Graeme Clark Collection
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ItemElectrophonically driven single unit responses of the anteroventral cochlear nucleus in cat [Abstract]( 1996)Electrical stimulation of the cochlea results in both direct and electrophonic excitation of auditory nerve fibres. It has been proposed that electrophonic stimulation results from the creation of a mechanical disturbance on the basilar membrane which has properties similar those resulting from acoustic stimuli. Auditory nerve compound action potential (CAP) forward masking studies1 show the level of frequency specific electrophonic stimulation is highly correlated with the spectral energy of the electrical stimulus waveform. The level of spectral energy in pulsatile biphasic electrical stimuli decreases toward low frequencies suggesting the level of electrophonic stimulation will be diminished in the low frequency region of the cochlea.
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ItemThe effects of action potential propagation delay times and an absolute refractory period upon the synchronization index in the integrate and fire neuron model and a comparison with neurons in the auditory pathway( 2000)The effects of action potential (AP) propagation delay times and the absolute refractory period upon the synchronization index are analysed for the integrate and fire neuron model, and the results are compared with recordings from auditory ganglion neurons and cochlear nucleus neurons. In the model the noisy periodic synaptic input to the neuron is summed and an AP is generated when the membrane potential reaches threshold. The output phase distribution (phase histogram) is calculated at the site at which the APs are generated. The AP propagation delay times along an axon are modelled using a periodically wrapped Gaussian distribution, with the width fitted from experimental data. This distribution is convolved with the calculated phase distribution to obtain the phase distribution at the axon terminal.
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ItemThe effect of rate of stimulation of the auditory nerve on phoneme recognition(Australian Speech Science and Technology Association, 2000)Five patients implanted with the Nucleus CI-24M cochlear implant were tested on consonant and vowel perception with three different average rates of stimulation: 250 pulses/s per channel, 807 pps/ch and 1615 pps/ch. There were no significant differences in phoneme recognition scores when learning effects were taken into account. Information transmission analysesof consonant confusion matrices revealed that, with higher rates of stimulation, manner of articulation features were better perceived but place of articulation features were more poorlyperceived. The results and analyses suggest that high rates of stimulation provide improved information about temporal information and frication in speech, but mask the spectral detail required for the perception of place of articulation.
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ItemDelay analysis in an investigation of auditory temporal coding( 2000)Delay analysis is a method for analysing phase-locked responses to periodic stimuli which is widely used in the study of auditory cells, as it provides an estimate of the delay present in a system from steady-state data. While the usual formulation utilises the assumption that the delay is constant across frequencies, in the auditory system delay varies with frequency. In this paper two new formulations of delay analysis are introduced, and are applied to the analysis of auditory temporal coding. In rats anaesthetised with urethane (1.3g/kg i.p.), in vivo extracellular recordings were made in the auditory nerve, cochlear nucleus and trapezoid body using glass microelectrodes filled with 1M potassium acetate (50-70MΩ).
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ItemLatest results for adults & children using cochlear implants & future implications( 2000)The overall success of a cochlear implant procedure is most often quantified by assessing how well implantees can understand speech. This is because a primary aim of the application of cochlear implants is to improve communication, and it is relatively straightforward to obtain accurate measures of speech recognition. The quality of cochlear implant hearing is not well described by measuring an audiogram, as the detection of sound is similar across all implantees if the device is functioning correctly. A commonly used measure of speech understanding is the CID everyday sentence test where the number of correctly identified words within sentences is assessed. In the early days of cochlear implants, subjects could only recognize a few words without lipreading but the most recent results show average scores of 80% without lipreading for this sentence test, after 6 months of experience with the device.
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ItemWhere to now? - Impact of New Technologies on use of cochlear implants( 2000)The history of cochlear implant use by adults and children with profound hearing loss although relatively short (20 some years), has been characterised by continual technological innovations which have enhanced the performance, packaging, and clinical use of these devices. In particular, the development of the Nucleus multiple channel cochlear implant has included a series of speech processing hardware and speech processing strategy, implemented by Cochlear Limited, and based on research findings that have resulted in an increase in mean speech perception benefits for adults and children.
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ItemParameter selection and programming recommendations for the ACE and CIS speech processing strategies [Abstract]( 1999)The Nucleus 24 Cochlear Implant system with the SPrint processor provides access to multiple speech processing strategies and a wide range of programming parameters. Strategy comparison studies have suggested that the optimal parameter set and coding strategy varies from individual to individual. It is necessary, however, to establish some default programming parameters and fitting guidelines. Therefore we have investigated the effect of stimulation rate and the number of channels or maxima in the ACE or CIS strategies, as well as the optimal programming strategy for subjects with a limited number of available electrodes. Speech perception was tested using monosyllabic words and sentences in noise, with the evaluation protocol designed to take into account learning effects. Take-home experience was provided with all programs, and subjects were asked to complete a comparative performance questionnaire regarding program preference. Six or eight subjects were enrolled in each study.
