Graeme Clark Collection
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ItemEvaluation of a new Spectral Peak coding strategy for the Nucleus 22 channel cochlear implant system( 1994)Sixty-three postlinguistically deaf adults from four English-speaking countries participated in a 17-week field study of performance with a new speech coding strategy, Spectral Peak (SPEAK), and the most widely used strategy, Multipeak (MPEAK), both of which are implemented on wearable speech processors of the Nucleus 22 Channel Cochlear Implant System; MPEAK is a feature-extraction strategy, whereas SPEAK is a filterbank strategy. Subjects' performance was evaluated with an experimental design in which use of each strategy was reversed and replicated (ABAB). Average scores for speech tests presented sound-only at 70 dB SPL were higher with the SPEAK strategy than with the MPEAK strategy. For tests in quiet, mean scores for medial vowels were 74.8 percent versus 70.1 percent; for medial consonants, 68.6 percent versus 56.6 percent; for monosyllabic words, 33.8 percent versus 24.6 percent; and for sentences, 77.5 percent versus 67.4 percent. For tests in noise, mean scores for Four-Choice Spondees at +10 and +5 dB signal-to-noise ratio (S/N) were 88.5 percent versus 73.6 percent and 80.1 percent versus 62.3 percent, respectively; and for sentences at +15 dB, +10, and +5 dB S/N, 66.5 percent versus 43.4 percent, 61.5 percent versus 37.1 percent, and 60.4 percent versus 31.7 percent, respectively. Subjects showed marked improvement in recognition of sentences in noise with the new SPEAK filterbank strategy. These results agree closely with subjects' responses to a questionnaire on which approximately 80 percent reported they heard best with the SPEAK strategy for everyday listening situations.
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ItemCochlear pathology following chronic electrical stimulation of the auditory nerve: II Deafened kittens( 1994)The present study examines the effects of long-term electrical stimulation of the auditory nerve on cochlear histopathology and spiral ganglion cell survival in young sensorineural deafened cats. Eight kittens were deafened using kanamycin and ethacrynic acid, and implanted with bipolar or monopolar scala tympani electrodes. Following recovery from surgery the animals were unilaterally stimulated using charge balanced biphasic current pulses for 450-1730 hours over implant periods of up to four months. Charge densities varied from 0.6-0.9 µC.cm ^-2 geom. per phase for monopolar electrodes to 12-26 µC.cm ^-2 geom. per phase for the bipolar electrodes. Electrically-evoked auditory brainstem responses (EABRs) were periodically monitored during stimulation to confirm that the stimulus levels were above threshold, and to monitor any change in the response of the auditory nerve. Following completion of the stimulation program cochleae were prepared for histological examination. EABRs exhibited relatively stable thresholds for both stimulated and implanted, unstimulated control cochleae for the stimulus duration. While the growth in response amplitude as a function of stimulus current remained stable for the bipolar control and monopolar stimulated cochleae, the five cochleae chronically stimulated using bipolar electrodes exhibited a moderate to large increase in response amplitude. These increases were associated with a more widespread fibrous tissue response which may have altered the current distribution within these cochleae. Implanted control cochleae exhibited significantly less tissue response within the scala tympani. Importantly, we observed no statistically significant difference in the spiral ganglion cell density associated with chronic electrical stimulation when compared with unstimulated control cochleae. While the present study supports the safe application of cochlear implants in young profoundly deafened children, it does not corroborate previous studies that have reported electrical stimulation providing a trophic effect on degenerating auditory nerve fibres.
