Graeme Clark Collection

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    Effects of intracochlear factors on spiral ganglion cells and auditory brain stem response after long-term electrical stimulation in deafened kittens
    Araki, Susumu ; Kawano, Atsushi ; Seldon, H. Lee ; Shepherd, Robert K. ; Funasaka, Sotaro ; Clark, Graeme M. ( 2000)
    Using an animal model, we have studied the response of the auditory brain stem to cochlear implantation and the effect of intracochlear factors on this response. Neonatally, pharmacologically deafened cats (100 to more than 180 days old) were implanted with a 4-electrode array in both cochleas. Then, the left cochlea of each cat was electrically stimulated for total periods of up to 1000 hours. After a terminal 14C-2-deoxyglucose (2DG) experiment, the fraction of the right inferior colliculus with a significant accumulation of 2DG label was calculated. Using 3-dimensional computer-aided reconstruction, we examined the cochleas of these animals for spiral ganglion cell (SGC) survival and intracochlear factors such as electrode positions, degeneration of the organ of Corti, and the degree of fibrosis of the scala tympani. The distribution of each parameter was calculated along the organ of Corti from the basal end. There was a positive correlation between SGC survival and the level of fibrosis in the scala tympani, and a negative correlation between SGC survival and the degree of organ of Corti degeneration. Finally, there was a negative correlation between the 2DG-labeled inferior colliculus volume fraction and the degree of fibrosis, particularly in the 1-mm region nearest the pair of electrodes, and presumably in the basal turn.
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    Electrode discrimination by early-deafened subjects using the Cochlear Limited multiple electrode cochlear implant
    Busby, P. A. ; Clark, Graeme M. ( 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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    Pitch estimation by early-deafened subjects using a multiple-electrode cochlear implant
    Busby, P. A. ; Clark, Graeme M. ( 2000)
    Abstract not available due to copyright.
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    Calculation of interspike intervals for integrate-and-fire neurons with Poisson distribution of synaptic inputs
    Burkitt, A. N. ; Clark, Graeme M. ( 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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    Analysis of synchronization in response of neurons to noisy periodic synaptic input
    Burkitt, A. N. ; Clark, Graeme M. ( 2000)
    Unavailable due to copyright.
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    Renewal-process approximation of a stochastic threshold model for electrical neural stimulation
    Bruce, Ian C. ; Irlicht, Laurence S. ; White, Mark W. ; O'Leary, Stephen J. ; Clark, Graeme M. ( 2000)
    In a recent set of modelling studies we have developed a stochastic threshold model of auditory nerveresponse to single biphasic electrical pulses (Bruce et al., 1999c) and moderate rate (less than 800 pulses per second) pulse trains (Bruce et al., 1999a). In this article we derive an analytical approximation for the single-pulse model, which is then extended to describe the pulse-train model in the case of evenly timed, uniform pulses. This renewal process description provides an accurate and computationally efficient model of electrical stimulation of single auditory nerve fibers by a cochlear implant that may be extended to other forms of electrical neural stimulation.
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    Multichannel auditory brainstem implantation: the Australian experience
    Briggs, R. J. S. ; Fagan, P. ; Atlas, M. ; Kaye, A. H. ; Sheehy, J. ; Hollow, R. ; Shaw. S. ; Clark, Graeme M. (Cambridge University Press, 2000)
    The multichannel auditory brainstem implant (ABI) provides the potential for hearing restoration in patients with neuro bromatosis type 2 (NF2). Programmes for auditory brainstem implantation have been established in two Australian centres. Eight patients have been implanted under the protocol of an international multi-centre clinical trial. Three patients had ABI insertion at the time of first side tumour removal, four at second side tumour removal and one after previous bilateral surgery where there was some residual tumour. The translabyrinthine approach was used in all cases. Successful positioning of the electrode array was achieved in seven of eight patients, all of whom achieved auditory perception with electrical stimulation. Intra-operative electrically evoked auditory brainstem response testing was successful in four patients and was useful in confirming correct electrode position. In six cases postoperative psychophysical and auditory perception testing demonstrated that useful auditory sensations were achieved. Five of these patients regularly used the implant. In one patient electrode placement was unsuccessful and only non-auditory sensations occurred on stimulation. In the remaining patients nonauditory sensations were minimal and avoidable by selective electrode programming. Auditory brainstem implantation should be considered in patients with NF2. The greatest benefit is seen in patients without debilitating disease who have non-aidable hearing in the contralateral ear.
