Graeme Clark Collection
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ItemImproved sound processing for cochlear implants( 2002)Four signal processing schemes currently under development aim to improve the perception of sounds/ especially speech, for children and adults using the Nucleus cochlear implant system. The schemes are (1) fast-acting input-signal compression, (2) Adaptive Dynamic Range Optimisation (ADRO), (3) TESM, a scheme that emphasises transients in signals, and (4) DRSP, a strategy that applies different stimulation rates to selected sets of electrodes.
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ItemFactors affecting speech perception outcomes for older children using multichannel cochlear implants( 2002)Experience with cochlear implantation in early-deafened teenagers or young adults has been somewhat disappointing, however, in recent years a proportion of older children have demonstrated excellent speech perception performance. There appears to be a wide gap between the good and poor performers within this group. It is important to investigate the possible factors influencing performance so that adolescents and their families are able to make informed decisions regarding cochlear implant surgery. This study considered a number of factors in a group of 25 children implanted in Melbourne between the ages of 8 and 18 years. Each subject completed open set speech perception testing using BKB sentences before and after implantation 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 postoperative communication mode. Multivariate analysis suggested that three factors have a significant predictive value for post-implant speech perception: pre-operative open-set sentence score, duration of profound hearing loss and equivalent language age. 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.
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ItemA comparative study of phase-contrast and conventional x-ray imaging in human temporal bone samples( 2001)This study compared a new x-ray modality, phase-contrast radiography, with conventional radiography for imaging in human temporal bones and also investigated its potential application in the development of electrode arrays for advanced cochlear implants. Nucleus standard electrode arrays and peri-modiolar Contourn.4 electrode arrays were implanted into the cochleae of 10 human temporal bones. Both conventional and phase-contrast radiographs were taken of ~ach temporal bon~. The phase-contrast radiographs showed significant improvements over conventional radiographs in the detail of temporal bone images. These improvements included enhanced contrast at the edge of canal type features, inherent image magnification, higher spatial resolution, and ability to use detectors such as Imaging Plates. The results demonstrate that phase-contrast imaging can have important advantages in visualisation of anatomical details of both the inner ear structures and the microelectrode. It can provide a clearer definition of electrode location in relation to cochlear walls. This study demonstrates the feasibility of applying phase-contrast radiography to studies of the human temporal bone. However, its usefulness in the imaging of larger objects or perhaps even with patients in a clinical setting will require further investigation.
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ItemApplication of advanced radiographic technology in cochlear implant research( 2001)The effective development of peri-modiolar or other advanced electrode arrays for cochlear implants requires detailed analysis of the insertion procedure and electrode positioning in the cochlea. Routine x-ray techniques cannot provide sufficient detail to meet this need. A new micro-focus x-ray imaging system has been built for our research. The system consists of a x-ray tube with a sub 10-micron focal spot mounted below an adjustable work surface and an image intensifier placed approximately 100 cm above the x-ray aperture. A variety of intracochlear electrode arrays and human temporal bones were studied using this system. The micro-focus x-ray imaging system allows for micro-fluoroscopy to visualise the real time implantation procedure. It also enables capturing of images onto reusable phosphor imaging plates or films for subsequent viewing or analysis. Images are produced at up to 95 times magnification with superior resolution and enhanced contrast. This new radiographic technology plays an important role in development of safe and effective advanced intracochlear electrode arrays.
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ItemInhibition underlies the encoding of short voice onset times in the ventral cochlear nucleus( 2001)Recent experiments in our laboratory have shown that voice onset time (VOT), the time between consonant release and the first glottal pulse of an ensuing vowel, is effectively encoded by neurons within the ventral cochlear nucleus (VCN). In this investigation we examined the possible neural mechanisms which may underlie this VOT encoding. In male rats anaesthetised with urethane (2.5g1kg i.p), microelectrodes containing 1M potassium acetate, were inserted into the VCN. Speech stimuli consisting of 3 syllables spoken naturally by a male and female were presented at double rate and 3 intensities (/bεt/, /dεt/, and /gεt/ at 45, 65 and 75 dB SPL). Intracellular recordings were made in 12 neurons, eight of which had a response to pure tones typical of spherical bushy neurons, responding in a primary-like (PL) fashion. The remaining cells were classified as either globular bushy (n=2) or stellate cells (n=2). In PL neurons, the VOT period was associated with hyperpolarisation. The duration and amplitude of this hyperpolarising influence was greater for female speech. These PL units showed better encoding of VOT than other cell types in which hyperpolarisation was less evident and action > potentials were often evoked during this period at the highest intensity level. We propose that this hyperpolarisation is due to stimulation of inhibitory sidebands by the high frequency frication noise within the VOT period. This inhibition reduces the probability of action potential generation during the VOT period and enhances the salience of the voice onset enabling more effective encoding of VOT than seen in the auditory nerve.
