Graeme Clark Collection
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ItemPhysiological and histopathological effects of chronic intracochlear electrical stimulation(Monash University Press, 1983)Direct and r.f. currents are known to result in destruction of neural tissue. However, it is now apparent that non-destructive electrical stimulation can be achieved by the use of biphasic pulsatile stimuli (Lilly, 1960; Mortimer et al., 1970; Hughes et al., 1980). Although maximum biologically safe stimulation regimes have yet to be clearly defined, the evidence of a number of investigators suggests that charge density per phase and charge injection per phase are important parameters when establishing biologically safe levels of electrical stimulation (Pudenz et al., 1975; Pudenz et al., 1977; Brown et al., 1977; Babb et al., 1977). Furthermore, considerable attention has been given to ensure that the stimulus is not producing adverse electrochemical reactions that could result in physical or toxic injury to the biological environment. Brummer et al. (1977) have defined the upper limit of electrochemically safe electrical stimulation for platinum electrodes as charge balanced biphasic pulses at a maximum charge density of 300 ?C/cm2 geom./phase.
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ItemThe auditory brainstem response in hearing and deaf cats evoked by intracochlear electrical stimulation(Monash University Press, 1983)This study was performed to investigate in detail the auditory brainstem response (ABR) for intracochlear electrical stimulation. Brainstem response audiometry is a simple, noninvasive procedure with the responses under many stimulus conditions being readily understood in terms of single auditory nerve discharge properties. The amplitude and latency behaviour of the Nl brainstem response correlates well with that recorded directly from the auditory nerve (Huang & Buchwald, 1978). In addition, the brainstem response can be divided into frequency-specific components corresponding to tonotopical locations in the cochlea, as exhibited in the method of derived responses (e.g. Parker &Thornton, 1978). It is therefore well suited to both physiological and clinical investigation of auditory function and therefore should be useful in evaluating auditory function under conditions of electrical stimulation of the cochlea.
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ItemElectrical stimulation of the human cochlea: psychophysical and speech studies(Plenum Publishing Corporation, 1981)This report describes psychophysical and speech studies conducted on two of our post-lingually deaf patients implanted with the nature of the hearing sensations produced by the individual electrodes, and to investigate the feasibility of the transmission of speech information to higher centres by means of cadences of stimulation using on electrode at a time. Two totally deaf patients (MC1 and MC2) participated in these 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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ItemThe effect of pulsatile intracochlear electrical stimulation on intracellularly recorded cochlear nucleus neurons(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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ItemTemporal coding in auditory neurons to electrical stimulation(Monduzzi Editore, 1997)Different electrically evoked response properties are elicited by similar acoustically differentiated AVCN units. Discharge entrainment and synchrony of some AVCN units is maintained throughout the electrical stimulus duration at all rates, similar to that described for auditory nerve fibres. Other units exhibit a decline in discharge entrainment over the duration of the electrical stimulus with increasing rate. Within this group of units, some exhibit a highly synchronous response while others show a decline in the response synchrony with increasing stimulus rate.
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ItemCochlear implant research directions(Monduzzi Editore, 1997)Frequency Coding: Initial cochlear implant research (Clark, 1969) showed that with electrical stimulation of the auditory nerve there is an electroneural "bottle-neck" limiting the flow of information from sound to the central auditory nervous system. This electroneural "bottle-neck" is due to the difficulty in simulating with electrical stimulation the temporal as well as the place coding of frequency. One of the main aims of our research is to improve cochlear implant performance by widening the "bottle-neck" with better simulation of the temporal and place coding of frequency. Temporal coding is considered to be due to a direct relationship between the intervals between action potentials and the period of the sound wave. Temporal coding is thought to apply to low frequencies, but its importance for high frequencies is still not clear. Place coding is due to excitation of specific sites within the cochlea and the central auditory pathways 'so that a frequency scale is preserved anatomically (i.e. the brain is organized tonotopically).
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ItemAn improved model of electrical stimulation of the auditory nerve(Monduzzi Editore, 1997)Mathematical models are a useful means of formally describing and investigating pertinent features of complex systems such as the human auditory system. These features may be deduced from physiological and psychophysical experiments utilising animal models or humans, and from engineering studies. Historically, models of the auditory nerve's (AN) response to electrical stimulation have ignored randomness in single-fiber activity which has been recorded in physiological studies. These models, however, have been unable to accurately predict a number of important psychophysical phenomena. In this study, a model that incorporates random activity of the AN is presented, and is shown to predict psychophysical performance. These results indicate that random activity is indeed an important part of the response of the AN to electrical stimulation.
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ItemA digital computer model of electrical stimulation in the human cochlea for auditory prosthesis research( 1990)A three-dimensional model of electrical stimulation in the human cochlea has been developed and implemented on a digital computer. The model was used to estimate the distributions of electric potential and current density in the human cochlea in response to electrical stimulation using scala tympani electrodes. The computed distributions were used to investigate the relative merits of two scala tympani electrode designs. The results showed that the electrode design consisting of a medial electrode pair in the scala tympani is a more viable alternative than a lateral electrode pair for patients suffering from profound-to-total hearing impairment.
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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.