Graeme Clark Collection
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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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ItemPhysiological and histopathological effects of chronic monopolar high rate stimulation of the auditory nerve [Abstract]( 1999)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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ItemAn analysis of high rate speech processing strategies using the Nucleus 24 cochlear implant [Abstract]( 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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ItemAn analysis of high rate speech processing strategies using the Nucleus 24 cochlear implant [Abstract]( 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.
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ItemElectrical stimulation of the auditory nerve: chronic monopolar stimulation using very high stimulus rates [Abstract]( 1998)Speech processing strategies based on high rate electrical stimulation have been associated with recent improvements of speech perception among cochlear implant users. In the present study we investigated the effects of chronic monopolar stimulation using very high rates (14493 pulses\s). Under general anaesthesia (ketamine (20 mg/kg) and xylazine (3.8 mg/kg) i.p.) six normal hearing cats were implanted bilaterally with a three channel platinum (Pt) scala tympani electrode array, while a return Pt-electrode was placed outside the bulla. Chronic electrical stimulation using charge-balanced biphasic current pulses was delivered unilaterally via a transcutaneous leadwire connected to a backpack-stimulator for up to 2000 h. The animals hearing status was periodically monitored using acoustically evoked compound action potentials (CAP's) and brainstem responses (ABR's). In addition the electrically evoked ABR (EABR) was also recorded to ensure that the chronic stimulus was above threshold. Stimulus current and electrode voltage waveforms were monitored twice daily and access resistance (Ra) and electrode impedance (Zc) calculated. ABR and CAP thresholds were elevated immediately following implantation, but generally showed evidence of partial recovery (0-40 dB). Further deterioration of thresholds on the stimulated side (10-30 dB) was subsequently observed, while control-thresholds remained more stable. Ra (1.3-1.8 kΩ) and Zc (2.2-3.8Ω) typically increased in the first few weeks of electrical stimulation up to Ra:5.6 kΩ and Zc:8.1 kΩ, before decreasing slightly to a constant plateau. These initial results indicate changes in the electrode-tissue interface and tissue growth within the cochlea. They also indicate that chronic stimulation at these high rates may decrease residual hearing.
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ItemAdvances in cochlear implant speech processing [Abstract]( 1997)Our early research emphasized there was a restriction on the amount of speech and other acoustic information that could be transmitted to the nervous system by electrical stimulation of the auditory nerve. It also showed the need to use multiple-channel stimulation, and minimize channel interaction. As a result our research over the last 30 years has been directed towards optimizing the information presented to the auditory nervous system. This has involved extracting the energy of the first and second formants (FO/F2-WSP II; FO/FI/F2-WSP III; Multipeak-MSP) as well as the outputs of high band pass fixed filters (Multipeak - MSP) and coding these outputs as cochlear place of stimulation. The voicing frequency was coded as rate of stimulation. Our most recent speech processing strategy (SPEAK) extracts a specified number of .maximal outputs from a series of band pass filters, rather than selecting the peaks of energy which was the case with the other strategies. The voltages from the maximal outputs are used to stimulate appropriate electrodes on a place coding basis. The stimuli are presented at a constant stimulus rate to reduce channel interaction. Voicing is conveyed as amplitude variations.
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ItemA stimulation of spatio-temporal firing across auditory nerve fibres( 1997)Present cochlear implant speech processing strategies give recipients a perception of sound inferior to that of the normal hearing population. Since it is beyond current technology to achieve an electrically evoked auditory-nerve output identical to that of normal hearing, stimulation strategies are limited to approximating certain features of the neural firing patterns. The importance of the spatio-temporal firing patterns of an ensemble of auditory nerve fibres to speech perception has been stated in previous studies (1,2). This paper utilises a composite model of the cochlea and hair-cell/auditory nerve transduction using artificial and speech signals as input to produce a spatio-temporal excitation pattern which represents the fluctuating firing probability of the auditory neurons. A model of electrical stimulation of the auditory nerve is then used to show how stimulation strategies currently used produce neural firing patterns qualitatively different to those produced by normal hearing. Our investigations indicate that it is possible to generate electrical stimulation parameters that cause the spatio-temporal responses of the neural population to better approximate normal hearing. These responses enable us to identify stimulation parameters required to obtain the chosen neural firing patterns. A number of examples illustrate the utility of this method, revealing the spatio-temporal responses for varying numbers of neurons and electrode displacements.
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ItemTemporal coding in auditory neurons to electrical stimulation [Abstract]( 1997)The temporal response of the auditory pathway following intracochlear electrical stimulation will reflect the level of encoded temporal information, which is important for the further developmentof cochlear implant speech processing strategies, and in tum lead to a better understanding of temporal coding of acoustic stimuli Temporal coding of sound frequencies is based on the phase or time locked neural response seen to low frequency acoustic stimuli. The ability of neurons to respond in a time locked manner may determine the degree of encoded temporal frequency information. Electrophysiological studies have shown that the degree of response synchrony to charge balanced biphasic electrical stimuli is far greater than that seen to acoustic stimuli. We have investigated the temporal response properties of single units in the anteroventral cochlear nucleus (AVCN) in the cat to rates of electrical stimulation up to 800 pulses/s.
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ItemPreliminary speech perception results for children with the 22-electrode Melbourne / cochlear hearing prosthesis( 1992)The Cochlear 22-electrode cochlear prosthesis was first implanted in a child at the Cochlear Implant Clinic at the University of. Melbourne and Royal Victorian Eye & Ear Hospital in 1985. Since that time 42 children have received the device in Melbourne. Analysis of patient details shows that the majority of these children have a congenital as opposed to on acquired aetiology of hearing loss. In all but 3 cases, the children use 15 or more electrodes. In order to assist with evaluation of, benefits to speech perception across the very heterogeneous group of children, a six level hierarchical classification scheme for speech perception performance levels was created. All of the children achieved a minimum of Category 2 (discrimination of suprasegmental information). In total, 59 % of the children achieved Category 5 or 6 (open set recognition for unfamiliar materials). Analysis showed that the majority of these children had more than one year of experience. In contrast, the majority of children in Category 2 are those with less than one year of experience with the device.
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ItemImprovements in speech processing for the nucleus cochlear implant [Abstract]( 1992)The Nucleus Cochlear implant allows ~ variety of multiple channel speech processing strategies to be developed and trialled. The initial strategy first developed by the University of Melbourne presented the second formant frequency as place and voicing as rate of stimulation. The strategy was subsequently improved by presenting the first format on a place basis as well. Since that time the addition of more spectral information coded as place of stimulation and temporal information presented as variations in amplitude have resulted in better open-set CNC word and sentence scores for electrical stimulation alone. One of the improved strategies selects four pairs of electrodes from the 22 electrode array each glottal pulse to present the first and second formants as well as the output from two high Frequency band pass filters. The other strategy stimulates six of the 22 electrodes representing the six maximal outputs from 16 bandpass filters. The clinical results have shown that both the above strategies results in better open-set speech perception for electrical stimulation in quiet and in noise. 80 % scores have been obtained for open-sets of CNC words and 90 % for open-sets of words in sentences for some of the patients using the latter strategy.