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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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.