Graeme Clark Collection
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ItemElectrode discrimination by early-deafened subjects using the Cochlear Limited multiple electrode cochlear implant( 2000)Objective: The aims of this study were to determine whether electrode discrimination by early-deafened subjects using the Cochlear Limited prosthesis varied at different locations on the electrode array, was influenced by the effects of auditory deprivation and experience with electric stimulation, and was related to speech perception. Design: Difference limens for electrode discrimination were measured in 16 early-deafened subjects at three positions on the array: electrodes 18 (apical), 14 (mid), and 8 (basal). Electrodes were stimulated using random variations in current level to minimize the influence of loudness cues. Assessed were correlations between the difference limens, subject variables related to auditory deprivation (age at onset of deafness, duration of deafness, and age at implantation) and auditory experience (duration of implant use and the total time period of auditory experience), and speech perception scores from two closed-set and two open-set tests. Results: The average difference limens across the three positions were less than two electrodes for 75%, of subjects, with average limens between 2 and 6.5 electrodes for the remaining 25% of subjects. Significant differences across the three positions were found for 69% of subjects. The average limens and those at the basal position positively correlated with variables related to auditory deprivation, with larger limens for subjects implanted at a later age and with a longer duration of deafness. The average limens and those at the apical position negatively correlated with closed-set speech perception scores, with lower scores for subjects with larger limens, but not with open-set scores. Speech scores also negatively correlated with variables related to auditory deprivation. Conclusions: These findings showed that early-deafened subjects were generally successful in electrode discrimination although performance varied across the array for over half the subjects. Discrimination performance was influenced by the effects of auditory deprivation, and both electrode discrimination and variables related to auditory deprivation influenced closed-set speech perception.
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ItemPitch estimation by early-deafened subjects using a multiple-electrode cochlear implant( 2000)Abstract not available due to copyright.
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ItemCalculation of interspike intervals for integrate-and-fire neurons with Poisson distribution of synaptic inputs( 2000)We present a new technique for calculating the interspike intervals of integrate-and-fire neurons. There are two new components to this technique. First, the probability density of the summed potential is calculated by integrating over the distribution of arrival times of the afferent postsynaptic potentials (PSPs), rather than using conventional stochastic differential equation techniques. A general formulation of this technique is given in terms of the probability distribution of the inputs and the time course of the postsynaptic response. The expressions are evaluated in the gaussian approximation, which gives results that become more accurate for large numbers of small-amplitude PSPs. Second, the probability density of output spikes, which are generated when the potential reaches threshold, is given in terms of an integral involving a conditional probability density. A.N. Burkitt and G.M. Clark, 'Calculation of Interspike Intervals for Integrate and Fire Neurons with Poisson Distribution of Synaptic Inputs ', Neural Computation, 12:8 (August, 2000), pp. 1789-1820. © 2000 by the Massachusetts Institute of Technology. http://www.mitpressjournals.org.ezp.lib.unimelb.edu.au/toc/neco/12/8
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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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ItemHow much residual hearing is too much?( 2000)The value of cochlear implants as an established clinical option for profoundly hearing-impaired adults and children has been supported by significant research results over a number of years (U.S. National Institutes of Health Consensus Statement 1995). As a direct consequence of the level of benefits shown for cochlear implant users on measures of speech perception, research has focused on investigating whether severely hearing impaired adults and children would be suitable candidates for cochlear implantation. I n considering the candidature of any individual, both medical and audiological suitability are investigated. The primary concern is to establish to what degree the patient would benefit from use of the cochlear implant.