Graeme Clark Collection
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ItemChronic electrical stimulation of the auditory nerve at high rates: I. Effect on residual hearing [Abstract]( 1996)In addition to direct excitation of auditory nerve fibres, cochlear implant patients with small amounts of residual hearing may receive important additional auditory cues via electrophonic activation of hair cells 1. Before incorporating electrophonic hearing into speech processing strategies, the extent of hair cell survival following cochlear implantation must first be determined. We have recently demonstrated widespread survival of hair cells apical to electrode arrays implanted for periods of up to three years, the present report describes the effects of chronic electrical stimulation on hair cell survival.
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ItemChronic electrical stimulation of the auditory nerve at high rates: II. Cochlear pathophysiology [Abstract]( 1996)A major factor in the improved performance of cochlear implant patients has been the use of high stimulus rate speech processing strategies. While these strategies show clear clinical advantage, we know little of their long-term safety. Indeed, recent studies have indicated that high stimulus rates at intensities above clinical limits, can result in neural damage as a result of prolonged neuronal hyperactivity. The present study was designed to evaluate the effects of chronic electrical stimulation of the auditory nerve at high rates, using intensities within clinical limits.
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ItemSpatial representation of the cochlea within the inferior colliculus of neonatally deafened kittens following chronic electrical stimulation of the auditory nerve [Abstract]( 1995)The orderly tonotopic representation of the cochlea is accurately reproduced within the central auditory system of normal hearing animals. Any degradation of this representation as a result of a neonatal hearing loss or chronic electrical stimulation during development could have important implications for the use of multichannel cochlear implants in young children. In the present study we have used 2-deoxyglucose autoradiography (2-00) to examine the topographic representation of the cochlea within the inferior colliculus (IC) of neonatally deafened kittens following periods of chronic intracochlear electrical stimulation.
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ItemImplantation of the new nucleus C1-3 receiver stimulator and electrode array [Abstract]( 1997)There is an important need to fix the cochlear implant electrode array 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 sites have been in the region of the posterior root of the zygoma and the floor of the antrum. Fixation has been by Dacron mesh ties platinum wire ties, or clips inserted with-special instruments. Biological cements have previously been tried but found to be toxic. The most ideal site is in the region of the cochleostomy.
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ItemTemporal coding for sound and tempor-spatial patterns of electrical stimulation [Abstract]( 1997)The anterior division of the ventral cochlear nucleus (AVCN) is the first relay station of the auditory pathway. It receives auditory information via the auditory nerves emanating from the cochlea. Electrical stimulation via current cochlear implants [ ] does not lead to responses at the cochlear nucleus that exactly match tho elicited by comparable auditory stimulation. Complex temporal patterns of electrical stimulation may provide a better simulation of the acoustic input.
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ItemIntracellular responses of ventral cochlear nucleus neurones to acoustic stimulation in the rat( 1997)The ventral cochlear nucleus (VCN) is the first relay station of the auditory pathway. Presently, little is known about the acoustically evoked intracellular response of neurones in the VCN. We investigated the effect of acoustic stimulation on neurones in the rat VCN using in vivo intracellular recordings and dye-filling. In male rats anaesthetised with urethane (1.3g/kg i.p) microelectrodes containing 1M potassium acetate, or with 4% neurobiotin, were inserted into the VCN. Stable impalements were made from 37 neurones classified as having a Primary-like (n=13), Primary-like with notch (n=4) and Chopper (n=20) response to acoustic stimulation (50 ms pure tones, 5 ms r/f time, 0.2 Hz repetition).
