Graeme Clark Collection
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ItemThe perceptual dimensions of single-electrode and nonsimultaneous dual-electrode stimuli in cochlear implantees( 1996)Abstract not available due to copyright.
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ItemThe importance of different frequency bands to the speech perception of cochlear implantees [Abstract]( 1996)It is well known that cochlear implantees exhibit a wide range of speech perception ability. Understanding the reason for this variability may lead to improved speech processors. This study investigates whether implantees rely on different areas of the speech spectrum for speech cues, compared to normally hearing listeners, and whether poor performers rely on different spectral areas than better performers. Six subjects with the Mini System 22 implant and using the SPEAK strategy participated in this experiment. Scores for monosyllabic words were obtained using the full speech spectrum and with selected frequency bands removed from the subjects’ speech processor maps. The Articulation Index (AI) is a measure of the proportion of speech information available to a listener, and the relative contribution to AI from different frequency bands is termed the Importance Function. The five frequency bands studied in this experiment were determined to be of equal importance to normally hearing listeners for the speech material used. The scores for each implantee were transformed into AI values, and hence the relative importance of the bands was determined. This relative importance was compared between the implantee group and normally hearing listeners to determine the way in which speech perception by electrical stimulation varies from that by acoustical stimulation. Comparisons were also made between individual implantees to determine whether correlations exist between their speech perception ability and their use of cues in different parts of the spectrum. Further research will determine whether the differences among implantees are correlated with their ability to perceive changes in stimulation place or temporal characteristics.
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ItemLoudness summation for two channels of stimulation in cochlear implants: effects of spatial and temporal separation( 1995)An experiment with four implantees with the Mini System 22 device was undertaken to measure the loudness summation across two channels of stimulation, with stimuli in which the current pulses were delivered alternately to each channel. The effects of varying spatial separation, temporal separation, and extent of stimulation were investigated. It was found that the absolute amount of summation varied among subjects, and was in general independent of electrode separation, except for a reduction at zero separation. Widening of the spatial extent of the stimulation did not have a consistent effect. There was a reduction in summation for all subjects at zero electrode separation when the time between the two pulses was increased from less than I millisecond to 2 milliseconds. In conclusion, loudness summation did not appear to be highly dependent on parameters that affect the spatial current spread in the cochlea. Further study of the effect of temporal parameters on loudness may help to quantify interaction between stimulation channels.
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ItemComparison of current speech coding strategies( 1993)This paper reports on two studies carried out at the University of Melbourne jointly with Cochlear Pty Ltd. The studies demonstrated substantial speech perception improvements over the current Multipeak strategy in background noise.
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ItemThe pitch of amplitude-modulated electrical stimuli in cochlear implantees [Abstract]( 1993)The ability of cochlear implantees to detect amplitude modulation of pulsatile electrical stimulation, suggests that some speech feature information may be conveyed effectively by this means. For example, modulations at the fundamental frequency of speech may provide a voice pitch percept to implantees, particularly in speech processing strategies which generate constant-rate stimulation. The pitch evoked by sinusoidally modulated current pulse trains on a single electrodes has been studied. Modulation frequencies of 100, 150 and 200Hz, and carrier pulse rates varying from 200 to 1200Hz, were used. The results showed that the pitch of the stimulation was related to the modulation frequency, provided that either the carrier rate was a multiple of the modulation frequency, or the carrier rate was sufficiently high (at least four times the modulation frequency for the stimuli studied here). Furthermore, when the modulated stimuli were matched in pitch to non-modulated pulse trains, it was. found that the rate of the matched non-modulated stimuli was close to but somewhat higher than the modulation frequency. This difference depended on the carrier rate and varied among subjects.
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ItemMusical timbre perception investigated using forward-masking( 1999)There is growing general interest in the perception of musical sounds by cochlear implantees. This study was aimed at the perception of one specific aspect of musical timbre, the shape of steady-state frequency spectra. The relationship of the physical and internal spectral shapes was investigated using a forward-masking technique. In addition, the ability of subjects to identify and discriminate selected musical sounds was tested in two related experiments.
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ItemPreliminary results on spectral shape perception and discrimination of musical sounds by normal hearing subjects and cochlear implantees( 1997)This paper presents an overview of an ongoing research project investigating the perception of musical timbre by people with normal hearing, impaired hearing, and cochlear implants. The investigation of musical timbre has been limited to the perception of steady-state frequency spectra from 10 different sources, including sampled acoustic instruments, sung vowels, and synthetic waveforms. Subjects were tested in three different tasks: I) the discrimination of spectra when presented in all possible pairs; 2) the measurement of the internally-perceived frequency spectra using a forward-masking paradigm; and 3) the identification of the spectra by name with the restricted set of sound sources from which they were sampled. Preliminary results from the normally hearing subjects show the spectra to be 99.8% distinguishable, and that significant detail is evident in the internal spectral envelopes from different sounds. There was around 50%-correct identification of stimuli by name with the original sound sources from which they were sampled. The experimental work with hearing impaired and cochlear implant subjects has commenced.
