Graeme Clark Collection
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ItemA multiple-channel cochlear implant: an evaluation using open-set CID sentences( 1981)A multiple-channel cochlear implant and speech processor have been used in two postlingually deaf adult patients with a total hearing loss, to enable them to perceive varying degrees of running speech. The results have been confirmed with open-set CID everyday sentence tests. Using the implant alone, the patients obtained 8% and 14% scores with pre-recorded material, and 34% and 36% scores for "live" presentations. This was equivalent to the perception of 35% of connected discourse. When the implant was used in conjunction with lipreading, improvements of 188% and 386% were obtained over lipreading alone, and the scores were 68% and 98% which were equivalent to the perception of 60% and 95% of connected discourse.
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ItemSpeech processor design for a multiple-channel cochlear implant( 1980)This paper outlines the strategy adopted for a laboratory-based speech processor used to provide speech information to patients with a multiple-channel cochlear implant It also presents the results of vowel and consonant recognition studies and speech test using open sets of words and sentences.
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ItemHearing restoration with the multichannel auditory brainstem implant( 1997)Restoration of useful hearing is now possible in patients with bilateral acoustic neuromas by direct electrical stimulation of the cochlear nucleus. Our first experience with the Multichannel Auditory Brainstem Implant is reported. A forty four year old female with bilateral acoustic neuromas and a strong family history of Neurofibromatosis Type II presented with profound bilateral hearing impairment. Translabyrinthine removal of the right tumour was performed with placement of the Nucleus eight electrode Auditory Brainstem Implant. Intraoperative electrically evoked auditory brainstem response monitoring successfully confirmed placement over the cochlear nucleus. Postoperatively, auditory responses were obtained on stimulation of all electrodes with minimal non-auditory sensations. The patient now receives useful auditory sensations using the "SPEAK" speech processing strategy. Auditory brainstem Implantation should be considered for patients with Neurofibromatosis Type II in whom hearing preservation tumour removal is not possible.
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ItemDelayed neurotrophin treatment supports auditory neuron survival in deaf guinea pigs [Abstract]( nd)The cochlear implant provides auditory cues to patients with a severe profound hearing loss by direct electrical stimulation of the auditory nerve. As such, the total number and integrity of the surviving auditory neuron population may govern the benefits that patients can derive from the implants. Therefore, the rescue of auditory neurons from degeneration following the loss of hair cells is of great therapeutic significance. Neurotrophic factors are known to be important for the development and maintenance of the auditory system I, and have also been rep6rted to act as survival factors for auditory neurons in animal models of deafness. However, while studies have demonstrated that the application of neurotrophins into the inner ear shortly following deafening can prevent auditory neuron degeneration2,3, much less is known about the survival effects of delayed neurotrophin treatment, which is a clinically more realistic model. This study therefore examined the effects of delayed neurotrophin treatment on auditory neuron survival following deafening. Specifically, we aimed to determine if any or all of the neurotrophins -BDNF, NT -3, NT-4/5 and NGF -could rescue neurons from degeneration after a period of two weeks of deafuess. Normal hearing guinea pigs were bilaterally deafened J using a combination of the aminoglycoside kanamycin and the loop diuretic frusemide. Two weeks later the left cochleae were implanted with a cannula attached to a mini-osmotic pump, which delivered 10Ilg of neurotrophin over a period of 28 days. The right cochleae acted as deafened and untreated controls. Despite the delayed treatments, each of the four neurotrophins prevented the degeneration of auditory neurons that is normally seen following loss of hair cells. When compared to normal hearing animals, the neuronal survival rates of deafened, neurotrophin-treated animals ranged between 79 87%; in contrast, deafened, untreated controls displayed only 52% neuronal survival. Current work is also investigating the expression patterns of the neurotrophin Trk receptors in relation to these findings, and these results will also be discussed. The results of this study provide further support to the theory that neurotrophic factors may be able to be used as therapeutic agents for the benefit of the hearing impaired community.
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ItemCochlear implants: a personal scientific journey [Abstract]( 2002)Electrical stimulation of the auditory system to reproduce hearing commenced through academic curiosity, and the hope of helping deaf people. It received direction from neurophysiology, and later psychophysics and speech science. In the 1960s and 1970s there were many questions requiring answers before cochlear implants could become a practical reality. Key concerns were: (1) the cochlea was too complex for electrical stimulation to reproduce the coding of sound; (2) multiple electrodes inserted into the cochlea for the place coding of frequency could damage the auditory nerves to be stimulated; (3) speech was too complex to be reproduced by electrical stimulation; and (4) children born deaf would not develop the appropriate neural connectivity for speech understanding. The first questions were addressed on the experimental animal. Speech research on patients was only possible with the advent of silicon chip technology allowing the development of an implantable receiver-stimulator package. Initial research established proof of principle that connected discourse was possible with multiple electrode stimulation of the auditory nerve in severely and profoundly deaf people. The research has been developed industrially for the benefits to be provided on a widespread basis through clinics worldwide. Further research has resulted in continuing improvements so that the average profoundly deaf person can hear as well as someone with severe hearing loss using a hearing aid. There is still much research required to achieve high fidelity sound, hearing in noise, and totally implantable devices.