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ItemIncreased survival of auditory neurones treated with LIF( 1997)Degeneration of spiral ganglion cells (SOC) is one of the most common correlates of sensorineural hearing loss (1). Several lines of evidence show that the continued supply of growth factors is responsible for maintaining auditory neurone integrity (2). In the present study SOC cultures were used as a model of auditory innervation to test the ability of the cytokine leukaemia inhibitory factor (LIF) and the neurotrophin NT -3 to promote neuronal survival individually or in combination. The data demonstrate that LIF promotes the survival of SOC in a concentration-dependent manner, with a significant increase in neuronal survival at concentrations as low as 0.1 ng/ml compared to untreated wells ( p< 0.05), and a maximum neuronal survival at 10 ng/ml. In addition, when used in combination LIF and NT-3 were more effective in promoting neuronal survival than either factor individually, with a significant increase in survival at concentrations of 0.1ng mI[to the power of]-1/0.1 ng mI[to the power of]-1 (LIF/NT-3). To our knowledge this is the first study reporting that LIF has trophic activity on SOC. Moreover, the data suggest that a combination of several growth factors may provide a better approach when developing pharmacological therapies for auditory neuron repair.
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ItemIntracochlear factors contributing to psychophysical percepts following cochlear implantation( 1997)Variations of performance of cochlear implant patients may be related to several factors. In this anatomical study we focus on determining how intracochlear factors affect postoperative psychophysical percepts of the 22-channel cochlear implant system. We have used 3dimensional (3D) computer reconstruction of cochleas of former Nucleus 22-channel implant patients to quantitatively map intracochlear pathology relative to electrode positions. and relate the type and quantity of pathology to the T and C levels and dynamic ranges of individual electrodes. Preliminary results of this study were presented at the International Symposium on Cochlear Implants, Speech and Hearing Research in Melbourne, October, 1994 (I ).
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ItemTowards a SSEP hearing screening test for neonates [Abstracts]( 1997)Newborn hearing screening tests utilizing evoked otoacoustic emissions (EOAEs) and/or the auditory brainstem response (ABR) recording have been recommended by the NIH(1993). The University of Melbourne experience with steady-state evoked potentials (SSEPs) in newborns suggests that they too, have potential as a screening tool (Rickards et al, 1984). In the present study, SSEPs were recorded from neonates in a pilot study of how the technology may be applied to newborn hearing screening. Eighty-eight neonates were tested using transient-and distortion product EOAEs, ABRs and SSEPs. Only those infants who had TEOAEs, DPOAEs, and ABRs that met a rigid and objective "pass" criteria were included in the study. SSEPs were evoked by amplitude modulated tones using carrier frequencies at 500, 1000, 2000, and 4000 HZ, and modulation frequencies between 75 and 95 Hz presented at levels between 40 and 80 dB SPL. Out of 324 individual SSEP tests, 12% were no-stimulus control trials. Phase-coherence statistics were used to determine when a significant or "passing" result had been obtained. Preliminary results show that a 94% pass rate is achieved for a CF of 2 KHZ, presented at 60 dB SPL, but that pass rates are lower (79% to 89%) for 500, 1KHz and 4KHZ CF's at the same stimulus levels. For lower stimulus levels, pass rates do not exceed 80% for any carrier frequency. Phase-coherence estimates were made for 32, 64, ahd 96 samples at each carrier and modulation frequency combination. We will report the phase coherence estimates for each CF, MF and level combination as a function of sample size, in order to suggest a protocol that may be efficient in newborn hearing screening applications.
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ItemPsychophysics and speech perception with a premodiolar electrode array( 1997)A perimodiolar electrode array, designed to lie closer to the modiolus than the standard Coclear Limited array and developed by the Cooperative Research Centre for Cochlear Implant, Speech and Hearing Research in conjunction with Cochlear Limited, has recently been implanted in an adult patient. While excellent speech results have been obtained using the standard array, it is hypothesised that the improved place coding expected from an array lying closer to the modiolus would result in improved speech perception. Analysis of a modified Stenvers view X-ray revealed that the most apical electrode was inserted to an angle of 311 0 and that the apical half of the array was considerably closer to the modiolus than would have been the case for a standard array. Thresholds and comfortable levels were correlated with the lateral position of the stimulated electrode in the scala tympani, being much reduced for the apical electrodes. Forward masking and electrode discrimination are being measured in order to study the spread of neural excitation and the distinctness of percepts for different electrodes.