Graeme Clark Collection
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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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ItemSpeech cues for cochlear implantees: spectral discrimination [Abstract]( 1997)The ability of cochlear implantees to understand speech varies over a wide range. While some implantees achieve scores close to 100 % open set word tests, other require visual cues to achieve a significant score on these tests. The focus of this research is to investigate reasons for the wide range of ability and therefore to improve the speech processors used by individual implantees. This study first investigated whether the relative importance of various frequency regions of the speech spectrum differs for implantees of different performance levels, and for implantee groups compared to normally hearing subjects.
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ItemTemporal integration of pulsatile stimuli distributed across multiple electrodes [Abstract]( 1995)Several speech processing strategies in current use present pulsatile stimulation nonsimultaneously on multiple intracochlear electrodes. In most cases the rate of stimulation is constant, and variations in the pulse amplitudes are presumed to contribute to the overall delivery of information to the implant user. However, it is not known whether such amplitude modulations are combined across nearby electrode positions, or whether they produce separable percepts when presented on multiple electrodes. This question, addressing the issue of channel interactions in multiple-electrode implants, is important to the development of advanced sound processing strategies. It was investigated in a psychophysical experiment using amplitude-modulated pulse trains presented at an overall carrier rate of 1000 Hz, with pulses being delivered alternately to two electrodes. The modulation had a period of 10 ms, within which two pulses (one on each electrode) had a high amplitude, while the remaining eight had a low amplitude. The time was varied between the two high pulses in each period (which can be regarded as a phase shift). The smallest time delay (1 ms) produced an overall 100 Hz modulation pattern, whereas the largest delay (5 ms) produced a 200 Hz modulation pattern. All of the stimuli produced identical 100 Hz patterns on each of the component electrodes, and so would be distinguishable only if the combined pattern were being perceived. The hypothesis was that this combined pattern would be perceived only for small electrode separations. This hypothesis was confirmed experimentally in five subjects by using a four-interval forced-choice task where the subject was asked to identify the one stimulus which was different. Phase differences of 1 - 2 ms could be detected provided that the electrode separation was within about 3 - 4 mm. A subsequent experiment using single-interval pitch estimation tasks showed that the differences in the temporal patterns were perceived in the same way as rate pitch differences. In conclusion, our experiments showed that, when modulated pulsatile stimuli are delivered to nearby electrode positions, implantees may perceive the modulations of the combined stimuli and not just the modulations on the separate electrodes.
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ItemMusical pitch perception by a user of the Nucleus 22-electrode cochlear implant [Abstract]( 1995)The study of how musical sounds are perceived by users of cochlear implants is both interesting and rewarding. In particular, it has the potential to provide detailed information on the perception of pitch with electrical stimulation of the auditory nerve. However, it is difficult to find subjects whose understanding of musical terminology and memory of conventional musical pitch relationships are adequate for this research. In our Melbourne laboratory, we have been fortunate to locate one implant user who received several years' formal training in the tuning of musical instruments before he lost his hearing. This subject is capable of estimating musical intervals in an open-set context, and can also adjust the interval between two "notes" to match a given target. Experiments involving both estimation and production of musical intervals have been carried out. Several parameters of the electrical stimulation have been varied to create notes of different pitches. These include the rate of steady pulse trains delivered to a fixed electrode position; the place of stimulation with a constant pulse rate; selected combinations of rate and place; and the frequency of sinusoidally amplitude-modulated (SAM) pulse trains delivered to a fixed electrode position. The results show that when pulse rate is the variable parameter, the rate ratio for a given musical interval closely approximates that for acoustic signals, provided that the electrode used is in the apical region of the cochlea. When the stimulation is moved to more-basal positions, the rate ratios become larger than the corresponding acoustic frequency ratios. Changing the place of stimulation with a constant pulse rate causes pitch changes that are comparable with the presumed place-to-characteristic frequency mapping of the subject's electrode array. Combinations of place and rate variation result in more complex effects, in which the pitch associated with the place of stimulation generally dominates, but is affected by the rate. The pitch of SAM pulse trains, although probably weaker than that of unmodulated low-rate pulse trains, is related to the modulation frequency. The ratio of modulation frequencies required for a given musical interval is larger than the corresponding ratio of acoustic frequencies in normal hearing.
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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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ItemAn improved speech processor for a 22-electrode cochelar implant [Abstract]( 1992)A spectral maxima speech processing strategy ( SMSP) has been developed as a result of research to improve the speech perception performance of a multiple-channel cochlear implant. With this speech processing strategy. the six spectral maxima from the outputs of 16 band pass filters are used to stimulate the cochlea on a place basis at a constant rate. This SMSP strategy has been compared with the MSP-MULTIPEAK strategy, the present speech processor provided by Cochlear Pty. Limited, on four postlinguistically deaf adults. The study showed that the SMSP strategy was significantly better than the MSP-MULTJPEAK for the recognition of closed-set vowels and consonants, and open-set monosyllable words and sentences in background noise.
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ItemMultichannel cochlear implants in children: an overview of experimental and clinical results at the University of Melbourne [Opening Lecture]( 1992)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 The University of Melbourne in association with Cochlear Pty Ltd, electrically stimulates selective regions of the auditory nerve using an array of 22 platinum (Pt) electrodes located in the scala tympani. Its development followed basic experimental studies and the development and evaluation of a prototype device in the 1970's. Following safety studies and a successful clinical trial, the Melbourne/Cochlear multichannel implant was approved for use in adults by the United States Food and Drug Administration (FDA) in 1985. More than 3000 patients throughout the world have since been implanted with this device, many being able to understand a significant amount of unfamiliar, connected speech without lipreading Following miniaturization of the implant, it became suitable for use with children. In 1990, after additional biological safety and clinical investigations, the FDA approved the use of the Melbourne/Cochlear multichannel implant for profoundly deaf children above the age of two years. And in 1991, the device received the medical device implantation approval certificate from the Japanese Government. The present paper presents an overview of our recent biological safety studies and clinical experience in children, and discusses the likely future development of these devices.