Graeme Clark Collection
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ItemBrainstem encoding of short voice onset times in natural speech( 2001)An auditory nerve study has shown that short voice onset times (VOTs) in synthetic consonant-vowel syllables are not accurately encoded by the fibres' discharge rate. We have re-examined this issue within the ventral Cochlear nucleus (VCN), using natural speech and a fine-grain analysis of single unit responses. We recorded extracellularly from 93 VCN neurons in rats anaesthetised with urethane (2.5 g/kg ip). After identifying a cell's response type and best frequency (BF), 3 syllables spoken by a male were presented at double rate and 3 intensities (/bεt/, /dεt/, and /gεt/, at 45, 65, and 75 dB SPL). These three syllables differ in their VOTs (the interval between consonant release and the onset of glottal pulses associated with voicing) due to the different points of articulation of the three initial stop consonants. In many neurons (particularly onset cells), these syllables evoked a clear response to consonant release, followed by an interval of inactivity or reduced activity before the periodic response to the vowel's voicing frequency commenced. This interval of reduced or no activity corresponded to a given syllable's VOT. The responses of all cells (BFs: 0.9-19 kHz) to the 9 different syllable-SPL combinations were plotted as Grand Average post-stimulus time histograms. In 8/9 combinations, syllable onset was associated with a statistically significant peak in activity and the next significant peak in discharge rate occurred at the time of voice onset (± I ms). These results indicate that the prominent responses to consonant release and voice onset, produced by the synchronous firing of neurons with a wide range of BFs, accurately encode short VOTs.
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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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ItemPsychophysics and speech perception with a premodiolar electrode array( 1997)A perimodiolar electrode array, designed to lie closer to the modiolus than the standard Coclear Limited array and developed by the Cooperative Research Centre for Cochlear Implant, Speech and Hearing Research in conjunction with Cochlear Limited, has recently been implanted in an adult patient. While excellent speech results have been obtained using the standard array, it is hypothesised that the improved place coding expected from an array lying closer to the modiolus would result in improved speech perception. Analysis of a modified Stenvers view X-ray revealed that the most apical electrode was inserted to an angle of 311 0 and that the apical half of the array was considerably closer to the modiolus than would have been the case for a standard array. Thresholds and comfortable levels were correlated with the lateral position of the stimulated electrode in the scala tympani, being much reduced for the apical electrodes. Forward masking and electrode discrimination are being measured in order to study the spread of neural excitation and the distinctness of percepts for different electrodes.
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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).
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ItemCochlear implantation in young children: long-term effects of implantation on normal hair cells and spiral ganglion cells in the monkey model [Abstract]( 1992)Recent independent results obtained by profoundly deaf children implanted with the Melbourne 22-channel cochlear implant (1) have provided further impetus. for examining thefeasibility of implanting children under two and children with profound deafness. Safety st1,ldies, in appropriate animal models, must first establish the safety of this procedure.