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ItemHearing, vocalization and the external ear of a marsupial, the northern quoll, Dasyurus hallucatus( 1994)As part of a continuing study of the development of the marsupial auditory system, auditory brainstem responses (ABR) were recorded and an ABR audiogram was constructed for five female Northern Quolls (Dasyurus hallucatus), which are nocturnal carnivores. The best frequency for hearing lies between 8 and 10 kHz, and at 50 dB SPL there is a range from about 0.5 to 40 kHz. Vocalizations of adult quolls and pouch-young were recorded with a digital audio tape recorder, and the power spectra of representative calls were compared with the ABR audiogram. The common adult vocalizations have most energy at the lower end of the hearing range, whereas frequencies that are dominant in the isolation calls of the pouch-young lie close to the best frequency of hearing. Samples of nocturnal sounds of the habitat of the quoll were also recorded and analyzed. Power spectra have peak energy at frequencies between 2 and 5 kHz, with a smaller contribution above 10 kHz. The spectrum contains relatively little power at the best frequency of hearing. Measurements of the sound pressure level at the external ear canal as a function of stimulus frequency and location in space suggest that the directional amplifying properties of the pinna will operate most effectively on sound frequencies at the upper end of the quoll's hearing range, a region that may be important in prey detection. Comparisons are made with other mammalian nocturnal carnivores and with other marsupials. We speculate that, for nocturnal carnivores, one role of the low-frequency part of the hearing range concerns the recognition of adult conspecifics, the mid-frequency range is important for the detection of pouch-young, and the upper range may be particularly concerned with prey/predator detection.
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ItemChronic electrical stimulation of the auditory nerve at high stimulus rates: preliminary results( 1994)The present preliminary report describes the electrophysiological response of the cochlea during long-term stimulation. The data indicate that electrical stimulation at a rate of 1000 pulses per second does not appear to adversely affect the implanted cochlea.
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ItemPartial hearing loss in the macaque following the co-administration of kanamycin and ethacrynic acid( 1994)Co-administration of kanamycin (KA) with the loop diuretic ethacrynic acid (EA) rapidly produces a profound hearing loss in the cat while maintaining normal renal function [Xu et al., Hear. Res. 70, 205-215 (1993)]. In the present paper we have applied this deafening procedure to the old world monkey Macaca fascicularis (macaque). Following the co-administration of KA and EA, the hearing loss in the macaque developed far slower than we observed in the cat. Moreover, unlike the cat, there was evidence of a partial recovery in the animal’s hearing, resulting in a bilaterally symmetrical high frequency hearing loss. The extent of this hearing loss was dependent on the dose of the EA administered. Finally, the most unexpected result of the present study was the degree of acute nephrotoxicity experienced by these animals following the drug administration. The sensitivity of this species to renal failure restricted the dose of EA that could be safely administered. In conclusion, the co-administration of KA and EA cannot reliably produce a profound hearing loss in the macaque. While it can produce a dose dependent high frequency hearing loss the animal will also experience acute renal failure that requires careful management.
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ItemCochlear implantation in young children: histological studies on head growth, leadwire design, and electrode fixation in the monkey model( 1994)For safe cochlear implantation in children under 2 years of age, the implant assembly must not adversely affect adjacent tissues or compromise head growth. Furthermore, growth changes and tissue responses should not impair the function of the device. Dummy receiver-stimulators, interconnect plugs, and leadwire-lengthening systems were implanted for periods of 36 months in the young monkey to effectively model the implantation of the young child. The results show that implanting a receiver-stimulator package has no adverse effects on skull growth or the underlying central nervous system. The system for fixing the electrode at the fossa incudis proved effective. There was marked osteoneogenesis in the mastoid cavity, resulting in the fixation of the leadwire outside the cochlea. This study provides evidence for the safety of cochlear implantation in young subjects.
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ItemCochlear implantation in children: labyrinthitis following pneumococcal otitis media in unimplanted and implanted cat cochleas( 1994)Pneumococcal otitis media is frequent in young children and could lead to labyrinthitis post-implantation. To assess the risk and methods of minimizing it by a graft to the round window around the electrode entry point, we have used a cat animal model of pneumococcal otitis media. Twenty-one kittens were used in the study. Thirty-two cochleas were implanted when the kittens were 2 months of age. Fourteen cochleas were implanted without using a graft (12 were available for study); 9 had a fascial graft, and 9 a Gelfoam® graft (7 were available for study). The implanted kittens had their bullae inoculated with Streptococcus pneumoniae 2 months after implantation and were sacrificed 1 week later. There were also 9 unimplanted control ears which were inoculated when the animals were 4 months of age. Labyrinthitis occurred in 44% of unimplanted control, 50% of implanted ungrafted, and 6% of implanted grafted (fascia and Gelfoam®) cochleas. There was no statistically significant difference between the unimplanted control and the implanted cochleas (p < 0.05). There was, however, a difference between the implanted-ungrafted and implanted grafted cochleas, but not between the use of fascia and Gelfoam® to graft the round window entry point. As a result, the data indicates that cochlear implantation does not increase the risk of labyrinthitis following pneumococcal otitis media, but it is desirable to use fascia as a graft to the round window around the electrode entry point.