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ItemChronic electrical stimulation of the auditory nerve using high-surface area platinum electrodes [Abstract]( 1999)Electrical stimulation using high surface area (HiQ) platinum (Pt) electrodes exhibit a lower electrode impedance, polarisation and direct current (DC) in vitro compared with standard (ST) Pt electrodes of the same geometric size. In the present study we investigated whether HiQ electrodes maintain these advantages in vivo. This could be important for the development of new arrays with an increased number of smaller electrodes. Under general anaesthesia (Ketamine 20 mg/kg and Xylazine 4mg/kg i.p.) five normal hearing cats were implanted bilaterally with a two-channel Pt scala tympani electrode array (4 HiQ, I ST array). Chronic electrical stimulation using charge balanced biphasic current pulses was delivered unilaterally via a transcutaneous leadwire connected to a backpack-stimulator for periods of up to 2400 hours. DC, stimulus current and electrode voltage waveforms were monitored twice daily and access resistance (Ra), electrode impedance (Zc) and polarisation (Zc-Ra) calculated (kΩ). Mean HiQ data were compared to ST data using Students t-test (*=p<0.05, **=p<0.001). Immediately following implantation both HiQ and ST-electrodes exhibited low impedance values (Ra: 1.06 vs 1.00, Zc: 1.24 vs 2.12*, and Zc-Ra: 0.18 vs 1.08**). Subsequently impedance increased, largely due to a rise in Ra (5.35 vs 6.8: Zc: 6.96 vs 9.80, and Zc-Ra: 1.61 vs 3.00*). At the end of the experiment the array was explanted and tested in saline (Ra: 0.63 vs 0.74*, Zc: 0.73 vs 1.90**, Zc-Ra: 0.10 vs 1.16**). These initial results suggests that intracochlear tissue growth increased the access resistance of both electrode designs. However, HiQ electrodes maintained not only a significantly lower polarisation, they also showed a lower average residual DC (23 vs 130 nA**) throughout the experiment.
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ItemSpatial spread of neural excitation in cochlear implants: comparison of measurements made using NRT and forward masking [Abstract]( 1998)Recently developed technology allows intracochlear potentials to be measured in cochlear implant recipients, using telemetry. Neural response telemetry (NRT) enables the measurement of compound action potentials evoked by stimulation of cochlear implant electrodes. These objective measures can now be compared with related psychophysical measures in humans. We will present data, from both NRT and forward masking, on spatial spread of neural excitation due to stimulation of cochlear implant electrodes. The response fields from more apical neurons will spread quite broadly to the sensing electrodes of an implanted array, resulting in misleadingly broad NRT estimates of the spatial spread of neural excitation. Forward masking, which might itself lay claims to some degree of "objectivity", will not suffer from this limitation. Comparison of data from the two measures will help to determine the limitations of NRT as a tool for measuring spatial spread of neural excitation.
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ItemThe bionic ear in the second and third millennia [Abstract]( 1999)The development of the Bionic Ear or cochlear implant required answers to a number of questions: I) the cochlear hair cells and their nerve connections were too complex and numerous to be replaced by a small number of electrodes for reproducing the coding of sound; 2) a cochlear implant would destroy the very nerves it was hoped to stimulate; 3) speech was too complex to be presented to the nervous system by electrical stimulation for speech understanding to occur; 4) there would not be enough residual hearing nerves in the cochlea after die back due to deafness to transmit essential speech information; 5) children born deaf would not develop the neural connections in the auditory pathways for hearing with electrical stimulation. The questions have been answered by basic and clinical studies. Research on the experimental animal showed that electrical stimulation could not reproduce the synchrony of neural responses or frequency discrimination above 300 pulses/s, (less than the 3kHz range for speech understanding). Therefore, a multiple-electrode system was proposed to reproduce place as well as temporal coding of frequency. Experimental studies showed that current could be localised in the cochlea for place coding, and biological research established that an intracochlear electrode would be safe. Initial implants on adults confirmed the limitations frequency and intensity coding seen in the experimental animal, and a speech processing strategy was developed that extracted the important elements of speech for transmission through the "electro-neural" bottle-neck to the brain. This enabled the patients to understand running speech, and with the further improvements in this speech processing strategy the results are comparable to severely deaf people using a hearing aid. Similar good results have been obtained in children born deaf. There are a number of challenges for the third millenium to further improve the performance of cochlear implants. These challenges are: I) better simulation of the way the brain codes sounds; 2) invisible cochlear implants; 3) the use of nerve growth factors to protect the auditory nerve from die back after deafness or result in its regeneration; and 4) the use of neurotrophins to re-establish the neural plasticity seen in young children to develop the appropriate neural connections for the coding of frequency.