However, improvement over time has been evident with consistent use of the device. Long-term benefit for these patients will depend to a large extent on motivational and social factors. Results for a small number of young deaf children have been encouraging. The age at onset of profound deafness, intelligence, educational management and family support are all important factors affecting the potential benefit of a cochlear implant for a child.
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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.
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ItemSteady state evoked potentials to amplitude modulated tones [Abstract]( 1982)Evoked responses have been recorded from the scalp of normal human subjects to continuous sinusoidally-modulated amplitude-modulated tones. Phase locking the computer to the modulation envelope enabled an averaging technique to be used to improve the signal to noise ratio. The responses were found to be periodic with the same fundamental frequency as the modulation envelope. Fourier analysis was used to quantify the amplitude and phase of the first and second harmonic components of the responses. The variation of the response amplitude with SPL depended on the modulation and carrier frequency. At modulation frequencies of less than 20Hz and carrier frequencies of less than 1KHz, the amplitude of a response increases up to 50dBSPL but remains invariant at higher levels. At higher modulation frequencies, the response amplitude is uniform to 60-80dBSPL and then increases very rapidly, sometimes by as much as a factor of 20 for a 20dB increase in SPL. This is particularly true of high carrier frequencies. These amplitude growth functions can be explained in part by neural tuning curves. Phase locked responses ran be recorded down to 30dBSPL at most modulation rates and carrier frequencies. Estimates of latencies of these potentials were made by measuring the phase change of the first and second harmonic components whilst changing the modulation frequency. The latencies varied with modulation frequency, carrier frequency and SPL. Latencies of both first and second harmonic components fall into 14 discrete groups from 3msec up to 104msec, with the majority of responses having latencies of 9msec to 33msec. The origin of some of these responses is likely to be the auditory cortex.
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ItemSingle versus multiple-channel electrical stimulation of the auditory nerve in speech processing for a totally deaf patient [Abstract]( 1982)Auditory neurophysiological studies have provided evidence that frequency is coded on both a place and time basis. Psychophysical studies on patients with a profound or total postlingual hearing loss have established that electrical stimulation of the auditory nerve can also convey pitch sensations that depend on toe place or site of auditory nerve stimulation. Pitch perception is also related to the rate of stimulation (Tong et al., 1980). The present study has been undertaken to determine whether a totally deaf patient can integrate both the place and time information from frequency coding in understanding speech signals. Audiological tests were performed to evaluate a totally deaf patient's perception of phonemes, words and sentences for single-channel stimulation that conveyed the fundamental frequency (FO) as rate of stimulation; and multiple-channel stimulation that presented the fundamental frequency as rate, and the second formant (F2) as place of stimulation. The results shown in the table indicate that multiplechannel stimulation provided significantly better scores than single-channel stimulation when using electrical stimulation alone. Furthermore, all scores were significantly better for multiple channel stimulation when combined with lip reading, except the MRT word test. This is an abstract of a paper from the Proceedings of the Australian Physiological and Pharmacological Society published by Australian Physiological and Pharmacological Society. This version is reproduced with the permission of the publisher.