Graeme Clark Collection

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Now showing 1 - 10 of 58
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    Electrophonically driven single unit responses of the anteroventral cochlear nucleus in cat [Abstract]
    Morrison, N. A. ; Brown, M. ; Clark, Graeme M. ( 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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    A "Combionic Aid": Combined speech processing for a cochlear implant in one ear and speech processing hearing aid in the other ear [Abstract]
    Dooley, Gary J. ; Blamey, Peter J. ; Seligman, Peter M. ; Clark, Graeme M. ( 1993)
    Independent use of a cochlear implant in one ear and a hearing aid in the other is not acceptable for many implant users with some residual hearing. Psychophysical evidence suggests that there are substantial interactions between acoustic and electrical signals including masking and loudness summation. These effects may contribute to the difficulty in using two independent devices and it is desirable to control the parameters of the electrical and acoustical signals far more accurately than is possible with two independent devices with separate microphones. In order to achieve this control we have developed a Combionic aid incorporating an implant and an 'in1planlcompatible' hearing aid controlled from the same speech processor. The new processor is particularly flexible and can implement a wide variety of speech processing strategies for combined acoustic and electrical stimulation. A benchtop prototype has been tested with five patients using a range of different speech tests. In general, patients do better when they use acoustic and electrical information simultaneously than they do with either alone. Some patients on some tests perform significantly better with the bimodal aid than they do with independent hearing aids and implant processors worn together. Wearable devices have now been built and evaluations of these devices are continuing.
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    Parameter selection and programming recommendations for the ACE and CIS speech processing strategies [Abstract]
    Plant, Kerrie L. ; Whitford, Lesley A. ; Psarros, C. E. ; Vandali, A. E. ; Clark, Graeme M. ( 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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    Chronic electrical stimulation of the auditory nerve using high-surface area platinum electrodes [Abstract]
    Tykocinski, M. ; Liu, X. ; Clark, Graeme M. ( 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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    Spatial spread of neural excitation in cochlear implants: comparison of measurements made using NRT and forward masking [Abstract]
    Cohen, L. T. ; Saunders, E. ; Cone-Wesson, Barbara ; Clark, Graeme M. ( 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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    The bionic ear in the second and third millennia [Abstract]
    Clark, Graeme M. ( 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.
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    An analysis of high rate speech processing strategies using the Nucleus 24 cochlear implant [Abstract]
    Vandali, A. E. ; Grayden, D. B. ; Whitford, L. A. ; Plant, K. L. ; Clark, Graeme M. ( 1998)
    The effects of varying the stimulation rate on speech perception was evaluated in five postlinguistically deaf adult users of the Nucleus 24 Cochlear Implant System. Three different rates of electrical stimulation, 250, 807, and 1615 pulses per second per channel were employed. For the high stimulus rate the analysis frequency was the same as for the medium rate condition. The study investigated the effect of varying rate of stimulation when using the electrode selection technique of the SPEAK strategy. The study used a repeated ABC experimental design, in order to account for learning effects and to minimize ordering effects. Speech perception was evaluated using both monosyllabic words (open-sets of CNC words in quiet) and sentence materials (open-sets of CUNY sentences at signal-to-noise ratios from +20 to 0dB). In addition, the subjects' perception of closed-sets of 19 vowels and 24 consonants, presented in the H/VID and A/C/A context, were also investigated. The recognition and perception of distinctive features were assessed across strategies and patients. Preliminary speech perception results have shown no statistically significant difference in performance between the low and medium stimulation rates. However, significantly poorer results were observed for the high rate condition for some tests with some individuals. Individual differences may be explained by the effects of rate of stimulation on speech features.
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    Factors influencing gap detection in children using cochlear implants [Abstract]
    Busby, P. A. ; Clark, Graeme M. ( 1998)
    Two studies on gap detection were conducted with children implanted with the Cochlear Limited multiple-electrode prosthesis. In the first study, gap thresholds were measured at three pulse rates (200, 500, and 1000 pulses/s) and two stimulus durations (500 and 1000 ms) in 15 subjects. Average gap thresholds ranged from 1.8 to 32.1 ms. There was, essentially, no effect of pulse rate and in all but two subjects, no effect of stimulus duration. For these two subjects, thresholds were higher for the 1000 ms stimulus duration. In the second study, the relationships between gap thresholds and subject characteristics, and between gap thresholds and speech perception scores were examined using data from 27 subjects. A significant negative correlation between age at onset of deafness and gap thresholds was found, indicating that subjects with an earlier onset of deafness had larger gap thresholds, and most of the variability in gap thresholds was found for the congenitally deaf subjects. Gap thresholds did not correlate with other subject variables, such as duration of deafness and duration of implant use. Significant negative correlations were found between gap thresholds and word scores for open-set BKB sentences in the auditory-visual condition and the lipreading enhancement scores for the same test, indicating that subjects with poorer gap thresholds had lower speech scores. Gap thresholds did not correlate with word scores for BKB sentences in the audition alone condition, and scores for closed-set monosyllabic word tests.
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    The development of a precurved cochlear implant electrode array and its preliminary psychophysical evaluation [Abstract]
    Cohen, L. T. ; Saunders, E. ; Treaba, C. ; Pyman, B. C. ; Clark, Graeme M. ( 1998)
    A precurved banded electrode array may provide a better interface with the auditory neural pathways for cochlear implants, and provide better speech perception. A prototype arrray has been further evaluated for ease of insertion, siting within the cochlea and induction of any cochlear trauma. The arrays were inserted into the human cochlear under simulated surgery. The bones were embedded in Araldite, X-rayed and sectioned. X-ray reconstruction analyses of the position of the implanted array showed its insertion to be favourable. Cochlear implants with precurved arrays have been implanted in three patients. Psychophysical evaluation and X-ray analyses have shown that as electrode distance from the modiolus decreased: threshold current decreased; dynamic range increased; current spread as measured by forward masking studies, was more focused; electrode discrimination with loudness jitter (being abetter representation of the dynamic speech signal) improved; JNDs for loudness, expressed as a function of dynamic range, decreased.
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    An analysis of high rate speech processing strategies using the Nucleus 24 cochlear implant [Abstract]
    Vandali, A. E. ; Grayden, D. B. ; Whitford, L. A. ; Plant, K. L. ; Clark, Graeme M. ( 1998)
    Speech comprehension for a group of five users of the Nucleus 24 Cochlear Implant system was explored at three rates of electrical stimulation, 250, 807, and 1615 pulses per second per channel. For the high stimulus rate the analysis frequency was the same as for the medium rate condition. The study investigated the effect of varying rate of stimulation when using the electrode selection technique of the SPEAK strategy. This has been undertaken using a repeated ABC experimental design to account for learning and minimize ordering effects. Speech perception was assessed using open-sets of CNC words in quiet and open-sets of CLTNY sentences at signal-to-noise ratios from +20 to 0dB. Closed-sets of 19 vowels and 24 consonants were also presented, in the H/V/D and A/C/A context. The recognition and perception of distinctive features were assessed across strategies and patients. Preliminary speech perception results have shown no statistically significant difference in performance between the low and medium stimulation rates. However, significantly poorer results were observed for the high rate condition for some tests. Individual differences may be explained by the effects of rate of stimulation on speech features.