Graeme Clark Collection
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ItemEffects of intracochlear factors on spiral ganglion cells and auditory brain stem response after long-term electrical stimulation in deafened kittens( 2000)Using an animal model, we have studied the response of the auditory brain stem to cochlear implantation and the effect of intracochlear factors on this response. Neonatally, pharmacologically deafened cats (100 to more than 180 days old) were implanted with a 4-electrode array in both cochleas. Then, the left cochlea of each cat was electrically stimulated for total periods of up to 1000 hours. After a terminal 14C-2-deoxyglucose (2DG) experiment, the fraction of the right inferior colliculus with a significant accumulation of 2DG label was calculated. Using 3-dimensional computer-aided reconstruction, we examined the cochleas of these animals for spiral ganglion cell (SGC) survival and intracochlear factors such as electrode positions, degeneration of the organ of Corti, and the degree of fibrosis of the scala tympani. The distribution of each parameter was calculated along the organ of Corti from the basal end. There was a positive correlation between SGC survival and the level of fibrosis in the scala tympani, and a negative correlation between SGC survival and the degree of organ of Corti degeneration. Finally, there was a negative correlation between the 2DG-labeled inferior colliculus volume fraction and the degree of fibrosis, particularly in the 1-mm region nearest the pair of electrodes, and presumably in the basal turn.
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ItemElectrode discrimination by early-deafened subjects using the Cochlear Limited multiple electrode cochlear implant( 2000)Objective: The aims of this study were to determine whether electrode discrimination by early-deafened subjects using the Cochlear Limited prosthesis varied at different locations on the electrode array, was influenced by the effects of auditory deprivation and experience with electric stimulation, and was related to speech perception. Design: Difference limens for electrode discrimination were measured in 16 early-deafened subjects at three positions on the array: electrodes 18 (apical), 14 (mid), and 8 (basal). Electrodes were stimulated using random variations in current level to minimize the influence of loudness cues. Assessed were correlations between the difference limens, subject variables related to auditory deprivation (age at onset of deafness, duration of deafness, and age at implantation) and auditory experience (duration of implant use and the total time period of auditory experience), and speech perception scores from two closed-set and two open-set tests. Results: The average difference limens across the three positions were less than two electrodes for 75%, of subjects, with average limens between 2 and 6.5 electrodes for the remaining 25% of subjects. Significant differences across the three positions were found for 69% of subjects. The average limens and those at the basal position positively correlated with variables related to auditory deprivation, with larger limens for subjects implanted at a later age and with a longer duration of deafness. The average limens and those at the apical position negatively correlated with closed-set speech perception scores, with lower scores for subjects with larger limens, but not with open-set scores. Speech scores also negatively correlated with variables related to auditory deprivation. Conclusions: These findings showed that early-deafened subjects were generally successful in electrode discrimination although performance varied across the array for over half the subjects. Discrimination performance was influenced by the effects of auditory deprivation, and both electrode discrimination and variables related to auditory deprivation influenced closed-set speech perception.
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ItemPredicting speech perception results for children using multichannel cochlear implants [Abstract]( 2002)It is most helpful in counselling families considering cochlear implantation to have some ability to predict outcomes for individual hearing-impaired children. Speech perception results for open-set words and sentences have been collected for all children implanted with the Nucleus device in Melbourne. Assessments are available at approximately six month intervals following implantation, Data was collected for each child regarding type of hearing loss, duration and age at onset of profound hearing loss, age at implantation, pre and post-implant communication mode, developmental delay, speech coding scheme and implant experience. These data were used as predictor variables in step-wise multiple linear regression analyses with the speech perception scores as the dependent variables. Shorter duration of profound hearing loss, later onset of profound hearing loss, exclusively oral communication mode following implantation, and longer implant experience were associated with significantly (p < 0.00 I) improved open-set speech perception. The use of the SPEAK signal coding scheme was shown to provide significantly better speech perception performance for children (p < 0.00 I). Developmental delay was associated with poorer speech perception outcomes (p < 0.0 I). Over 50% of the variance in speech perception scores was accounted for by these variables. The study suggests that younger implantation for congenitally deaf children leads to improved speech perception results. On the other hand, the development of auditory language skills in implanted children may be as important as age at implantation in enhancing long term outcomes. Regression equations derived from these results can be used to predict outcomes for cochlear implant candidates with a reasonable accuracy.
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ItemCochlear implants: a personal scientific journey [Abstract]( 2002)Electrical stimulation of the auditory system to reproduce hearing commenced through academic curiosity, and the hope of helping deaf people. It received direction from neurophysiology, and later psychophysics and speech science. In the 1960s and 1970s there were many questions requiring answers before cochlear implants could become a practical reality. Key concerns were: (1) the cochlea was too complex for electrical stimulation to reproduce the coding of sound; (2) multiple electrodes inserted into the cochlea for the place coding of frequency could damage the auditory nerves to be stimulated; (3) speech was too complex to be reproduced by electrical stimulation; and (4) children born deaf would not develop the appropriate neural connectivity for speech understanding. The first questions were addressed on the experimental animal. Speech research on patients was only possible with the advent of silicon chip technology allowing the development of an implantable receiver-stimulator package. Initial research established proof of principle that connected discourse was possible with multiple electrode stimulation of the auditory nerve in severely and profoundly deaf people. The research has been developed industrially for the benefits to be provided on a widespread basis through clinics worldwide. Further research has resulted in continuing improvements so that the average profoundly deaf person can hear as well as someone with severe hearing loss using a hearing aid. There is still much research required to achieve high fidelity sound, hearing in noise, and totally implantable devices.