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ItemShort-term auditory memory in children using cochlear implants( 2001)There are many factors contributing to the variance in language performance of children using cochlear implants. Typically studies have investigated the predictive value of demographic factors such as duration of profound deafness. It is possible that profound auditory deprivation prior to implantation may have caused auditory processing deficits at a cortical level and, in particular, a deficit in short-term, sequential auditory memory. The aim of the study was to assess short-term sequential auditory memory ability in young children using cochlear implants and to investigate the relationship of this ability to receptive language performance.
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ItemPost mortem study of the intracochlear position of the nucleus standard 22 electrode array( 2000)The final position of an intracochlear cochlear implant electrode array can vary depending on the pathology, the insertion technique used and the type of electrode array used. The distance of the electrodes from the target neural elements and the presence of intracochlear fibrous tissue or new bone formation are believed to affect the performance of the device. A post mortem study was conducted to assess these factors.
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ItemPhysiological and histopathological effects of chronic monopolar high rate stimulation on the auditory nerve( 2000)Speech processing strategies based on high rate electrical stimulation have been associated with improvements in speech perception among cochlear implant users. The present study was designed to evaluate the electrophysiological and histopathological effects of long-term intracochlear monopolar stimulation at the maximum stimulus rate of the current Nucleus Cochlear implant system (14493 pulses/s) as part of our ongoing investigations of safety issues associated with cochlear implants
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ItemSafety studies and preliminary patient trails with a new perimodiolar electrode array( 2000)A new perimodiolar electrode array, the curly-with stylet (CwS), has been developed and undergone safety trials in human cadaver temporal bones. The array was developed as a result of animal and modelling studies which indicated that there are potential advantages in situating the electrode array in closer proximity to the neural elements. Preliminary studies with four patients in Melbourne implanted with a developmental pre-curved array had supported the predictions of lower current requirements and possibly more focussed spread of excitation.
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ItemEffects of sensorineural hearing loss on the refractory properties of auditory nerve fibers( 2000)We hypothesised that the loss of the peripheral processes and the partial demyelination of auditory nerve fibres (ANFs) following a sensorineural hearing loss would increase their refractory properties. Normal control, and long-term (2.5 months) systemically deafened rats were anaesthetised (urethane, 1.3 g/kg i.p.), a bipolar stimulating electrode was implanted into the scala tympani and glass microelectrodes (30-80 MΩ) used to record single ANF activity. Stimuli (pairs of 100 µs/phase charge balanced biphasic pulses with interpulse intervals (IPIs) of 0.34-10 ms) were presented at 6 dB above threshold using a repetition interval of 250 ms. Absolute refractory period (ARP) was defined as the IPI at which the probability of eliciting a spike to the second stimulus was 0.1. In the present results, based on recordings from 62 fibres, ANFs were distinguished from cochlear nucleus (CN) neurones by their significantly shorter median latencies (AN: 0.575ms vs CN: 1.137ms; Whitney-Mann Rank Sum, p<0.0001). There were no significant differences between minimum ANF latencies from normal and deafened animals. Although the median ARP was greater in deafened versus normal animals, this difference was not statistically significant (normals: median0.658ms, interquartile range 0.554-0.913ms; deafened: 0.772ms and 0.616-1.073ms; p=0.16). Finally, the spike latency associated with the second pulse of a pair systematically increased with decreasing IPI, contrasting with the stable latency of the response to the leading pulse. Although pathological changes to ANFs may increase their refractory properties, at this duration of deafness these changes were not significant.