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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ItemA "Combionic Aid": Combined speech processing for a cochlear implant in one ear and speech processing hearing aid in the other ear [Abstract]( 1993)Independent use of a cochlear implant in one ear and a hearing aid in the other is not acceptable for many implant users with some residual hearing. Psychophysical evidence suggests that there are substantial interactions between acoustic and electrical signals including masking and loudness summation. These effects may contribute to the difficulty in using two independent devices and it is desirable to control the parameters of the electrical and acoustical signals far more accurately than is possible with two independent devices with separate microphones. In order to achieve this control we have developed a Combionic aid incorporating an implant and an 'in1planlcompatible' hearing aid controlled from the same speech processor. The new processor is particularly flexible and can implement a wide variety of speech processing strategies for combined acoustic and electrical stimulation. A benchtop prototype has been tested with five patients using a range of different speech tests. In general, patients do better when they use acoustic and electrical information simultaneously than they do with either alone. Some patients on some tests perform significantly better with the bimodal aid than they do with independent hearing aids and implant processors worn together. Wearable devices have now been built and evaluations of these devices are continuing.
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ItemSteady state evoked potentials: an objective measure of residual hearing in young cochlear implant candidates [Abstract]( 1993)Precise determination of hearing thresholds in prospective cochlear implant candidates is essential. As the minimum age of implantation for youl1g children has been reduced, the use of objective measures of hearing has become an important part of their pre-operative evaluation. Steady-state evoked potentials are scalp potentials elicited in response to sinusoidally amplitude and/or frequency modulated tones. A system has been developed at The University of Melbourne which allows the presence: of such a response to be detected automatically thus permitting an objective. frequency specific estimate of hearing to be made in sleeping or awake subjects. This paper investigates the use of SSEPs in determining hearing thresholds in young profoundly deaf children. Responses in such patients are compared with those: obtained in neonates, and a group of sleeping adult subjects with varying degrees of sensori-neural hearing loss. Results indicate that the SSE? procedure is well suited as a measure assessing residual hearing in profoundly deaf children suitable for a cochlear implant in that it can provide an accurate estimate of auditory thresholds using frequency specific stimuli presented at high levels.
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ItemSpeech perception benefits for children using the 22-channel Melbourne/cochlear hearing prosthesis [Abstract]( 1993)In 1985; the first child was implanted with the Cochlear 22-channel cochlear prosthesis at the University of Melbourne Royal Victorian Eye & Ear Hospital Cochlear Implant Clinic. There are now 42 children who have received the device in Melbourne. Analysis of patient details for these children show a very heterogeneous group, with a wide range in age, hearing thresholds, duration of deafness and aetiology. The major aetiologies found were either a congenital profound deafness.; or a hearing loss due to meningitis. In all but 3 cases, the children are using 15 or more electrodes in the array. Speech perception benefits have been analyzed according to a six-level hierarchical classification scheme. All of-the children achieved a minimum benefit of discrimination of suprasegmental information (Category 2), and 59% of the children achieved open-set understanding of unfamiliar speech material without the aid of lip-reading (Categories 5 & 6). Detailed analysis suggests that the majority of children achieving open-set speech perception benefits had more than one year of experience with their implant. and less than seven years of profound deafness prior to implantation.
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ItemFactors associated with open-set speech perception in children using the Cochlear multiple-channel prosthesis [Abstract]( 1993)Since 1985, nearly 100 children have received the 22-channel cochlear prosthesis from the Melbourne and Sydney cochlear implant clinics. These two clinics account for the bulk of casesin Australia, and have similar management philosophies and selection criteria. The patient population represents a variety of etiologies, and ranges in age from 2 - 18 years of age. Bothcongenital and postlinguistic hearing losses are included. In order to assess benefit to speech perception in such a diverse group, the children's results have been tabulated according to a six level hierarchical scale of speech perception achievement. The scale ranges from category I,detection of sound only, to category 6, which includes significant perception scores for open-setwords and sentences. Analysis of the results shows that the majority of the children are achieving open-set speech perception benefits, and that results continue to improve with additional experience with their devices. There are a number of contributing factors to these open-set speech� perception results which have impact both on selection issues and on habilitation with different age ranges �of patients.