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ItemEffects of rate and pulse manipulations of the spectral maxima speech processor upon speech production( 1993)Postlingually deafened adults experience many changes in their speech as a consequence of late-onset deafness. Previous studies have indicated changes in fundamental frequency, duration, intensity, and vowel formant frequencies in postlingually deafened adults. Postlingually deafened adults who receive multichannel cochlear implants demonstrate improved control of fundamental frequency and intensity. Shifts in formant frequencies to values similar to normal hearing. speakers also appears in some adult implant users. In order to examine how adult implant users adjust their vowel production in response to map changes, we examined vowel production following manipulations to the processing strategy of the Spectral Maxima Speech Processor. This processor uses a speech processing scheme in which six spectral maxima from the outputss of 16 bandpass filters stimulate the cochlea on a place basis at a constant rate. The rate of sampling of the filterbank output is 250 Hz, so six biphasic pulses are presented every 4 msec and there is no attempt to extract fundamental frequency or to find the formant peaks in the speech signal. Two manipulations to the scheme were examined. In the first condition, the rate of sampling remained at 250Hz but eight biphasic pulses were presented rather than six. In the second condition, six biphasic pulses were presented but the rate of sampling of the filterbank output was increased to 400 Hz. Speech samples also were acquired using the standard spectral maxima processor .and with no auditory feedback when the processor was turned off. Speech samples from three �subjects were acquired immediately after receiving the manipulated speech processors and after two weeks experience with the various processors. Preliminary data indicate one subject experienced increased fundamental frequencies while using the 400 Hz high rate strategy. No significant changes were observed in fundamental frequency between the normal SMSP processor and the eight pulse variation. Elimination of feedback resulted in significantly lower fundamental frequencies. The high rate and eight pulse variations resulted in significantly higher first formant frequencies. Second formant frequencies also appear to be influenced by the processing strategies. Comparisons of values produced immediately after receiving a new strategy versus those produced after two weeks use show shifts in primarily first formant values. Data will be presented for all three subjects and discussed in regard to sensitivity to variations in speech processing schemes and the influence of experience with manipulated schemes. (Work supported by the N1H-NIDCD).
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ItemMultichannel cochlear implants in children: an overview of experimental and clinical results [Abstract]( 1991)During the last decade there has been great progress in the clinical management of profound, postlinguistically deafened adults through the use of multichannel cochlear implants. The device developed by Cochlear Pty. Ltd. in association with the University of Melbourne, electrically stimulates selective regions of the residual auditory nerve using an array of 22 Pt electrodes located within the scala tympani. A speech processing strategy has been developed to provide patients with both voice pitch, and first and second formant information. Following experimental safety studies and successful clinical trials, this device was approved for use in adults by the United States FDA in 1985. In 1990, following further miniaturization of the implant, the FDA approved the device for use in profoundly deafened children above the age of two years. The present paper presents an overview of our recent biological safety studies and clinical experience with cochlear implants in children, and discusses the likely future development of these devices. Our biological safety studies were designed to evaluate the safety and design requirements of cochlear implantation in children, and more recently has focussed on issues for implantation in very young children (< 2 years old). These studies included the measurement of growth in the human temporal bone and the development of lead wires that can accommodate such growth, the development of an electrode fixation technique close to the cochlea, the effect of cochlear implantation on skull growth, the effect of long-term electrical stimulation on the maturing auditory system and the stimulating electrodes, and the effect of middle ear infection on cochlear implantation. Our clinical experience is based on twenty-five children that have now been implanted in our clinic. They include (i) postlinguistically deafened children; (ii) congenitally or early-deafened young children; and (iii) congenitally or early deafened adolescents. Clinical testing has shown improvements in speech perception, speech production and language in all three groups. Postlinguistically deafened children show similar speech perception results to postlinguistically deafened adults. For the congenitally deaf, younger children tend to show better results than the adolescents. Significantly, these clinical results are consistent with results from 142 children obtained from clinics throughout the world. These experimental and clinical results support the use of cochlear implants in young children. Further clinical improvements can be expected in the future with advances in both hardware and speech processing strategies.