Graeme Clark Collection
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ItemPsychophysical and speech perception studies: a case report on a binaural cochlear implant subject( 1993)Abstract not available due to copyright.
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ItemComparison of half-band and full-band electrodes for intracochlear electrical stimulation( 1993)It was hypothesized that intracochlear stimulating electrodes oriented toward the modiolus would require a lower stimulus current to elicit a threshold neural response than longitudinal band electrodes. Electrically evoked auditory brain stem responses (EABRs) and electrode impedances were recorded with full-band and oriented half-band scala tympani electrodes in anesthetized, deafened cats. To elicit a threshold EABR, the stimulus current required for stimulation through half-band electrodes oriented toward the modiolus was not significantly different from the current required for stimulation through full-band electrodes. The impedances of full-band electrodes were significantly lower than those of half-band electrodes. Considering the significantly higher impedance and current density of half band electrodes in comparison to full-band electrodes, as well as the critical orientation of half-band electrodes during implantation, we believe that full-band electrodes have advantages over half-band electrodes for scala tympani implantation.
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ItemEvaluation of expandable leadwires for paediatric cochlear implants( 1993)The development of cochlear implants for use in very young children (1-2 years old) will require techniques designed to accommodate temporal bone growth. Previous anatomic studies have shown that the leadwire of a cochlear implant must be capable of expanding up to 20 mm between the round window and the implanted receiver-stimulator in response to skull growth. In the present study morphologic and biomechanical evaluation of five expandable leadwire designs was conducted following their implantation in young cats. Two helical shaped leadwire designs frequently exhibited extensive fibrous tissue adhesions and broke during long-term implantation. In contrast, thin, flexible Silastic envelopes were effective in minimizing tissue adhesions. Residual V- and Z-shaped leadwires, placed in these envelopes, showed little evidence of fibrous tissue adhesions following implantation periods of up to 2 years. Moreover, these leadwires readily expanded both during the growth of the animal and when biomechanical expansion studies performed at the completion of the implant period. These expandable leadwire designs appear to be appropriate candidates for use in pediatric cochlear implants.
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ItemProfound hearing loss in the cat following the single co-administration of kanamycin and ethacrynic acid( 1993)Co-administration of kanamycin (KA) with the loop diuretic ethacrynic acid (EA) has previously been shown to produce a rapid and profound hearing loss in guinea pigs. In the present study we describe a modified technique for developing a profound hearing loss in cats. By monitoring the animal's hearing status during the intravenous infusion of EA the technique minimizes the effects of individual variability to the drug regime. Seven cats received a subcutaneous injection of KA (300 mg/kg) followed by intravenous infusion of EA (1 mg/min). Click-evoked auditory brainstem responses (ABRs) were recorded to monitor the animal's hearing during the infusion. When the ABR thresholds rose rapidly to levels in excess of 90 dB SPL the infusion of EA was stopped. This occurred at EA doses of 10-25 mg/kg, indicating considerable individual variability to the deafening procedure. However, there was a strong negative correlation (r = - 0.93) between the EA dose and body weight which accounted for much of this variability. Subsequent ABR monitoring showed that this profound hearing loss was both bilateral and permanent. Significantly, blood urea and creatinine levels, monitored for periods of up to three days after the procedure, remained within the normal range. Furthermore, there was no clinical evidence of renal dysfunction as indicated by weight loss or oliguria. Cochlear histopathology, examined after a two months to three year survival period, showed an absence of all inner and outer hair cells in the majority of cochleas. The extent of loss of spiral ganglion cells was dependent on their distance from the round window and the period of survival following the deafening procedure. Clearly, the degeneration of spiral ganglion cells continued for several years following the initial insult. Finally, we observed no evidence of renal histopathology. In conclusion, the co-administration of KA and EA produces a profound hearing loss in cats without evidence of renal impairment. Monitoring the animal's hearing status during the procedure ensures that the dose of EA can be optimised for individual animals. Moreover, it may be possible to adapt this procedure to produce animal models with controlled high frequency hearing losses.
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ItemPaediatric cochlear implantation: radiologic observations of skull growth( 1993)We investigated the effects of long-term implantation of auditory prostheses on skull growth in young animals. Four monkeys were implanted with dummy cochlear implants at 6 months of age. To simulate implantation in children, the bed for the receiver-stimulator or interconnecting plug was drilled across a calvarial suture down to the underlying dura. Plain skull oentgenograms were periodically taken to monitor head growth for up to 3 years after implantation. These longitudinal measurements revealed no significant asymmetric skull growth. Postmortem measurements using computed tomographic scans confirmed these results and showed no significant difference in the intracranial volumes between the implanted and control sides of each animal or between experimental and nonimplanted control monkeys. These results suggest that long-term cochlear implantation in very young children will not cause any significant deformity of the skull.
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ItemComparison of current speech coding strategies( 1993)This paper reports on two studies carried out at the University of Melbourne jointly with Cochlear Pty Ltd. The studies demonstrated substantial speech perception improvements over the current Multipeak strategy in background noise.