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ItemThe postnatal growth of the temporal bone and its implications for cochlear implantation in children( 1993)The growth of the human temporal bone is of practical concern if young children are implanted. It is feared that the normal development of the temporal bone after implantation may displace the electrode array and jeopardize the success of the device. To evaluate the extent of growth 60 cadaver specimens of all ages were examined by direct anatomical measurements. The bones were dissected by imitating the cochlear implantation surgical procedure in the temporal bone laboratory. 19 anatomical/surgical landmarks with implications for cochlear implant surgery were identified and the distance between them measured. The inner ear and middle ear are adult size at birth. The external auditory canal and most parts of the temporal bone are subject to significant lateral growth. The size of the pneumatised mastoid was found to increase in all directions. In the facial recess however, no postnatal growth could be noted. Between birth and adulthood a considerable amount of growth is to be expected between the sino-dural angle and the round window, the landmarks representing the implantation site for the receiver/stimulator and the electrode entry site respectively. From an anatomical and surgical point of view, cochlear implantation in very young children proved to be feasible, provided the electrode array is secured close to the cochlea and the design accommodates for controlled leadwire lengthening.
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ItemThe postnatal growth of the temporal bone and its implications for cochlear implants in children( 1993)An understanding of the postnatal growth of the temporal bone is an important prerequisite for the development of cochlear implantation in very young children. Such information will have an important bearing on both the design of the implant and the surgical procedure. We have measured the postnatal growth of the temporal bone by direct anatomical measurements on 60 cadaver specimens with ages ranging from 2 months to 84 years. Nineteen anatomical landmarks with implications for cochlear implant surgery were identified on each bone and the distance between these points measured. The inner and middle ears were adult size at birth. The external auditory canal and most parts of the temporal bone were subject to significant lateral growth. The size of the pneumatized mastoid increased with age in all directions. Significantly, no postnatal growth was observed in the facial recess. The fossa incudis showed no growth relative to the round window and was found to be a convenient fixation site for the electrode array close to the cochlea. However, with the electrode leadwire fixed at a cortical site such as the osterosuperior point of McEwan's triangle, the leadwire would be subject to approximately 20 mm of growth between this point and the cochlea. These anatomical results indicate that a paediatric cochlear implant would require an expandable leadwire to accommodate these growth changes.
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ItemElectrical stimulation of the auditory nerve: comparison of half-band with full-band scala tympani bipolar electrodes( 1993)The Melbourne/Cochlear auditory prosthesis uses an intracochlear electrode array containing 22 circumferential full-band electrodes mounted on a Silastic carrier. It could be hypothesized that half-band electrodes, oriented towards the modiolus, would produce lower stimulus thresholds than conventional full-band electrodes. This hypothesis is based on the assumption that, compared with full-band electrodes, half-band electrodes would produce an electrical field in which a greater proportion of the current would excite a defined group of neurons. In order to verify this hypothesis we recorded electrically evoked auditory brainstem responses (EABRs) for both full- and half-band electrodes inserted in the scala tympani of deafened cats. EABR thresholds for half-band electrodes oriented towards the modiolus were not significantly different from thresholds evoked using full-band electrodes (p>0.05, paired t-test), whereas thresholds evoked using half-band electrodes oriented towards the outer scala wall were significantly higher (p<0.01) than either the modiolar half-band or the full-band electrodes. These physiological results suggest that the electrical field generated within the auditory nerve by modiolar oriented half-band electrodes does not differ significantly from that produced by full-band electrodes. On the basis of these results, together with the fact that half-band electrodes would have higher current densities and electrode impedances, and would require careful orientation during implantation, we consider that there is no benefit in incorporating half-band electrodes in the design of scala tympani electrode arrays.