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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ItemReduction in excitability of the auditory nerve in guinea pigs following acute high rate electrical stimulation [Abstract]( 1996)Electrical stimulation of neural tissue involves the transfer of charge to tissue via electrodes. Safe charge transfer can be achieved using biphasic current pulses designed to reduce the generation of direct current (DC) or the production of electrochemical products. However, neural stimulators must also use capacitors in series with electrodes, or electrode shorting between current pulses, to further minimize DC due to electrode polarization.
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ItemComparison and alternate designs for peri-modiolar electrode arrays: insertion trauma and position [Abstract]( 1997)While it has been shown that the straight but flexible banded electrode array can be safely inserted into the scala tympani of the human cochlea, histological studies have revealed that the array lies along the outer walll.2. Since a profound total hearing loss is generally associated with a moderate to complete degeneration of the spiral ganglion peripheral process, these electrodes lie some distance from their target neural population -the spiral ganglion soma -located within Rosenthal's canal.
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ItemThe interaction between the cytokine LIF and the neurotrophins on spiral ganglion cells [Abstract]( 1997)The survival of auditory neurones depends on the continuous supply of trophic factors. Hair cells within the cochlea are known to produce supply growth factors responsible for the survival and growth of The survival of auditory neurones depends on the continuous supply of trophic factors. Hair cells within the cochlea are known to produce supply growth factors responsible for the survival and growth of [ ] neurones. As a result of trauma, disease or ageing cochlear hair cells are lost, and consequently, the supply of growth factors is reduced, leading to secondary wave of auditory nerve degeneration.
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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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ItemElectrical stimulus induced changes in excitability of the auditory nerve( 1997)High rate electrical stimulation of the auditory nerve using stimulus intensities well above the clinical limits can induce a significant reduction in the excitability of the auditory nerve as measured by a decrement in the amplitude of the electrically evoked auditory brainstem response (EABR). Two potential mechanisms may be associated with this stimulus induced reduction in activity: 1) stimulus induced prolonged neuronal hyperactivity; and 2) the generation of adverse electrochemical productions from the electrode surface. The purpose of the present study was to assess the extent to which adverse electrochemical damage contributes to the stimulus induced reduction in auditory nerve excitability. Twenty-six adult guinea pigs anaesthetized with ketamine (40 mg/kg i.p.) and xylazine (4 mglkg i.p.), were bilaterally implanted and unilaterally stimulated for two hours using a stimulus intensity of two or four times EABR threshold. Stimulus rates of 200, 400, or 1000 pulses/s (pps) were delivered via a standard platinum scala tympani electrode or large surface area ("high Q") platinum electrode.
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ItemDecrement in auditory nerve function following acute high rate stimulation using various stimulus paradigms in guinea pigs [Abstract]( 1996)Previous experimental studies have shown that chronic electrical stimulation of the auditory nerve using charge balanced biphasic current pulses at rates of up to 500 pulses per second (pps) do not adversely affect the adjacent spiral ganglion population. More recently psychophysical trials have indicated that speech processing strategies based on high pulse rates (1000 pps or more) can improve speech perception in cochlea implant patients. In this paper we summarize the results following acute high rate stimulation using different stimulus paradigms.
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ItemDecrement in auditory nerve function following acute high rate stimulation in guinea pigs [Abstract]( 1995)Cochlear implants have been shown to successfully provide profoundly deaf patients with auditory cues for speech discrimination. Psychophysical studies suggested that speech processing strategies based on stimulus rates of up to 1000 pulses per second (pps) may lead to an improvement in speech perception, due to a better representation of the rapid variations in the amplitude of speech. However, "neural fatigue" has been known to occur following brief periods of electrical stimulation at rates high enough to ensure that stimuli occur within the neurons relative refractory period, and has been shown to depend on stimulus duration and rate of the evoked neural activity. Prolonged electrical stimulation at these high stimulus rates could, therefore, have an adverse effect on the neurons metabolism and result in cellular energy depletion.