Graeme Clark Collection
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ItemElectrical network properties and distribution of potentials in the cat cochlea [Abstract](Australian Physiological and Pharmacological Society, 1978)The-patterns of electrical resistance and capacitance in the cochlea formed by the anatomical organisation of the tissue structures and fluids are important in determining the distribution of electrical potentials which arise during normal acoustic stimulation (von Bekesy,1951). Cochlear potential distributions have in the past been measured by recording from the scalar fluids both the spread of cochlear microphonics and also potentials due to electrical stimulation. However, similar distributions in the hair cell-nerve ending region of the organ of Corti may not necessarily occur because of current shunting effects due to the electrical network patterns. To examine these current shunting effects, a three dimensional mathematical model of the electrical properties of the cat cochlea was constructed. This was formed from a two dimensional cochlear cross-section model similar to that proposed by Johnstone et al., (1966) for the guinea pig. Sixteen such sections were resistively coupled to form the three dimensional model. Results derived from this model predict that during electrical stimulation of the cochlea, the current in the organ of Corti region attenuates quite differently to the scalar voltage by a degree which depends on the stimulus electrode configuration.
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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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ItemAuditory evoked potentials and auditory sequential memory [Abstract]( 1974)The evaluation of intelligence in infants and young children is important in assessing their prognosis and institutional placement, and is difficult with present clinical methods. Therefore, the recent studies which show a correlation between visual evoked responses and intelligence are of value. It is also of importance to determine if auditory evoked potentials may be used in assessing children with communication disorders, as their defect frequently involves the auditory system, and they will often require evoked response audiometry to exclude loss of hearing from the diagnosis.
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ItemAuditory evoked responses to frequency and amplitude modulated sound( 1973)Auditory evoked responses to pure tone bursts have been described in a number of studies and have been characterized by the P1, N1 and P2 components. The presence of later components, namely N2, P3 and the Contingent Negative Variation, depends largely on the cerebral processing of the stimulus. These evoked responses have been recorded using tone bursts. However, neurophysiological studies have shown that the cortex codes complex sounds. Therefore, we performed a set of acute experiments on the cat, using frequency and amplitude modulated sounds. This was reported in a previous study (Richards and Clark, 1972) which showed that similar onset and later waves could be recorded from the cortex of the cat. In some areas of the cortex the later waves were in synchrony with the modulation envelope.
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ItemThe design of an ear bar system for auditory neruophysiological research( 1972)Absolute sound intensity measurements at the ear drum are important in auditory research. Previous attempts to measure sound intensity using a small bore probe tube coupled to a microphone have proved unsatisfactory. In the present investigation an alternative system employing two condenser microphones coupled to the input end of the ear bar was used to estimate the sound intensity at the ear drum. Consideration was also given to minimizing harmonic distortion and resonance in the system.
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ItemImplantation of the new nucleus C1-3 receiver stimulator and electrode array [Abstract]( 1997)There is an important need to fix the cochlear implant electrode array at a site close to the cochlea, so that the electrode will not slide out, or be subject to differential movement with growth changes. Fixation sites have been in the region of the posterior root of the zygoma and the floor of the antrum. Fixation has been by Dacron mesh ties platinum wire ties, or clips inserted with-special instruments. Biological cements have previously been tried but found to be toxic. The most ideal site is in the region of the cochleostomy.
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ItemTemporal coding for sound and tempor-spatial patterns of electrical stimulation [Abstract]( 1997)The anterior division of the ventral cochlear nucleus (AVCN) is the first relay station of the auditory pathway. It receives auditory information via the auditory nerves emanating from the cochlea. Electrical stimulation via current cochlear implants [ ] does not lead to responses at the cochlear nucleus that exactly match tho elicited by comparable auditory stimulation. Complex temporal patterns of electrical stimulation may provide a better simulation of the acoustic input.
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ItemIntracellular responses of ventral cochlear nucleus neurones to acoustic stimulation in the rat( 1997)The ventral cochlear nucleus (VCN) is the first relay station of the auditory pathway. Presently, little is known about the acoustically evoked intracellular response of neurones in the VCN. We investigated the effect of acoustic stimulation on neurones in the rat VCN using in vivo intracellular recordings and dye-filling. In male rats anaesthetised with urethane (1.3g/kg i.p) microelectrodes containing 1M potassium acetate, or with 4% neurobiotin, were inserted into the VCN. Stable impalements were made from 37 neurones classified as having a Primary-like (n=13), Primary-like with notch (n=4) and Chopper (n=20) response to acoustic stimulation (50 ms pure tones, 5 ms r/f time, 0.2 Hz repetition).