Graeme Clark Collection

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    Chronic monopolar high rate simulation of the auditory nerve: physiological and histopathological effects
    TYKOCINSKI, MICHAEL ; Linahan, Neil ; Shepherd, R. K. ; Clark, Graeme M. (Kugler Publications, 2001)
    There is clinical interest in the development of high rate speech processing strategies, since there are indications that these might enhance speech perception due to an improved representation of the rapid variations in amplitude of speech. Significant improvement in speech perception using high rate stimulation has been demonstrated in cochlear implant recipients. However, it is important that the long-term safety of high rate stimulation is clearly established prior to its general clinical application. This is especially important, since acute animal studies have shown that high rate stimulation can induce a reduction in the excitability of the auditory nerve. This was also associated with an increase in both threshold and latency of the electrically evoked auditory brainstem response (EABR). However, while a chronic stimulation study indicated that monopolar electrical stimulation of the auditory nerve at rates of 1000 pulses per second (pps)/channel (three channels) had no adverse effects on the spiral ganglion cell density (SGCO),5 there is limited data concerning higher rates. In the present study, we evaluated the electrophysiological and histopathological effects of chronic monopolar electrical stimulation of the auditory nerve using considerably higher stimulus rates than have been used in previous studies.
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    Application of MEMS to cochlear implants
    Zhang, A. L. ; Huigen, J. M. ; Clark, Graeme M. ( 1998)
    A cochlear implant restores some hearing by electrically stimulating residual auditory nerve fibers in the cochlea. The cochlear implant represents a major scientific and technological breakthrough and is now providing hearing for thousands of profoundly and totally deaf people around the world. In this paper, we review the present multiple-channel cochlear implant technology and explores potential applications of micro-electro-mechanical system (MEMS) technology. A new generation of electrode arrays based on the silicon micromachining technology is presented. Approaches in the use of MEMS technology for a middle ear acoustic sensor in a totally implantable prosthesis is also discussed, with key issues for its development highlighted.
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    Cochlear implants in the second and third millennia
    Clark, Graeme M. ( 1998)
    Much has been achieved in the Second Millennium in the development of cochlear implants for profoundly deaf people, but further advances in the Third Millennium should result in most severely to profoundly deaf people being able to communicate effectively in a hearing community.
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    Direct current measurements in cochlear implants: an in vivo and in vitro study
    Huang, Christie Q. ; Carter, Paul M. ; Shepherd, Robert K. ; Seligman, Peter M. ; Tabor, Bruce ; Clark, Graeme M. ( 1998)
    Direct current (DC) was measured both in vivo and in vitro in cochlear implant electrodes with stimulation at moderate to high pulse rates in monopolar and bipolar modes. In vivo DC was approximately 2-3 times higher than that measured in vitro. In vivo DC levels were <100 nA even at very high rates, although DC levels increased as a function of stimulus rate and charge intensity. DC levels were lower: in the monopolar than in the bipolar stimulation condition. Stimulation with a monopolar capacitively coupled extracochlear electrode showed even lower DC levels in the intracochlear .electrodes. Our results indicated that the Nucleus electrode shorting system is able to maintain a low level of DC during very high rate stimulation for both monopolar and bipolar modes.
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    Peri-modiolar electrode arrays: a comparison of electrode position n the human temporal bone
    Shepherd, R. K. ; Treaba, C. G. ; Cohen, L. ; Pyman, B. ; Huigen, J. ; Xu, J. ; Clark, Graeme M. (Monduzzi Editore, 1997)
    This paper describes a radiologic evaluation of three types of peri-modiolar arrays, comparing their trajectory within the scala tympani with a standard Mini-22 electrode. All peri-modiolar arrays were found to lie closer to the modiolus for much of their insertion length compared with the standard array. While one design showed evidence for the potential of increased insertion trauma, two designs produced satisfactory results. Although further electrode development, temporal bone and histopathologic studies arE required, it would appear that the benefits of peri-modiolar electrode arrays will be realised clinically.
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    Evoked potential assessment of children with severe/profound hearing loss: a comparison of steady-state evoked potential (SSEP) and behavioural hearing threshold levels in subjects with absent click evoked auditory brainstem responses (ABR)
    Rance, G. ; Dowell, Richard, C. ; Rickards, F. W. ; Clark, Graeme M. (Monduzzi Editore, 1997)
    Steady-state evoked potential testing offers a means of obtaining accurate, frequency specific estimates of hearing threshold in subjects with even severe to total hearing losses. As such, the technique can play an important role in the preoperative evaluation of young cochlear implant candidates.
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    Siting the receiver-stimulator of the CI-24M model of the Cochlear Limited multiple-channel cochlear implant and fixation of its electrode array
    PYMAN, BRIAN ; Clark, Graeme M. (Monduzzi Editore, 1997)
    The correct siting of the cochlear implant receiver-stimulator package is important. The package should not obtrude significantly above the surface of bone, and should be so that blows to the head do not in damage to the package, skull or brain. The cochlear implant electrode array must be fixed at a site close to the cochlea, so that the electrode will not slide out, or be subject to differential movement with growth changes. Fixation, with Dacron® mesh, platinum-iridium ties, or clips, has been in the region of the posterior root of the zygoma and the floor of the antrum. Our research studies demonstrate that a specially-designed collar around the array can be placed through the cochleostomy and provide the necessary locking to prevent retraction of the array. It has a ceramic surround to encourage union with neighbouring bone, and stress relief to reduce wire fracture at the point where the array leaves the cochlea.
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    Development of silicon microelectrodes for cochlear implant technology
    Parker, Joanna R. ; Harrison, H. Barry ; Clark, Graeme M. ; Patrick, Jim ; Reinhold, Olaf (Piscataway, 1997)
    Silicon fabrication technology is being explored as a possible solution to the manufacturing of advanced cochlear implant electrode arrays. Silicon probes have been produced with thickness of 5?m and coated in Parylene� polymer to provide strength. To enable handling they are given a backing of silicone rubber before surgical use. This paper presents some techniques used to produce such silicon microelectrodes.
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    The effect of pulsatile intracochlear electrical stimulation on intracellularly recorded cochlear nucleus neurons
    Paolini, Antonio, G. ; Clark, Graeme M. (Monduzzi Editore, 1997)
    The anterior division of the ventral cochlear nucleus (AVCN) is the first relay station of the auditory pathway. We examined responses of neurons in the A VCN to intracochlear electrical stimulation using in vivo intracellular recordings. Twin pulse stimulation results indicated that these neurones evoke action potentials which are able to follow pulsatile stimulation at high rates. This ability to respond to each pulse along the stimulus train diminished when stimulus duration was increased to 50 ms. At rates 400 Hz and below in all neurones tested a deterministic response was seen to this longer duration pulsatile stimulation. With increasing rate of stimulation the response become more stochastic with apparent loss of encoding ability. These results have in1pIications in the clinical application of cochlear implants operating at high stimulus rates.
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    The relationship between aetiology of hearing loss and outcome following cochlear implantation in a paediatric population
    O'Sullivan, P. G. ; Ellul, S. M. ; Dowell, B. C. ; Clark, Graeme M. (Monduzzi Editore, 1997)
    Eighty-eight children who underwent cochlear implantation at the University of Melbourne Cochlear Implant Clinic are reviewed. The aetiology of the hearing loss is classified and is compared to their best level of speech perception performance. The group whose hearing loss was not congenital in origin performed better than those who were congenital in origin. Of those whose hearing loss was congenital in nature those with rubella appeared to perform best.