Graeme Clark Collection
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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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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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ItemSpeech perception & indirect benefits for severely hearing impaired children using cochlear implants [Abstract]( 1998)The benefits to speech perception, speech production, communication, and quality of life were investigated for severely hearing impaired children using cochlear implants and hearing aids. For the youngest children, assessments of psychological status, and use of a battery of cognitive and language tests were important factors in determining performance benefits prior to the child developing speech. For older children, the effects of increased one-to-one oral habilitation, and systematic encouragement of parental involvement were also investigated. Results suggested that parental support for consistent device use was an important factor in producing outcomes. Children with residual hearing were more likely to achieve open-set perception. Each child was also assessed for changes in quality of life, through use of a standardized health utility instrument. Results from the group were collated, and teamed with a costing assessment for the procedure, which allowed a complete cost-utility analysis to be completed. Results suggested that indirect benefits, such as the effects on speech production, usual activities, lower concentration, and less distress were ranked as more important than changes to hearing in a number of the children. Costs per QALY were consistent with those reported in the literature, and suggested that the cochlear implant is highly cost-effective as a medical technology.
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ItemEvaluation of direct and indirect benefits in the selection of cochlear implant candidates [Abstract]( 1998)The value of cochlear implants as an established clinical option for profoundly hearing-impaired adults and children has been supported by significant research results over a number of years which clearly established the benefits available (U.S. National Institutes of Health Consensus Statement 1995). Benefit has traditionally been considered as the impact of the cochlear implant procedure on hearing, and in the case of children, on the use of that hearing to develop speech and language. However, as a result of continuing research, improvements have been realised both in hardware and speech processing. As a direct consequence of these improvements, the mean open-set implant-alone speech perception scores for implanted adults have continued to increase. In response to the increased mean scores shown by adult cochlear implant users in quiet, perception tests in background noise are now being used as a direct measure of the potential benefits of cochlear implants available to severely-to-profoundly hearing-impaired candidates. In addition, consideration in candidature should also be given to indirect benefits, such as reduction in the stress. of listening and lipreading, improved performance at work, enhanced opportunity to maintain or in children to develop speech which is understandable to the general community, and the social effects of reducing the isolating effects of profound deafness. Measurement of indirect benefit can be combined with a costing study, enabling evaluation of the implant procedure from a cost-utility standpoint, and comparison of outcomes using other technologies such as hearing aids. The use of new approaches to candidature, including new measures of direct and indirect benefit is reported using data from the Melbourne cochlear implant program.
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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.