Graeme Clark Collection
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ItemPsychophysical studies for two multiple-channel cochlear implant patients( 1982)Abstract not available due to copyright.
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ItemEvoked responses in humans to continuous amplitude modulated tones [Abstract]( 1982)The clinical use of the slow cortical auditory evoked responses and the auditory brainstem response is now widespread. Both of these responses look at the electrical changes in the brain following the onset of an acoustic stimulus and are known as transient responses. This paper will describe a technique of recording electrical potentials evoked during a continuous sinusoidally-modulated amplitude-modulated tone. This type of response is known as a steady-state response. The responses to this type of sound were found to be periodic, having the same fundamental frequencies as the modulation envelope. A Fourier transform was used to quantify the amplitude and phase of the first two harmonic components of the response. Responses can be recorded for modulation rates from 4Hz to 448Hz, for carrier frequencies from 250Hz to 4KHz and for sound pressure levels (SPLs) from 30dBSPL to 100dBSPPL. In general, the response amplitude increases with SPL. Estimates of latencies of these steady-state potentials can be made by measuring the phase of both harmonics as the modulation frequency is varied. Latencies suggest the auditory cortex as one of the sources of the response. The clinical implication of these results will be discussed.
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ItemA 12-consonant confusion study on a multiple-channel cochlear implant patient( 1982)A consonant confusion study was undertaken on a multiple-channel cochlear implant patient using a wearable speech processing device. This patient suffered from total bilateral deafness acquired postlingually. The consonants /b/, /p/, /m/, /v/, /f/, /d/, /t/,/n/, /z/, /s/, /g/, /k/ were presented in VCV context with the vowel /a/ as in father by a male and female speaker under three conditions: lipreading alone; electrical stimulation alone using the wearable speech processor and multiple-channel cochlear implant; lipreading in conjunction with electrical stimulation. No significant difference was detected between the results for the male and female speakers. The percentage correct scores for the pooled results of both speakers were lipreading alone - 30%; electrical stimulation alone - 48%; lipreading with electrical simulation - 70%. Performance was significantly better for lipreading with electrical stimulation than for lipreading alone and for electrical stimulation alone than for lipreading alone. An information transmission analysis demonstrated the effective integration of visual and auditory information for lipreading with electrical stimulation. There was a significant improvement in performance for the electrical stimulation alone condition over the 2 months of the study in contrast to no such improvement for lipreading alone.
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ItemA multiple-channel cochlear implant: a summary of results for two patients( 1982)This report summarizes the results of the psychophysical and speech studies conducted on two patients with multiple-channel cochlear implants over an 18 month period at the University of Melbourne. The psychophysical results indicated that a suitable speech-processing strategy would be to convert the second formant frequency of speech to electrode position, the voicing frequency to pulse rate of electrical stimulation, and the acoustic intensity to current level. An evaluation of this strategy by means of a laboratory-based speech processor showed encouraging results, and a wearable speech processor was subsequently developed so that the patients could be benefited in their everyday life.
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ItemImplants aid hearing( 1982)In August 1978 surgeons from the University of Melbourne carried out a cochlear implant operation at the Royal Victorian Eye and Ear Hospital on a totally deaf patient who had lost all hearing following a head injury. The operation involved inserting an array of 20 separately insulated electrode wires along the fluid-filled scala tympani of the inner ear and then implanting an attached receiver-stimulator device in the mastoid bone behind the ear.
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ItemThe perception of vowels by hearing impaired children [Abstract]( 1982)This paper reports the results of a series of closed-set vowel identification experiments with four congenitally hearing impaired children (age 13 years) with moderate to profound bilateral sensorineural losses. Material was presented under three test conditions: hearing alone, through currently worn hearing aids; lipreading alone; and hearing plus lipreading. Analysis of the results using multidimensional scaling and hierarchical clustering revealed a strong relationship between the perceptual organisation of the responses and the physical attributes of the method of signal presentation. In the case of acoustic signals, the results indicated a perceptual organisation describable in terms of the duration of the vowel and the relative frequencies of the first and second formant. For visual signals, the perceptual organisation correlated with characteristics of lip-shaping during production. These results indicated that the subjects attempted to use similar perceptual cues as the normally hearing population. The relationship between the perceptual organisation of the acoustic stimuli and the aided audiogram is also discussed.
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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.