Graeme Clark Collection
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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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ItemThe relationship between the output synchrony of cochlear nucleus neurons and the site of stimulation in the cochlea( 2001)A model has been developed to determine the relationship between the output synchrony of cochlear nucleus neurons and the site of stimulation in the cochlea. This is an Integrate and Fire Neuron Model in which noisy periodic synaptic inputs to the neuron are summed and a spike is generated when the membrane potential reaches threshold. The model describes the stochastic input that auditory nerve fibres provide to a cochlear nucleus neuron and the corresponding stochastic output. To investigate the relationship between the output synchrony of cochlear nucleus neurons (namely globular bushy cells) and the site of stimulation in the cochlea, phase differences between the periodic inputs of the model were incorporated, in order to mimic how the travelling wave consecutively activates auditory nerve fibres originating over a spatial spread of the basilar membrane. Analysis of the model found that output synchrony decreased with an increase in frequency and spatial spread. Furthermore, enhancement of the output synchrony relative to the input synchrony occurred for small spatial spreads of the basilar membrane over which input primary afferent fibres originate. Adding noise helped to make the model more realistic. As a result enhancement of synchrony occurred with a spatial spread of less than 1.25 mm and 0.75 mm for 0.5 kHz and I kHz respectively, while for the higher frequencies analysed (2 kHz and 5 kHz) enhancement of synchrony did not occur. This research has implications for the design of electrode arrays in cochlear implants. The number and geometry of the electrodes and the stimulus patterns to be used will depend on the degree of convergence of fibres and how phase information is processed by neurons in the brainstem.
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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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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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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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ItemThe role phase-contrast imagining in intra-cochlear electrode development( 2000)In order to improve the design of intracochlear multichannel electrode arrays, it is fundamental that we have knowledge of the exact anatomical , position of the electrode within the scala of the cochlea. Currently, conventional skull radiography is still the mainstay of post-operative radiological assessment of electrode positioning. The present work investigates the use of phase-contrast radiography, a new x-ray modality, to provide improved imaging of the inner ear and the intracochlear electrode array in the human temporal bone (TB).
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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