Graeme Clark Collection
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ItemHistopathology of the binaural cochlear implant subject [Abstract]( 2001)Binaural hearing improves speech reception in noise, and is necessary for sound localisation. Normal hearing subjects use both interaural time, and intensity, differences to localise sound. This study investigates why sound localisation in bilateral cochlear implantees is insensitive to interaural time differences (Hoesel 1993). We looked for evidence of neural degeneration in the auditory brainstem involved in binaural sound localisation, since this may have degraded the neural circuitry required to accurately code interaural time delays. Method: The brainstem of a bilateral cochlear implantee was prepared for light microscopy by embedding it in paraffin, sectioning at 10 mm and staining sections with thionine or Luxol fast blue (LFB). The histological sections were digitised with NIH Image and 3-dimensional reconstructions made of the cochlear nucleus (CN) and superior olivary complex (SOC) with AnalysePC. Within the CN and the SOC, cell number and size were estimated by the physical dissector technique following thionine staining, and myelination of the nerve fibres was estimated using the optical density method following LFB staining. Results: A reduction in cell size (from thionine staining) and myelination (from LFB staining) was seen in both the CN and the SOC. Conclusions: These finding are consistent with neural degeneration within the auditory pathways. This may have lead to a degradation of the neural circuitry required to accurately detect interaural time delays.
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ItemThe effects of stochastic neural activity in a model predicting intensity perception with cochlear implants: low-rate stimulation( 1999)Most models of auditory nerve response to electrical stimulation are deterministic, despite significant physiological evidence for stochastic activity. Furthermore, psychophysical models and analyses of physiological data using deterministic descriptions do not accurately predict many psychophysical phenomena. In this paper we investigate whether inclusion of stochastic activity in neural models improves such predictions. To avoid the complication of interpulse interactions and to enable the use of a simpler and faster auditory nerve model we restrict our investigation to single pulses and low-rate (<200 pulses/s) pulse trains. We apply signal detection theory to produce direct predictions of behavioural threshold, dynamic range and intensity difference limen. Specifically, we investigate threshold versus pulse duration (the strength-duration characteristics), threshold and uncomfortable loudness (and the corresponding dynamic range) versus phase duration, the effects of electrode configuration on dynamic range and on strength-duration, threshold versus number of pulses (the temporal-integration characteristics), intensity difference limen as a function of loudness, and the effects of neural survival on these measures. For all psychophysical measures investigated, the inclusion of stochastic activity in the auditory nerve model was found to produce more accurate predictions.
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ItemA stochastic model of the electrically stimulated auditory nerve: pulse-train response( 1999)The single-pulse model of the companion paper [1] is extended to describe responses to pulse trains by introducing a phenomenological refractory mechanism. Comparisons with physiological data from cat auditory nerve fibers are made for pulse rates between 100 and 800 pulses/s. First, it is shown that both the shape and slope of mean discharge rate curves are better predicted by the stochastic model than by the deterministic model. Second, while interpulse effects such as refractory effects do indeed increase the dynamic range at higher pulse rates, both the physiological data and the model indicate that much of the dynamic range for pulse-train stimuli is due to stochastic activity. Third, it is shown that the stochastic model is able to predict the general magnitude and behavior of variance in discharge rate as a function of pulse rate, while the deterministic model predicts no variance at all.
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ItemA stochastic model of the electrically stimulated auditory nerve: single-pulse response( 1999)Most models of neural response to electrical stimulation, such as the Hodgkin-Huxley equations, are deterministic, despite significant physiological evidence for the existence of stochastic activity. For instance, the range of discharge probabilities measured in response to single electrical pulses cannot be explained at all by deterministic models. Furthermore, there is growing evidence that the stochastic component of auditory nerve response to electrical stimulation may be fundamental to functionally significant physiological and psychophysical phenomena. In this paper we present a simple and computationally efficient stochastic model of single-fiber response to single biphasic electrical pulses, based on a deterministic threshold model of action potential generation. Comparisons with physiological data from cat auditory nerve fibers are made, and it is shown that the stochastic model predicts discharge probabilities measured in response to single biphasic pulses more accurately than does the equivalent deterministic model. In addition, physiological data show an increase in stochastic activity with increasing pulse width of anodic/cathodic biphasic pulses, a phenomenon not present for monophasic stimuli. These and other data from the auditory nerve are then used to develop a population model of the total auditory nerve, where each fiber is described by the single-fiber model.
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ItemCurrent distributions of intracochlear electrodes in cats [Abstract](Monash University Press, 1983)A non-invasive technique, "two electrode mapping" has been developed to measure the current distribution of a multichannel electrode array within the feline scala tympani. An electrode’s current distribution is an important determinant of its ability to excite discrete neural groups. The electrode array, a “banded electrode” is a series of platinum rings, supported by a cylinder of silastic which fits freely into the scala tympani. All cats had normal hearing pre-operatively and were implanted through the round window. Hearing was normal within 10 dB after implantation, necessitating the presentation of 30-40 dB white noise to the implanted ear throughout experimentation to mask the electrophonic component (Black et al., 1983).
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ItemIntracochlear electrical simulation of normal and deaf cats investigated using brainstem response audiometry( 1983)Brainstem response audiometry for intracochlear electrical stimulation of normal-hearing and deafened cats was investigated. In normal cochleas the brainstem response amplitude grew slowly near threshold as a current-amplitude dependent process, identified as electrophonic in origin. This terminated in a rapidly growing charge-dependent process at approximately 20 dB above threshold, identified as direct electrical stimulation of the auditory nerve. Small levels of white noise (25-35 dB SPL) were sufficient to mask most of the electrophonic response, leaving the direct stimulation process essentially unmodified. In cochleas damaged with d.c. currents and loud sounds, only a rapidly growing charge-dependent process was observed which grew similarly to that in normal-hearing cats but occurred at lower currents. This indicates that possibly the electrical properties of the cochlea were altered in the deafening process, suggesting the inadequacy of normal animals as deaf models for electrical stimulation. Using the technique of derived brainstem responses, it was shown that direct electrical stimulus components were localized to the vicinity of the stimulus electrode with electrophonic components distributed more widely. However, at high currents there was some evidence of the stimulus spreading into the internal auditory meatus.
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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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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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ItemResponses from single units in the dorsal cochlear nucleus to electrical stimulation of the cochlea( 1992)To help improve our understanding of how the brain responds to electrical stimulation of the auditory nerve we have examined the responses of dorsal cochlear nucleus (DCN) units to both acoustic stimulation and electrical stimulation of the cochlea. This work extended our previous studies which have compared the responses to electrical and acoustic stimulation In the auditory nerve (Javel et al 1987, Ann. Otol. Rhinol. laryngeal. Suppl. 128, 96:2630) and the ventral cochlear nucleus (Shepherd et al 1988, NIH Contract NO1-NS-72342, 5th Quarterly Progress Report).