Graeme Clark Collection

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    The histological and physiological effects of the auditory brainstem prosthesis of the auditory pathway
    Lui, Xuguang ; McPhee, Greg. ; Seldon, H. Lee ; Clark, Graeme M. (Monduzzi Editore, 1997)
    To rehabilitate profoundly deaf patients who cannot benefit from the cochlear implant due to bilateral interruption of the auditory nerve, particularly from neurofibromatosis II, the histological and physiological effects of an auditory brainstem prosthesis on the cochlear nuclei of guinea pigs were examined in order to establish the safety and the efficacy of this prosthesis.
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    Speech cues for cochlear implantees: spectral discrimination
    Henry, Belinda, A. ; McKay, Colette M. ; McDermott, Hugh, J. ; Clark, Graeme M. (Monduzzi Editore, 1997)
    The relationship between the ability of cochlear implantees to perceive speech and their ability to discriminate between stimulation on adjacent electrodes was investigated. Speech perception ability was assessed with monosyllabic words in 8 users Nucleus cochlear prosthesis. The ability of these subjects to discriminate between stimulation on adjacent electrodes, in of random loudness differences between electrodes was determined. Results were averaged in the apical, mid and basal regions of the cochlea. Regression analysis showed that speech perception ability and electrode discrimination ability were correlated in the apical to mid region of the cochlea. but the two factors were not correlated in the basal region. Therefore, these results suggest that implantees require the ability to discriminate between stimulation on adjacent electrodes in the apical to mid region of the cochlea in order to achieve high speech perception scores.
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    Speech perception of hearing aid users versus cochlear implantees
    Flynn, Mark C. ; Dowell, Richard, C. ; Clark, Graeme M. (Monduzzi Editore, 1997)
    Recent improvements in speech recognition abilities for profoundly deaf cochlear implantees suggests that some adults with a severe hearing impairment might benefit from a cochlear implant. Unfortunately, no studies have directly compared the speech perception abilities of the severely hearing impaired with those of cochlear implant users. An investigation of the speech perception performance of people with a severe hearing loss, who use hearing aids, was conducted in the Department of Otolaryngology at The University of Melbourne. Each participant (n=35) took part in a series of speech perception tests which included 11 vowel recognition tests, CNC words, CUNY sentences, and the Connected Speech Test. The results from these severely hearing impaired persons were compared to results from cochlear implant users. The group of severely hearing impaired adults performed better on tests of vowel and word perception but poorer on tests of open set sentence perception in comparison to a group of cochlear implant users.
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    Speech perception in implanted children: effects of speech processing strategy and residual hearing
    Meskin, T. ; Rance, G. ; Cody, K. ; Sarant, J. ; Larratt, M. ; Latus, K. ; Hollow, R. ; Rehn, C. ; Dowell, R.C. ; Pyman, B. ; Gibson, W.P.R. ; Clark, Graeme M. ; Cowan, Robert S. C. ; Barker, E. J. ; Pegg, P. ; Dettman, S. ; Rennie, M. ; Galvin, K. (Mendoza Editor, 1997)
    The ability of implanted children to adapt to different speech processing strategies has been demonstrated for the Nucleus implant system. Children previously experienced with the Multipeak speech processing strategy. were able to gain significant improvements in consonant, word and sentence perception using the Speak speech processing strategy. suggesting some degree of neural plasticity in neural-auditory coding. Of 192 implanted children with different degrees of preoperative residual hearing, 65% were found to obtain significant scores on open-set speech materials using electrical stimulation alone. Those children with more residual hearing had a greater probability of achieving open-set understanding and at a minimum level, perceived high frequency consonant information which would not have been available through conventional hearing aids.
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    Cochlear implant research directions
    Clark, Graeme M. (Monduzzi Editore, 1997)
    Frequency Coding: Initial cochlear implant research (Clark, 1969) showed that with electrical stimulation of the auditory nerve there is an electroneural "bottle-neck" limiting the flow of information from sound to the central auditory nervous system. This electroneural "bottle-neck" is due to the difficulty in simulating with electrical stimulation the temporal as well as the place coding of frequency. One of the main aims of our research is to improve cochlear implant performance by widening the "bottle-neck" with better simulation of the temporal and place coding of frequency. Temporal coding is considered to be due to a direct relationship between the intervals between action potentials and the period of the sound wave. Temporal coding is thought to apply to low frequencies, but its importance for high frequencies is still not clear. Place coding is due to excitation of specific sites within the cochlea and the central auditory pathways 'so that a frequency scale is preserved anatomically (i.e. the brain is organized tonotopically).
