Graeme Clark Collection
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ItemChronic monopolar high rate simulation of the auditory nerve: physiological and histopathological effects(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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ItemAdvances in computational modelling of cochlear implant physiology and perception(IOS Press, 2001)Models of cochlear implant physiology and perception have historically utilized deterministic descriptions of auditory-nerve (AN) responses to electrical stimulation, which ignore stochastic activity present in the response. Physiological models of AN responses have been developed that do incorporate stochastic activity [8][13][14][27][38][39], but the consequences of stochastic activity for the perception of cochlear implant stimulation have not been investigated until recently [3]. Such an investigation is prompted by inaccuracies in predicting cochlear implant perception by deterministic models. For example, studies of single-fiber responses, where only an arbitrary deterministic measure of threshold is recorded, do not accurately predict perceptual threshold versus phase duration (strength-duration) curves for sinusoidal stimulation [24] or for pulsatile stimulation [25][26]. Furthermore, strength-duration curves of cochlear implant users are not well predicted by deterministic Hodgkin Huxley type models [25] [30].However, the complexity of previous stochastic physiological models has made the computation of responses for large numbers of fibers both laborious and time-consuming. Furthermore, the parameters of these models are often not easily matched to the fiber characteristics of the auditory nerve in humans or other mammals. This has prompted us to develop a simpler and more computationally efficient model of electrical stimulation of the auditory nerve [1][2][4] which is capable of direct and rapid prediction of perceptual data[3]
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ItemA multi-threshold neural network for frequency estimation( 1996)Human perception of sound arises from the transmission of action-potentials (APs) through a neural network consisting of the auditory nerve and elements of the brain. Analysis of the response properties of individual neurons provides information regarding how features of sounds are coded in their firing patterns, and hints as to how higher brain centres may decode these neural response patterns to produce a perception of sound. Auditory neurons differ in the frequency of sound to which they respond most actively (their characteristic frequency), in their spontaneous (zero input) response, and also in their onset and saturation thresholds. Experiments have shown that neurons with low spontaneous rates show enhanced responses to the envelopes of complex sounds, while fibres with higher spontaneous rates respond to the temporal fine structure. In this paper, we determine an expression for the Cramer-Rao bound for frequency estimation of the envelope and fine structure of complex sounds by groups of neurons with parameterised response properties. The estimation variances are calculated for some typical estimation tasks, and demonstrate how, in the examples studied, a combination of low and high threshold fibres does not improve the estimation performance of a fictitious 'efficient' observer, but may improve the estimation performance of neural systems, such as biological neural networks, which are based on the detection of dominant interspike times.
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ItemThe influence of electrode geometry on the electrically evoked auditory brain stem response( 1988)The electrically-evoked auditory brainstem response (EABR) consists of a series of far-field potentials that reflect synchronous neural activity within the auditory brainstem in response to a transient electrical stimulus. The EABR appears relatively simply organized in terms of its amplitude and latency behaviour. The growth in amplitude of wave IV of the EABR, for example, reflects changes in the amplitude of the electrically-evoked VIII nerve compound action potential as a function of stimulus intensity. In addition, single unit population studies have shown a monotonic relationship between the growth in EABR amplitude and the number of nerve fibres being stimulated (Merzenich and White, 1977). The EABR can therefore, provide an insight into the response of the auditory nerve to electrical stimulation. We have used this technique to investigate the efficacy of electrical stimulation of the auditory nerve using a variety of stimulating electrode geometries.
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ItemElectrical stimulation of the auditory nerve: stimulus induced reductions in neural excitability [Abstract]( 1987)Electrical stimulation of the auditory nerve elicits highly synchronised neural activity (Javel et al., in press). As the stimulus current is increased the neural response becomes highly deterministic with every current pulse eliciting a spike even at stimulus rates of 600-800 pulses per second (pps). Our previous acute experimental studies have shown that high stimulus rates (> 200 pps) and high stimulus currents (> 1.0 mA) can result in temporary and sometimes permanent reductions in the excitability of the auditory nerve (Shepherd and Clark, 1986). The present study was designed to examine the mechanisms underlying these stimulus induced reductions in excitability. These results will have implications for the maximum safe and effective stimulus rates that can be employed in cochlear implants.