Graeme Clark Collection
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ItemIs deep insertion of the cochlea implant electrode array necessary and possible?( 1995)Hyaluronate (Healon TM) appears to be useful in achieving deeper electrode insertions than are generally achieved at present. However, biosafety studies are still being conducted. If found to be a safe technique, further work to explore different electrical stimulation strategies and speech processing schemes will also need to be undertaken. This work is ongoing.
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ItemCurrent distributions of intracochlear electrodes in cats [Abstract](Monash University Press, 1983)A non-invasive technique, "two electrode mapping" has been developed to measure the current distribution of a multichannel electrode array within the feline scala tympani. An electrode’s current distribution is an important determinant of its ability to excite discrete neural groups. The electrode array, a “banded electrode” is a series of platinum rings, supported by a cylinder of silastic which fits freely into the scala tympani. All cats had normal hearing pre-operatively and were implanted through the round window. Hearing was normal within 10 dB after implantation, necessitating the presentation of 30-40 dB white noise to the implanted ear throughout experimentation to mask the electrophonic component (Black et al., 1983).
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ItemClinical results with a multichannel pseudobipolar system( 1983)A receiver-stimulator and multiple-electrode array were implanted in the scala tympani of a 46-year-old totally deaf man on August 1, 1978. This patient had lost all hearing after a head injury 18 months prior to surgery. Pure-tone Bekesy and speech audiometry showed no hearing in either ear at the maximum output levels of the audiometer, and no vibrotactile responses were elicited. A second patient, a 63-year-old man, received an implant on July 17, 1979. This patient had a progressive sensorineural hearing loss extending over 30 years due to bomb blast and chronic infection. He had no help from a hearing aid for 13 years prior to surgery. Pure-tone Bekesy audiometry under headphones revealed no hearing in the left ear, and in the right he had thresholds of 125 dB SPL at 0.125 kHz; 115 dB at 0.25 kHz, and 117 dB at 0.5 kHz. There was no speech discrimination in either ear under headphones or in a monitored sound field. His right ear was fitted with a hearing aid (Calaid G12G) by the National Acoustics Laboratory, but at its maximum output level he received only a limited gain at 5.0 kHz and could not be aided. (From Introduction)
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ItemDesign and fabrication of the banded electrode array( 1983)A multiple-channel electrode array must meet certain design requirements; these are listed in TABLE 1. First, there should be no trauma associated with the surgical insertion, and if there is a need to replace the array, this procedure should also be atraumatic. Second, it should be biologically inert. This means that it should be biocompatible with the tissues. When placed in the cochlea, the array should also not predispose the patient to local infection, and this is particularly important in children, in whom recurrent middle ear infections could spread to the inner ear. There should also be no risk of carcinogenicity with long-term implantation. Third, the electrode array should be designed so that the stimulus current can be localized to discrete groups of nerve fibers, and it should also be possible to stimulate as many groups as possible from the total remaining nerve population. Fourth, with long-term stimulation, there should be no significant corrosion of the electrodes used, and the electrical stimulation should not lead to damage of the tissues in the cochlea, especially the residual nerve fibers. Fifth, the electrode array should be mechanically robust and stable. It should not be prone to break as a result of repeated stress by the acceleration of the head during everyday movements. The array should also be capable of being fixed in place so that it will not shift its position. Sixth, it is desirable that the means of fabrication of the multiple-channel array should be simple and inexpensive. (From Introduction)
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ItemInsertion study using new peri-modiolar electrode array designs [Abstract]( 1999)Intracochlear multi-channel cochlear implants have been shown to successfully provide auditory information for profoundly deaf patients by electrically stimulating discrete populations of auditory nerve fibers via a scala tympani (ST) electrode array. Histological and radiological examination of implanted human temporal bones showed that the current straight Nucleus® array is usually positioned against the outer wall of the ST. An electrode array close to the modiolus could be expected to reduce stimulation thresholds and result in a more localized neural excitation pattern.
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ItemThe development of a precurved cochlear implant electrode array and its preliminary psychophysical evaluation [Abstract]( 1998)A precurved banded electrode array may provide a better interface with the auditory neural pathways for cochlear implants, and provide better speech perception. A prototype arrray has been further evaluated for ease of insertion, siting within the cochlea and induction of any cochlear trauma. The arrays were inserted into the human cochlear under simulated surgery. The bones were embedded in Araldite, X-rayed and sectioned. X-ray reconstruction analyses of the position of the implanted array showed its insertion to be favourable. Cochlear implants with precurved arrays have been implanted in three patients. Psychophysical evaluation and X-ray analyses have shown that as electrode distance from the modiolus decreased: threshold current decreased; dynamic range increased; current spread as measured by forward masking studies, was more focused; electrode discrimination with loudness jitter (being abetter representation of the dynamic speech signal) improved; JNDs for loudness, expressed as a function of dynamic range, decreased.
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ItemHistopathology following electrode insertion and chronic electrical stimulation(Raven Press, 1985)We have examined a number of safety issues associated with cochlear implants. This work has been primarily designed to evaluate the histopathological effects of intracochlear electrode implantation and chronic electrical stimulation. The results of these studies may be summarized as follows: 1) The insertion of the banded free-fit scala tympani array into human cadaver temporal bones produces minimal damage, occurring primarily to a localized region of the spiral ligament. This damage would not result in significant neural degeneration and thus, would not compromise the efficacy of the multiple channel device; 2) chronic intracochlear electrical stimulation for continuous periods of 500 to 2000 hours, using charge balanced biphasic current pulses developing charge densities of 18-32 }?C/cm2. geom./phase, does not adversely affect the spiral ganglion cell population; 3) labyrinthine infection severely reduces the viable spiral ganglion cell population; 4) the formation of new bone present in approximately half of the animals we have implanted --is not associated with electrical stimulation per se; 5) scanning electron microscope studies of electrodes subjected to long periods of intracochlear electrical stimulation reveals minimal platinum dissolution when compared with unstimulated control electrodes, and electrodes that have been stimulated for similar periods in inorganic saline.
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ItemDistribution of electrically stimulated nerve fibres in the cat cochlea( 1982)An implant electrode array for a cochlear hearing prosthesis has been developed with mechanical properties which allow atraumatic implantation into the human scala tympani. It consists of small platinum electrode bands welded around a flexible silicon rubber tube (Clark et al, 1979). The present study examines the properites of this electrode in electrically simulating the auidtory nerve. The electrode was inserted through the round window for a distance of 5-6mm into the scala tympani of the cat. Brainstem evoked responses and those from the round window were recorded when stimulating with square biphasic current pulses (0.1 msec/phase). Since there was usually less than 10-20 dB hearing loss in the implanted ear, it was possible to selectively mask components of these responses with high-pass filtered noise. The noise masked the response component arising from fibres in the cochlear region corresponding to the noise band. Responses were recorded in the presence of noise with different cut-off frequencies F1, F2 therefore yielded a response band-limited to the region f1-2. In this way it was possible to measure the amount of electrically stimulated activity in a number of different frequency bands. This technique is identical to that of derived response audiometry using acoustic stimulation. The input-output characteristics of the cochlea to a variety of acoustic transients were measured to exclude the possibility of either electrophonic hearing or altered basilar membrane characteristics contaminating the results. The figure shows the distributions of excited fibres using an electrode with an extended ground system running longitudinally in the cochlea. They were measure as the amplitude of the band-limited responses. Results were similar for bipolar electrodes and these electrodes are thus equally suitable for our present cochlear implant prosthesis.