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    Auditory central nervous system plasticity: application to cochlear implantation
    Clark, Graeme M. (Monduzzi Editore, 1997)
    There are two types of plasticity in the central auditory nervous system. The first type occurs during the development of neural connections within a critical period after birth. The second type of plasticity results from a change in the central representation of neurons in the mature animal after neural connectivity has been established. Evidence of a critical period for plasticity central auditory system has been demonstrated in the ferret where a marked loss of neurons in the cochlear nucleus occurs after ablation of the cochlea five days after birth (Moore, 1990). However, ablation of the cochlea 24 days post-partem (i.e. a week before the onset of hearing) has little effect.
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    Advances in cochlear implant speech processing
    Clark, Graeme M. (Monduzzi Editore, 1997)
    A cochlear implant is a device which restores some hearing in severely-to-profoundly deaf people when the organ of Corti has not developed or is destroyed by disease or injury to such an extent no comparable hearing can be obtained with a hearing aid. When the organ of Corti is severely malfunctioning or absent, sound vibrations cannot be transduced into temporo-spatial patterns of action potentials along the auditory nerve for the coding of frequency and intensity. As a result, a hearing aid which amplifies sound is of little or no use. Our early research (Clark, 1969) emphasized that with electrical stimulation there was an electro-neural "bottle-neck" restricting the amount of speech and other acoustic information that could be presented to the nervous system. It also showed the need to use multiple-channel stimulation presented non-simultaneously, to minimize channel interaction (Clark, 1987).
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    An improved model of electrical stimulation of the auditory nerve
    Bruce, I. ; Irlicht, L. S. ; White, M. ; O'Leary, S. J. ; Dynes, S. ; Javel, E. ; Clark, Graeme M. (Monduzzi Editore, 1997)
    Mathematical models are a useful means of formally describing and investigating pertinent features of complex systems such as the human auditory system. These features may be deduced from physiological and psychophysical experiments utilising animal models or humans, and from engineering studies. Historically, models of the auditory nerve's (AN) response to electrical stimulation have ignored randomness in single-fiber activity which has been recorded in physiological studies. These models, however, have been unable to accurately predict a number of important psychophysical phenomena. In this study, a model that incorporates random activity of the AN is presented, and is shown to predict psychophysical performance. These results indicate that random activity is indeed an important part of the response of the AN to electrical stimulation.
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    Pitch and vowel perception in cochlear implant users
    Blamey, Peter J. ; Parisi, Elvira S. ( 1994)
    Two methods of determining the pitch or timbre of electrical stimuli in comparison with acoustic stimuli are described. In the first experiment, the pitch of pure tones and electrical stimuli were compared directly by implant users who have residual hearing in the non-implanted ear. This resulted in a relationship between frequency in the non-implanted ear and position of the best-matched electrode in the implanted ear. In the second experiment, one- and two-formant synthetic vowels, with formant frequencies covering the range from 200 to 4000 Hz, were presented to the same implant users through their implant or through their hearing aid. The listeners categorised each stimulus according to the closest vowel from a set of eleven possibilities, and a vowel centre was calculated for each response category for each ear. Assuming that stimuli at the vowel centres in each ear sound alike, a second relationship between frequency and electrode position was derived. Both experiments showed that electrically-evoked pitch is much lower than that produced by pure tones at the corresponding cochlear location in normally-hearing listeners. This helps to explain why cochlear implants with electrode arrays that rarely extend beyond the basal turn of the cochlea have achieved high levels of speech recognition in postlinguistically deafened adults without major retraining or adaptation by the users. The techniques described also have potential for optimising speech recognition for individual implant users.
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    Using an automatic word-tagger to analyse the spoken language of children with impaired hearing
    Blamey, P. J. ; Grogan, M. L. ; Shields, M. B. ( 1994)
    The grammatical analysis and description of spoken language of children with impaired hearing is time-consuming, but has important implications for their habilitation and educational management. Word-tagging programs have achieved high levels of accuracy with text and adult spoken language. This paper investigates the accuracy of one automatic word tagger (AUTASYS 3.0 developed for the International Corpus of English project, ICE) on a small corpus of spoken language samples from children using a cochlear implant. The accuracy of the tagging and the usefulness of the results in comparison with more conventional analyses are discussed.