Graeme Clark Collection
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ItemElectrical stimulation of the auditory nerve: stimulus induced reductions in neural excitability [Abstract]( 1987)Electrical stimulation of the auditory nerve elicits highly synchronised neural activity (Javel et al., in press). As the stimulus current is increased the neural response becomes highly deterministic with every current pulse eliciting a spike even at stimulus rates of 600-800 pulses per second (pps). Our previous acute experimental studies have shown that high stimulus rates (> 200 pps) and high stimulus currents (> 1.0 mA) can result in temporary and sometimes permanent reductions in the excitability of the auditory nerve (Shepherd and Clark, 1986). The present study was designed to examine the mechanisms underlying these stimulus induced reductions in excitability. These results will have implications for the maximum safe and effective stimulus rates that can be employed in cochlear implants.
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ItemResults of multiple-electrode cochlear implants in children( 1987)Children in Australia and United States of America are now being implanted with the Nucleus 22 electrode intracochlear prosthesis utilizing the F0/F1F2 coding strategy. A total of 32 adolescents (10-17 years) and 24 preadolescents (2-9 years) have been implanted as of 31 August, 1987. No significant postoperative complications were recorded, the speech processors were successfully programmed, and all are users of the device. For the 56 children, the average length of postoperative stimulation time is 2.8 months. Because the majority of children have such short experience with the device we report herein two children from the University of Melbourne (A) and two children from the United States (U) who have been using the Nucleus system for 12 months or more. Child 1A has only 10 electrodes in the cochlea; therefore, the number of channels programmed for the children is 10, 17, 18 and 18, respectively. Child Al and A2 were deafened by meningitis at 3-3 and 3 years of age, respectively. Child U3 became profoundly deafened from a progressive sensorineural loss at age 11 and Child U4 was deafened by recurrent cochlear hydrops at age 13 years.
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ItemVentral cochlear nucleus neurone responses to electrical stimulation of the auditory nerve in the cat [Abstract]( 1987)Discharge patterns of cat auditory nerve fibres stimulated by constant-current biphasic pulses have been previously described (Javel et al., in press). However details of neural activity elicited central to the auditory nerve by such pulses are not yet determined. The scope of this study was to describe the activity elicited by such pulses, in the auditory neurones thought to receive direct auditory nerve fibre input -those of the ventral cochlear nucleus (VCN) exhibiting a "primary-like" (PRI-like) response. The objective was to compare the electrically-and acoustically-elicited responses of these neurones. The observed differences will be used to direct future efforts in the choice of electrical stimuli which would produce discharge patterns more akin to those elicited acoustically.
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ItemSurgery for multielectrode cochlear implants( 1987)For the surgery of the NUCLEUS Cochlear Implant (CI) in general anaesthesia we use a skin cut beginning at the bottom of the entrance to the outer ear canal, following the posterior circumference to a point nearly 12 o'clock. From here the incision runs superiorly to the tragus until two or three centimetres above the pinna base and in a wide smooth circle in direction to the occiput. The wide circle is necessary to get a distance of about 2 cm away from the package and also to guarantee the blood supply by the occipital artery and by the postauricular artery as well.
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ItemSignal processing in quiet and noise( 1987)It has been shown that many profoundly deaf patients using multichannel cochlear implants are able to understand significant amounts of conversational speech using the prosthesis without the aid of lipreading. These results are usually obtained under ideal acoustic conditions but, unfortunately, the environments in which the prostheses are most often used are rarely perfect. Some form of competing signal is always present in the urban setting, from other conversations, radio and television, appliances, traffic noise and so on. As might be expected, implant users in general find background noise to be the largest detrimental factor in their understanding of speech, both with and without the aid of lipreading. Recently, some assessment of implant patient performance with competing noise has been attempted using a four-alternative forced-choice spondee test (1) at Iowa University. Similar testing has been carried out at the University of Melbourne with a group of patients using the Nucleus multichannel cochlear prosthesis. This study formed part of an assessment of a two formant (F0/FI/F2) speech coding strategy (2). Results suggested that the new scheme provided improved speech recognition both in quiet and with competing noise. This paper reports on some more detailed investigations into the effects of background noise on speech recognition for multichannel cochlear implant users.
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ItemThe histopathology of the human temporal bone following cochlear implantation in a patient: a summary( 1987)The macroscopic and microscopic evaluation of the implanted and unimplanted temporal bones in a patient who had a cochlear prosthesis for 27 months prior to his death from cardiac disease has shown that the device is biocompatible, and does not lead to any significant adverse effects. The cause of deafness was meningitis.
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ItemSpeech recognition abilities in profoundly deafened adults using the Nucleus 22 Channel Cochlear Implant System( 1987)Research in the area of cochlear prostheses to restore a level of hearing sensation to the profoundly deaf has been ongoing at a number of centers throughout the world since the 1960's. 3, 4, 7, 8,. Work on a multichannel cochlear implant that utilizes a speech feature extraction coding strategy and multi-sited, sequential, bipolar stimulation to enhance pitch perception began at the University of Melbourne under the direction of Professor Graeme Clark in the 1970's. Collaboration with Nucleus Limited, a multi-national biomedical corporation from Australia, led to the development of the current version of the prosthesis. The Nucleus 22 Channel Cochlear Implant System has been described in detail elsewhere. 1, 5
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ItemSpeech processing for electrical stimulation of the auditory nerve( 1986)The development of cochlear prostheses which provide hearing .sensation to those previously totally deaf by means of electro-neural stimulation has brought new hope for normal communication to a portion of the deaf community that had previously been beyond help by conventional hearing aids. A cochlear prosthesis provide hearing sensation by exciting nerve fibres in the auditory nerve using small electrical current passed through one or more electrode placed in or around the cochlea. Once this artificial link in the auditory information pathway has been established there still remain considerable challenge in the selection of appropriate coding of information to be transmitted along it. In this paper we consider the design of signal processing necessary for an effective speech perception, prosthesis via the electrical stimulation of the auditory nerve.
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ItemA formant-estimating speech processor for cochlear implant patients(Speech Science and Technology Conference, 1986)A simple formant-estimating speech processor has been developed to make use of the "hearing" produced by electrical stimulation of the auditory nerve with a multiple-channel cochlear implant. Thirteen implant patients were trained and evaluated with a processor that presented the second formant frequency, fundamental frequency, and amplitude envelope of the speech. Nine patients were trained and evaluated with a processor that presented the first formant frequency and amplitude as well. The second group performed significantly better in discrimination tasks and word and sentence recognition through hearing alone. The second group also showed a significantly greater improvement when hearing and lipreading was compared with lipreading alone in a speech tracking task.
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ItemA model of auditory-visual speech perception(Speech Science and Technology Conference, 1986)A mathematical model relating the probabilities of correctly recognizing speech features, phonemes, and words was tested using data from the clinical trial of a multiple-channel cochlear implant. A monosyllabic word test was presented to the patients in the conditions hearing alone (H), lipreading alone (L), and hearing plus lipreading (HL). The model described the data quite well in each condition. The model was extended to predict the HL scores from the feature recognition probabilities in the H and L conditions. The model may be useful for the evaluation of automatic speech recognition devices as well as hearing impaired people.