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ItemCochlear implants for adults and children(Martin Dunitz, 2002)Cochlear implants which use multiple-electrode speech-processing strategies are now established clinical entity for children and adults, as a result, preoperative selection and (re)habilitation are key issues. It is hard to realize that it was only in the 1960s and 1970s when many scientists and clinicians said that successful cochlear implants were not possible in the foreseeable future. The questions that had to be addressed by a multi disciplinary research effort are discussed, and the solutions achieved from the University of Melbourne's perspective are presented. However, the main aim of this chapter is to focus on preoperative selection, and (re)habilitation, including the results obtained. These issues are discussed primarily with reference to data from the University of Melbourne's Cochlear Implant Clinic at the Royal Victorian Eye and Ear Hospital. As this is a book on audiological medicine only, an overview of surgical principles is presented. The surgical management of the patient is, of course, very important, so for more details the reader is referred elsewhere. Cochlear implantation has also been the subject of quite intense ethical debate, particularly over its use for children. For this reason, a discussion of ethical issues is included. Finally, the chapter concludes with a vision of research in the next Millennium.
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ItemHistopathology of the binaural cochlear implant subject [Abstract]( 2001)Binaural hearing improves speech reception in noise, and is necessary for sound localisation. Normal hearing subjects use both interaural time, and intensity, differences to localise sound. This study investigates why sound localisation in bilateral cochlear implantees is insensitive to interaural time differences (Hoesel 1993). We looked for evidence of neural degeneration in the auditory brainstem involved in binaural sound localisation, since this may have degraded the neural circuitry required to accurately code interaural time delays. Method: The brainstem of a bilateral cochlear implantee was prepared for light microscopy by embedding it in paraffin, sectioning at 10 mm and staining sections with thionine or Luxol fast blue (LFB). The histological sections were digitised with NIH Image and 3-dimensional reconstructions made of the cochlear nucleus (CN) and superior olivary complex (SOC) with AnalysePC. Within the CN and the SOC, cell number and size were estimated by the physical dissector technique following thionine staining, and myelination of the nerve fibres was estimated using the optical density method following LFB staining. Results: A reduction in cell size (from thionine staining) and myelination (from LFB staining) was seen in both the CN and the SOC. Conclusions: These finding are consistent with neural degeneration within the auditory pathways. This may have lead to a degradation of the neural circuitry required to accurately detect interaural time delays.
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ItemSynchronization of the neural response to noisy periodic synaptic input(M I T PRESS, 2001-12)The timing information contained in the response of a neuron to noisy periodic synaptic input is analyzed for the leaky integrate-and-fire neural model. We address the question of the relationship between the timing of the synaptic inputs and the output spikes. This requires an analysis of the interspike interval distribution of the output spikes, which is obtained in the gaussian approximation. The conditional output spike density in response to noisy periodic input is evaluated as a function of the initial phase of the inputs. This enables the phase transition matrix to be calculated, which relates the phase at which the output spike is generated to the initial phase of the inputs. The interspike interval histogram and the period histogram for the neural response to ongoing periodic input are then evaluated by using the leading eigenvector of this phase transition matrix. The synchronization index of the output spikes is found to increase sharply as the inputs become synchronized. This enhancement of synchronization is most pronounced for large numbers of inputs and lower frequencies of modulation and also for rates of input near the critical input rate. However, the mutual information between the input phase of the stimulus and the timing of output spikes is found to decrease at low input rates as the number of inputs increases. The results show close agreement with those obtained from numerical simulations for large numbers of inputs.
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ItemSpeech perception outcomes for older patients using the nucleus cochlear implant [Abstract]( 2002)With improved medical technology and increased life expectancy, more patients are receiving cochlear implants at a later age. It is important to examine speech perception outcomes for older patients, in order to be able to provide appropriate pre-operative counselling regarding post-operative expectations. Speech perception scores for patients at the Melbourne Cochlear Implant Clinic implanted at age 65 or more were examined, and compared to the adult population implanted under the age of 65. (All of these patients were users of SPEAK or later strategies.) The aims were to see if speech perception scores were significantly different between the two groups as a whole, and to see if a correlation existed between increasing age of implantation above 65 and speech perception scores. Preliminary results suggest that there is a small but significant effect of age on speech perception outcomes. The findings from this study will be used to help clinicians provide more accurate advice and counselling for older patients, regarding the potential outcomes from a cochlear implant.
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ItemLateral inhibition in ventral cochlear nucleus chopper neurons: contribution to coding of a speech feature [Abstract]( 2002)Lateral inhibition in the auditory system enhances excitatory responses by suppressing off-best frequency (BF) neural activity. Previous work has suggested that lateral inhibition activated by high frequency frication noise associated with stop consonant plays a role in coding voice onset time (VOT), the period between consonant release and onset of the ensuing vowel.
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ItemImproved sound processing for cochlear implants( 2002)Four signal processing schemes currently under development aim to improve the perception of sounds/ especially speech, for children and adults using the Nucleus cochlear implant system. The schemes are (1) fast-acting input-signal compression, (2) Adaptive Dynamic Range Optimisation (ADRO), (3) TESM, a scheme that emphasises transients in signals, and (4) DRSP, a strategy that applies different stimulation rates to selected sets of electrodes.