Graeme Clark Collection
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ItemHearing restoration with the multichannel auditory brainstem implant( 1997)Restoration of useful hearing is now possible in patients with bilateral acoustic neuromas by direct electrical stimulation of the cochlear nucleus. Our first experience with the Multichannel Auditory Brainstem Implant is reported. A forty four year old female with bilateral acoustic neuromas and a strong family history of Neurofibromatosis Type II presented with profound bilateral hearing impairment. Translabyrinthine removal of the right tumour was performed with placement of the Nucleus eight electrode Auditory Brainstem Implant. Intraoperative electrically evoked auditory brainstem response monitoring successfully confirmed placement over the cochlear nucleus. Postoperatively, auditory responses were obtained on stimulation of all electrodes with minimal non-auditory sensations. The patient now receives useful auditory sensations using the "SPEAK" speech processing strategy. Auditory brainstem Implantation should be considered for patients with Neurofibromatosis Type II in whom hearing preservation tumour removal is not possible.
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ItemElectrical stimulation of the auditory nerve with a cochlear implant and the temporal coding of sound frequencies: a brief review( 1997)There is considerable evidence that the brain translates (encodes) the frequency of a sound into both place of excitation (place encoding), and the pattern of intervals between action potentials (temporal encoding). Furthermore, temporal encoding is now thought to be due to a temporal as well as spatial pattern of action potentials in a small group of neurons. This pattern needs to be reproduced with a cochlear implant for improved speech processing. Our recent research has also demonstrated that the timing of excitatory postsynaptic potentials seen with intracellular recordings from brain cells, rather than extracellularly recorded action potentials, correlates better with the frequency of sound. These excitatory postsynaptic potentials are likely to be the link between the patterns of action potentials arriving at nerve cells and the biomolecular activity in the cell. This response also needs to be replicated with improved speech processing strategies.
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ItemEffects of chronic electrical stimulation on spiral ganglion neuron survival and size in deafened kittens( 1998)We have studied spiral ganglion cell (SGC) survival and soma size in neonatally pharmacologically deafened kittens. They were implanted with a four-electrode array in the left cochlea at 100 to 180 or more days of age. Eight animals were chronically stimulated approximately 1000 hours over approximately 60 days with charge-balanced, biphasic current pulses; three were unstimulated controls. Using three-dimensional computer-aided reconstruction of the cochlea, the SGC position and cross-sectional area were stored. SGC position was mapped to the organ of Corti by perpendicular projections, starting from the basal end. The basal region of the cochlea was divided into three 4-mm segments. SGC survival (number per 0.1 mm of the length of the organ of Corti) and soma size for stimulated cochleae were compared statistically with implanted but unstimulated cochleae. There was no evidence of an effect of electrical stimulation on SGC survival under this protocol and with this duration. On the other hand, the cell size on the stimulated side was significantly larger than the control side in the middle segment (4 to 8 mm from the basal end). SGCs undergo a reduction in size after prolonged auditory deprivation; however, these changes may be partially moderated after chronic intracochlear electrical stimulation.
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ItemElectrophonically driven single unit responses of the anteroventral cochlear nucleus in cat [Abstract]( 1996)Electrical stimulation of the cochlea results in both direct and electrophonic excitation of auditory nerve fibres. It has been proposed that electrophonic stimulation results from the creation of a mechanical disturbance on the basilar membrane which has properties similar those resulting from acoustic stimuli. Auditory nerve compound action potential (CAP) forward masking studies1 show the level of frequency specific electrophonic stimulation is highly correlated with the spectral energy of the electrical stimulus waveform. The level of spectral energy in pulsatile biphasic electrical stimuli decreases toward low frequencies suggesting the level of electrophonic stimulation will be diminished in the low frequency region of the cochlea.
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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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ItemTemporal coding in auditory neurons to electrical stimulation [Abstract]( 1997)The temporal response of the auditory pathway following intracochlear electrical stimulation will reflect the level of encoded temporal information, which is important for the further developmentof cochlear implant speech processing strategies, and in tum lead to a better understanding of temporal coding of acoustic stimuli Temporal coding of sound frequencies is based on the phase or time locked neural response seen to low frequency acoustic stimuli. The ability of neurons to respond in a time locked manner may determine the degree of encoded temporal frequency information. Electrophysiological studies have shown that the degree of response synchrony to charge balanced biphasic electrical stimuli is far greater than that seen to acoustic stimuli. We have investigated the temporal response properties of single units in the anteroventral cochlear nucleus (AVCN) in the cat to rates of electrical stimulation up to 800 pulses/s.
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ItemInvestigation of curved intracochlear electrode arrays [Abstract]( 1992)It has been demonstrated that the Melbourne/Cochlear multi-channel cochlear implant is safe and effective for use in profoundly-totally deaf patients. Recent studies have highlighted the importance of deaf insertion and placing the electrodes closer to the spiral ganglion neurons. In order to improve the electrode insertion depth and proximity to the modiolus, we have investigated curved electrode arrays. Prototypes of such arrays and their accessory inserter have been made. Trial insertions were performed on skeletonized cochleae of human temporal bones. The preliminary results showed that, when compared with conventional straight electrode arrays, the curved arrays could be inserted deeper and located closer to the modiolus. These findings indicate that the curved --.~ electrodes currently under investigation should result in a reduction in stimulus threshold and improve pitch perception and may also result in the use of more channels of 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).