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ItemEvaluation of leadwire fixation for paediatric cochlear implants [Abstract]( 1994)A paediatric cochlear implant should include a leadwire system that can readily expand in the presence of tissue adhesions and can be effectively fixed at a site close to the cochlea to ensure that the electrode array is not displaced during skull growth. In this study, leadwires were implanted in six young animals for a period of five months. During explantation, the mean force � standard deviation required to expand individual leadwire was found to be 12.5 � 5.0g. In order to evaluate the efficacy of leadwire fixation techniques, four fixation procedures were initially developed in human temporal bones and subsequently used to fix leadwires implanted in the temporal bones of eight animals for a period of four months. Leadwires were fixed by platinum wires at the fossa incudis or by platinum wires with a titanium barbed nail at the mastoid. The biomechanical evaluation revealed that the forces required to displace the leadwire from fixation points were 70.6 � 33.5g. Significantly, the forces required to withdraw a chronically implanted electrode array from an animal cochlea were 1.5 � 0.4g. The present results highlight the importance of an effective leadwire fixation technique for paediatric cochlear implants, particularly in preventing the displacement of an electrode array from the cochlea during skull growth.
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Item"Cochlear View" and its application in cochlear implant patients [Abstract]( 1994)Recent advances in multichannel intracochlear implantation have generated interests in correlating individual stimulating electrodes to pitch perception. An appropriate radiographic technique is required to precisely document the location of the implanted intracochlear electrode array. Anatomical studies, including the measurements of the temporal bone using high-resolution CT films and 3D reconstruction from the petrous bone sections, were conducted to define the spatial position of cochlea in the skull. Thus, a "Cochlear View" was designed and introduced for postoperative radiological evaluation of multichannel intracochlear implantation. In this paper, a detailed radiographic method and radiological interpretation of the "Cochlear View" are described. A plain radiograph of the "Cochlear View" was taken of 120 patients who had received the Nucleus multichannel implant. Studies have shown that a plain radiograph of the "Cochlear View" provides sufficient information to correctly evaluate the results of implantation, including the insertion depth and position of individual electrodes. It plays an important role in guiding the management of frequency mapping and acts as a useful reference for further research purposes.
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ItemPhysiological and histopathological response of the cochlea to chronic electrical stimulation of the auditory nerve at high stimulus rates [Abstract]( 1994)Previous research has shown that chronic electrical stimulation of the auditory nerve using charge balanced biphasic current pulses at rates of up to 500 pulses per second (pps) does not adversely affect the adjacent spiral ganglion population. More recently, a number of clinical trials have suggested that speech processing strategies based on high pulse rates (e.g. 1000 pps), can further improve speech perception. In the present study we evaluated the physiological and histopathological response of the cochlea following long-term stimulation using rates of 1000 pps. Thirteen normal hearing cats were bilaterally implanted with scala tympani electrodes and unilaterally stimulated using 25-50 �s per phase charge balanced biphasic current pulses presented at 1000 pps. Additional charge balance was achieved by shorting the electrodes between current pulses. Each animal was stimulated for periods ranging from 700 - 2100 hours at current levels within its dynamic range. Auditory brainstem responses to both acoustic (ABR) and electrical (EABR) stimuli were periodically recorded throughout the chronic stimulation program. At completion of the program the cochleas were prepared for histological examination. While all animals exhibited an increase in acoustic thresholds following surgery, click evoked ABR's returned to near normal levels in half the animals. Frequency specific stimuli indicated that the most extensive hearing loss occurred adjacent to the array (>12 kHz) while lower frequency thresholds appeared at or near normal Our EABR data showed that the majority of animals exhibited slight increases in threshold, although response amplitudes remained very stable for the duration of the stimulus program. The physiological data reported here will be correlated with cochlear histopathology. These initial findings suggest that chronic intracochlear electrical stimulation at high pulse rates, using a carefully designed charge balanced stimulator, does not appear to adversely affect the implanted cochlea.