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    Cognitive processing in children using cochlear implants: the relationship between visual memory, attention, and executive functions and developing language skills
    Surowiecki, Vanessa N. ; SARANT, JULIA ; MARUFF, PAUL ; Blamey, Peter J. ; Busby, Peter A. ; Clark, Graeme M. ( 2002)
    We performed this study to determine whether children using a cochlear implant performed differently from age- and gender-matched hearing aid users on 8 neuropsychological measures of visual memory, attention, and executive functioning. The study also examined whether differences in cognitive skills could account for some of the observed variance in speech perception, vocabulary, and language abilities of hearing-impaired children. In contrast to previous studies, our results revealed no significant cognitive differences between children who use a cochlear implant and children who use hearing aids. Partial correlation analysis indicated that the children’s visual memory skills, i.e., their recognition memory, delayed recall, and paired associative learning memory skills, correlated significantly with their language skills. When examined at a significance level of .01, attention and executive functioning skills did not relate to the children’s developing speech perception, vocabulary, or language skills. The results suggested that differences in visual memory skills may account for some of the variance seen in the language abilities of children using implants and children using hearing aids.
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    Summation of spatiotemporal input patterns in leaky integrate-and-fire neurons: Application to neurons in the cochlear nucleus receiving converging auditory nerve fiber input
    Kuhlmann, L ; Burkitt, AN ; Paolini, A ; Clark, GM (SPRINGER, 2002)
    The response of leaky integrate-and-fire neurons is analyzed for periodic inputs whose phases vary with their spatial location. The model gives the relationship between the spatial summation distance and the degree of phase locking of the output spikes (i.e., locking to the periodic stochastic inputs, measured by the synchronization index). The synaptic inputs are modeled as an inhomogeneous Poisson process, and the analysis is carried out in the Gaussian approximation. The model has been applied to globular bushy cells of the cochlear nucleus, which receive converging inputs from auditory nerve fibers that originate at neighboring sites in the cochlea. The model elucidates the roles played by spatial summation and coincidence detection, showing how synchronization decreases with an increase in both frequency and spatial spread of inputs. It also shows under what conditions an enhancement of synchronization of the output relative to the input takes place.
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    The effect of language ability and residual hearing on speech perception outcomes for older children using multichannel cochlear implants [Abstract]
    Dowell, Richard C. ; Dettman, Shani J. ; Hill, Katie ; Winton, Elizabeth ; Hollow, Rod ; Clark, Graeme M. ( 2002)
    Early-deafened teenagers or young adults have shown somewhat disappointing performance with cochlear implants in the past, however, in recent years a proportion of older children have demonstrated excellent speech perception performance. There is a great deal of variability in speech perception performance within this group. It is important to investigate the factors influencing performance so that adolescents and their families can make informed decisions regarding cochlear implant surgery. This study considered a number of possible predictive factors in a group of 25 children implanted in Melbourne between the ages of 8 and 18 years. Subjects completed open set speech perception testing using BKB sentences both pre-and postoperatively, and pre-operative language testing using the Peabody Picture Vocabulary Test. Data were collected regarding the type of hearing loss, age at implant, age at hearing aid fitting, audiometric details, and the pre-and post-operative communication mode. Multivariate analysis suggested that three factors were associated with postoperative speech perception performance. Results were improved for subjects with better pre-operative speech perception, better pre-operative language ability, and when the duration of profound hearing loss was shorter. These three factors accounted for 66% of the variance in this group. The results of this study suggest that children who have useful pre-implant speech perception, and higher age-equivalent scores on language measures, would be expected to do well with a cochlear implant. A shorter duration of profound hearing loss is also advantageous. Mean speech perception scores for the older group were not significantly different from younger children.