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ItemCochlear implants for adults and children(Martin Dunitz, 2002)Cochlear implants which use multiple-electrode speech-processing strategies are now established clinical entity for children and adults, as a result, preoperative selection and (re)habilitation are key issues. It is hard to realize that it was only in the 1960s and 1970s when many scientists and clinicians said that successful cochlear implants were not possible in the foreseeable future. The questions that had to be addressed by a multi disciplinary research effort are discussed, and the solutions achieved from the University of Melbourne's perspective are presented. However, the main aim of this chapter is to focus on preoperative selection, and (re)habilitation, including the results obtained. These issues are discussed primarily with reference to data from the University of Melbourne's Cochlear Implant Clinic at the Royal Victorian Eye and Ear Hospital. As this is a book on audiological medicine only, an overview of surgical principles is presented. The surgical management of the patient is, of course, very important, so for more details the reader is referred elsewhere. Cochlear implantation has also been the subject of quite intense ethical debate, particularly over its use for children. For this reason, a discussion of ethical issues is included. Finally, the chapter concludes with a vision of research in the next Millennium.
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ItemHistopathology of the binaural cochlear implant subject [Abstract]( 2001)Binaural hearing improves speech reception in noise, and is necessary for sound localisation. Normal hearing subjects use both interaural time, and intensity, differences to localise sound. This study investigates why sound localisation in bilateral cochlear implantees is insensitive to interaural time differences (Hoesel 1993). We looked for evidence of neural degeneration in the auditory brainstem involved in binaural sound localisation, since this may have degraded the neural circuitry required to accurately code interaural time delays. Method: The brainstem of a bilateral cochlear implantee was prepared for light microscopy by embedding it in paraffin, sectioning at 10 mm and staining sections with thionine or Luxol fast blue (LFB). The histological sections were digitised with NIH Image and 3-dimensional reconstructions made of the cochlear nucleus (CN) and superior olivary complex (SOC) with AnalysePC. Within the CN and the SOC, cell number and size were estimated by the physical dissector technique following thionine staining, and myelination of the nerve fibres was estimated using the optical density method following LFB staining. Results: A reduction in cell size (from thionine staining) and myelination (from LFB staining) was seen in both the CN and the SOC. Conclusions: These finding are consistent with neural degeneration within the auditory pathways. This may have lead to a degradation of the neural circuitry required to accurately detect interaural time delays.
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ItemElectrical stimulation of the auditory nerve with a cochlear implant and the temporal coding of sound frequencies: a brief review( 1997)There is considerable evidence that the brain translates (encodes) the frequency of a sound into both place of excitation (place encoding), and the pattern of intervals between action potentials (temporal encoding). Furthermore, temporal encoding is now thought to be due to a temporal as well as spatial pattern of action potentials in a small group of neurons. This pattern needs to be reproduced with a cochlear implant for improved speech processing. Our recent research has also demonstrated that the timing of excitatory postsynaptic potentials seen with intracellular recordings from brain cells, rather than extracellularly recorded action potentials, correlates better with the frequency of sound. These excitatory postsynaptic potentials are likely to be the link between the patterns of action potentials arriving at nerve cells and the biomolecular activity in the cell. This response also needs to be replicated with improved speech processing strategies.
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ItemSpeech results with a bilateral multi-channel cochlear implant subject for spatially separated signal and noise( 1999)Speech tests in noise were administered to a bilaterally implanted cochlear implant subject. Performance for simultaneous use of two identical implants, with the same speech processing strategy on two independent standard clinical processors, was compared with that of the better performing monaural side alone. Speech was presented at an angle of 45 degrees toward one ear, with noise at 45 degrees toward the contralateral side. Tests were also administered for speech and noise reversed in location. When the speech signal was on the same side as the subject's better performing ear, monaural and binaural tests resulted in similar scores. When the speech was on the opposite side, however, the binaural condition showed significantly better speech scores. The results indicate that binaural implants can provide improved performance in noise when speech and noise arc spatially separated.
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ItemThe effects of stochastic neural activity in a model predicting intensity perception with cochlear implants: low-rate stimulation( 1999)Most models of auditory nerve response to electrical stimulation are deterministic, despite significant physiological evidence for stochastic activity. Furthermore, psychophysical models and analyses of physiological data using deterministic descriptions do not accurately predict many psychophysical phenomena. In this paper we investigate whether inclusion of stochastic activity in neural models improves such predictions. To avoid the complication of interpulse interactions and to enable the use of a simpler and faster auditory nerve model we restrict our investigation to single pulses and low-rate (<200 pulses/s) pulse trains. We apply signal detection theory to produce direct predictions of behavioural threshold, dynamic range and intensity difference limen. Specifically, we investigate threshold versus pulse duration (the strength-duration characteristics), threshold and uncomfortable loudness (and the corresponding dynamic range) versus phase duration, the effects of electrode configuration on dynamic range and on strength-duration, threshold versus number of pulses (the temporal-integration characteristics), intensity difference limen as a function of loudness, and the effects of neural survival on these measures. For all psychophysical measures investigated, the inclusion of stochastic activity in the auditory nerve model was found to produce more accurate predictions.