Graeme Clark Collection
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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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ItemRehabilitation for multiple-channel cochlear prosthesis patients(Raven Press, 1985)The postoperative program for multiple-channel cochlear prosthesis patients can be divided into four main areas: 1) Psychophysical evaluation is carried out to optimize the patient's speech processor for their individual needs, Measurements required include threshold, dynamic range and pitch ranking for each of the implanted electrodes. 2) Counselling is very important to ensure patients are able to effectively operate their speech processor and that they are aware of factors in the environment which may affect performance (e.g., noise). 3) Auditory training and training in conjunction with lipreading for tasks grading from simple (e.g., discrimination of word length) to more difficult (e.g. consonant discrimination) is of benefit in making patients aware of their capabilities with the prosthesis and helping them to improve communication skills. However, highly specific training (e.g., closed set vocabulary) does not seem to be of general benefit to patients. Speech tracking provides a training procedure relevant to normal communication but has some limitations due to the degree of familiarity reached with a particular speaker. 4) Assessment of patients is carried out at this stage in great detail as it is necessary to collect data about the effectiveness of cochlear implants and also to provide information about possible improvements to speech processing strategies and external hardware. The amount of assessment required will decline as the procedure becomes established, but some investigation will continue to be necessary.
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ItemPsychophysics of multiple-channel stimulation(Raven Press, 1985)Eight patients implanted with multiple-channel cochlear prostheses have displayed good discrimination of sound sensations elicited at different sites within the cochlea. All patients rank the sensations from "sharp" to "dull" in an order which corresponds with basal to apical position in the cochlea. Detailed psychophysical studies have been carried out on two patients. These showed that discrimination of rate of (pulsatile) stimulation is good for frequencies up to 300 Hz and falls off sharply for frequencies above this. Electrode transitions (changes in position along the cochlea) are well discriminated for fast changes (25 msec), whereas rate transitions are not well discriminated for changes faster than 100 msec. From these results a speech processing strategy was formulated where second formant information is mapped to position in the cochlea and fundamental frequency mapped to rate of stimulation. Vowel and consonant confusion studies show consistent results for all patients using this processing strategy. A study involving two electrode stimuli demonstrated the possibility of presenting first formant information in addition to the second formant and fundamental frequency.
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ItemPatient results for a multiple-channel cochlear prosthesis(Raven Press, 1985)Six patients implanted with multiple-channel cochlear prostheses and using take-home, wearable speech processors, were assessed three months postoperatively using the Minimal Auditory Capabilities (MAC) battery. Results showed statistically significant improvement on virtually all tests over their preoperative performance with a hearing aid. Four patients showed significant results for open set speech testing. Lipreading tests, using word and sentence material, showed significant improvement for all patients when the cochlear prosthesis was used with lipreading compared to lipreading alone. All the above tests were carried out without training with recorded material of an unfamiliar speaker. Improvements in communication speed of 55% to 126% over lipreading alone were obtained for the six patients as assessed by the speech tracking procedure. These results are for scores averaged over eight sessions of tracking with the two conditions (with and without cochlear prosthesis). The order of conditions was alternated at each session to control practice effects. The wearable speech processor is used all day every day by five patients, and four hours a day by one patient. Reported benefit is not only for communication but also for the recognition of environmental sounds. Four patients have attempted using the telephone with some success in a restricted context situation. One patient uses the telephone routinely without using any special coding strategies. Reported problems with the cochlear prosthesis are primarily related to background noise. Results for these six patients are consistent with those obtained for two patients implanted with a prototype multiple-channel prosthesis in 1978-1979.
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ItemSelection of patients for multiple-channel cochlear implantation(Raven Press, 1985)Only profoundly, bilaterally deaf adults are considered for evaluation. It is necessary to determine that the patient's communication ability cannot be improved to any significant degree with conventional hearing aids currently available. Initial assessment consists of audiometry, hearing aid evaluation(s), otological and medical examination, and for patients with no recent experience with hearing aids, a hearing aid trial. Polytome x-rays of temporal bones is carried out to ensure that cochlea structures are not grossly abnormal. Electrical stimulation of the promontory is used to confirm the presence of residual auditory nerve fibers. Where there is an audiometric difference between ears, the poorer ear is chosen for implantation provided there are no other contraindications. Intensive counselling is carried out to enable patients to make a fully informed decision about implantation. Patients undergo a battery of speech discrimination and lipreading tests with their hearing aid after their hearing aid trial. This is to provide a baseline for comparison with postoperative results and to assess the benefit obtained from the hearing aid. Any significant improvement in test results when using a hearing aid over lipreading alone would be a contraindication for implantation. Medical assessment is carried out as for any major surgery, including pathology, respiratory function tests and cardiovascular assessment. Particular emphasis is placed on infection prevention immediately preoperatively and during surgery.