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ItemReading comprehension levels of profoundly, prelingually deaf students in Victoria( 1993)In the recent ministerial report on deaf education in Victoria, the predicted outcome of deaf school leavers was that they would only achieve a grade three or four level of reading. One hundred and ninety five profoundly, prelingually deaf students from grade four to year twelve in Victoria were tested using the Stanford Reading Comprehension test. Although the results showed a wide range of reading ability among the students, the overall mean reading grade equivalent was grade 5.0 and over 40% were reading at average or above average levels. Sixty-nine percent of deaf "school leavers " were reading at higher levels than the grade four level assumed in the Victorian ministerial report and the average "school leaver" reading grade equivalent was at a grade 6.0 level.
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ItemElectrical stimulation of the auditory nerve: the effect of electrode position on neural excitation( 1993)Histological studies have shown that the Melbourne/Cochlear electrode array lies along the outer wall of the scala tympani and is therefore some distance from the residual VIIIth nerve elements. In order to investigate the influence of electrode position on neural excitation we systematically varied the position of the electrode array within the cat scala tympani while recording electrically evoked auditory brainstem responses (EABRs). Using both normal hearing and long-term deafened animals, we observed significant reductions in EABR thresholds as the electrode array was moved from the outer wall towards the modiolus. Further threshold reductions were observed when the array was placed underneath the osseous spiral lamina (OSL) close to the peripheral dendrites. These changes were independent of the bipolar inter-electrode separation, and were observed over a wide range of cochlear pathologies varying from normal to a moderate spiral ganglion cell loss. Interestingly, the one animal exhibiting extensive neural loss showed no correlation between EABR threshold and electrode position. There was also a general decrease in the gradient of the EABR input-output function as the electrode array was moved closer to the neural elements. This was, however, only statistically significant when the electrode was positioned adjacent to the peripheral dendrites. Significant reductions in EABR threshold were also observed as the inter-electrode spacing of the bipolar electrodes was increased. The gradient of the EABR input-output function also increased with increasing inter-electrode spacing, although again, this was only significant when the electrode array was positioned close to the neural elements. The present results indicate that the optimum placement of a Melbourne/Cochlear electrode array is adjacent to the peripheral dendrites. However, such a site would be difficult to achieve in practice while minimizing insertion trauma. An array lying adjacent to the modiolus would be a safe alternative while ensuring a significant reduction in threshold compared with the existing site (outer wall). This placement should result in more localized neural excitation patterns, an increase in the number of bipolar electrodes available, together with an increase in their dynamic range. These changes may lead to further improvements in speech perception among cochlear implant patients.
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ItemTinnitus management in the profoundly and totally deaf( 1993)Tinnitus is a common symptom of many cochlear or auditory system pathologies. Since tinnitus is frequently associated with a sensorineural hearing loss, it is not surprising that a large proportion of profoundly and totally deaf patients describe tinnitus as a symptom. The clinical management of severe tinnitus in these patients is discussed with particular emphasis on the use of electrical stimulation. While cochlear implants appear to provide a measure of relief when being used, significant improvements in the management of severe tinnitus will only occur when we have a greater understanding of the underlying pathophysiology, diagnostic procedures that can accurately establish the site of tinnitus generation, and more objective clinical trial procedures that include the use of controls.
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ItemThe effect of handedness in tactile speech perception( 1993)This study examined differential performance of normally hearing subjects using a tactile device on the dominant versus non-dominant hand. The study evaluated whether tactual sensitivity for non-speech stimuli was greater for the dominant hand as compared with the non-dominant hand, and secondly, whether there was an advantage for speech presented tactually to the dominant hand, resulting from a preferential pathway to the language processing area in the left cerebral hemisphere. Evaluations of threshold pulse width, dynamic ranges, paired electrode identification, and a closed-set tactual pattern discrimination test battery showed no difference in tactual sensitivity measures between the two hands. Speech perception was assessed with closed sets of vowels and consonants and with open-set Harvey Gardner (HG) words and Arthur Boothroyd (AB) words. Group mean scores were higher in each of the tactually aided conditions as compared with the unaided conditions for speech tests, with the exception of AB words in the tactile plus lip-reading plus audition/lip-reading plus audition condition on the right hand. Overall mean scores on the closed-set vowel test and on open-set HG and AB words were significantly higher for the tactually aided condition as compared with the unaided condition. Comparison of performance between the dominant and non-dominant hand showed a significant advantage for the dominant hand on the closed-set vowel test only. No significant differences between hands in either tactually aided or unaided conditions were evident for any of the other speech perception tests. Factors influencing this result could have been variations in degree of difficulty of the tests, the amount of training subjects received, or the training strategy employed. Although an advantage to presenting speech through the dominant hand may exist, it is unlikely to be great enough to outweigh possible restrictions on everyday use.