Graeme Clark Collection
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ItemInner ear implants(Dekker, 2004)The cochlear implant is an electronic device that brings useful hearing to severely to profoundly deaf people through multiple-channel electrical stimulation of the auditory nerves in the inner ear. This is required if their inner ears are so badly damaged by injury and disease, or so inadequately developed, that they cannot provide sufficient hearing for communication, even when the sound is amplified with a hearing aid. By stimulating the nerve directly with patterns of electrical pulses, the implant bypasses the normal function of the sense organ of hearing in the inner ear to partially reproduce the coding of sound. It consists of a wearable speech processor that picks up sound with a microphone, analyzes the signal, and then sends it by radio waves to the implanted receiver stimulator, which decodes the message and stimulates the electrode wires inserted into the inner ear.
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ItemChronic electrical stimulation of the auditory nerve using non-charge-balanced stimuli( 1999)This study was designed to evaluate the pathophysiological response of the cochlea following long-term intracochlear electrical stimulation using a poorly charge-balanced stimulus regime, leading to direct current (DC) levels >0.1 µA. Four normal-hearing adult cats were bilaterally implanted with scala tympani electrode arrays and unilaterally stimulated for periods up to 2200 h. Stimuli consisted of 50 µs monophasic current pulses presented at 2000 pulses per second (pps) per channel, and resulted in DC levels of 0.4-2.8 µA. Both acoustic and electrical (EABR) evoked potentials were periodically recorded during the stimulation program. Frequency-specific stimuli indicated that an extensive and widespread hearing loss occurred over the 4-24 KHz region in all stimulated cochleae, although the 2 KHz region exhibited thresholds close to normal in some animals, despite long-term implantation and chronic stimulation. Longitudinal EABRs showed a statistically significant increase in threshold for three of the four animals. Histopathological evaluation of the cochleae revealed a highly significant reduction in ganglion cell density in stimulated cochleae compared with their controls. Spiral ganglion cell loss was significantly correlated with the degree of inflammation, duration of electrical stimulation, and the level of DC. In conclusion, the present study highlights the potential for neural damage following stimulation using poorly charge-balanced stimuli.
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ItemCochlear implants: future research directions( 1995)The future of cochlear implants for profoundly deaf people now seems assured, and further research should improve its benefits. The present benefits of cochlear implants have now been clearly demonstrated. The results have shown that many postlingually deaf adults get significant open-set speech recognition using electrical stimulation alone, and that profoundly deaf children with a cochlear implant get better speech perception than similar children who use hearing aids or tactile vocoders.
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ItemCochlear pathology following chronic electrical stimulation using non charge balanced stimuli( 1991)During the course of a chronic intracochlear electrical stimulation study using charge balanced biphasic current pulses, one animal inadvertently received a short period of direct current (DC) stimulation at a level of approximately 1 µA. Subsequent, the animal was chronically stimulated using a poorly charge balanced waveform that produced a DC level of approximately 2 µA. Extensive pathological changes were observed within the cochlea. These changes included widespread spiral ganglion cell loss and new bone growth that extended throughout all turns of the cochlea. Significant changes in the morphology of the electrically evoked auditory brainstem response (EABR) were associated with these pathological changes. EABRs recorded prior to the DC stimulation exhibited a normal waveform morphology. However, responses recorded during the course of the DC stimulation were dominated by a short latency response believed to be vestibular in origin. The response thresholds were also significantly higher than levels recorded before the DC stimulation. In contrast, the contralateral cochlea, stimulated using charge balanced stimuli, showed no evidence of adverse pathological changes. Furthermore, EABRs evoked from this cochlea remained stable throughout the chronic stimulation period. Although preliminary, the present results illustrate the adverse nature of poorly charge balanced electrical stimuli. These results have important implications for both the design of neural prostheses and the use of DC stimuli to suppress tinnitus in patients.
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ItemElectrical stimulation of the auditory nerve in deaf kittens: effects on cochlear nucleus morphology( 1991)The present study examines the effects of long-term electrical stimulation of the auditory nerve on the morphology of neurons in the cochlear nucleus in young, sensorineural deaf animals. Kittens, systemically deafened using kanamycin and ethacrynic acid, received bilateral cochlear implants and were stimulated unilaterally for periods of up to four months. After sacrifice, cross-sectional areas of neuron somata were measured with an image-analysis system and compared using nonparametric statistics. The areas of cell somata within the anteroventral cochlear nucleus (AVCN) on the stimulated side were significantly larger than those of corresponding somata on the control, unstimulated side (P < 0.001). However, there was no statistically significant difference among dorsal cochlear nucleus (DCN) neurons. These results indicate that long-term electrical stimulation of the auditory nerve can at least partially negate some effects of early postnatal auditory deprivation at the level of the cochlear nucleus.
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ItemAmplitude and pulse rate difference limens for electrical stimulation of the cochlea following graded degeneration of the auditory nerve( 1983)Experimentally deafened cats with differing populations of residual spiral ganglion cells were implanted with cochlear electrodes and were electrically stimulated. They were conditioned to respond to changes in electrical pulse rate amplitude, and both electrical pulse rate and amplitude difference limens were determined. It was found that although there were some variations in difference limens between animals, these showed no correlation with residual cell populations over the range 8-44%.
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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 effect of pulsatile intracochlear electrical stimulation on intracellularly recorded cochlear nucleus neurons(Monduzzi Editore, 1997)The anterior division of the ventral cochlear nucleus (AVCN) is the first relay station of the auditory pathway. We examined responses of neurons in the A VCN to intracochlear electrical stimulation using in vivo intracellular recordings. Twin pulse stimulation results indicated that these neurones evoke action potentials which are able to follow pulsatile stimulation at high rates. This ability to respond to each pulse along the stimulus train diminished when stimulus duration was increased to 50 ms. At rates 400 Hz and below in all neurones tested a deterministic response was seen to this longer duration pulsatile stimulation. With increasing rate of stimulation the response become more stochastic with apparent loss of encoding ability. These results have in1pIications in the clinical application of cochlear implants operating at high stimulus rates.
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ItemElectrical stimulus induced changes in excitability of the auditory nerve( 1997)High rate electrical stimulation of the auditory nerve using stimulus intensities well above the clinical limits can induce a significant reduction in the excitability of the auditory nerve as measured by a decrement in the amplitude of the electrically evoked auditory brainstem response (EABR). Two potential mechanisms may be associated with this stimulus induced reduction in activity: 1) stimulus induced prolonged neuronal hyperactivity; and 2) the generation of adverse electrochemical productions from the electrode surface. The purpose of the present study was to assess the extent to which adverse electrochemical damage contributes to the stimulus induced reduction in auditory nerve excitability. Twenty-six adult guinea pigs anaesthetized with ketamine (40 mg/kg i.p.) and xylazine (4 mglkg i.p.), were bilaterally implanted and unilaterally stimulated for two hours using a stimulus intensity of two or four times EABR threshold. Stimulus rates of 200, 400, or 1000 pulses/s (pps) were delivered via a standard platinum scala tympani electrode or large surface area ("high Q") platinum electrode.