Graeme Clark Collection
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ItemHearing restoration with the multichannel auditory brainstem implant( 1997)Restoration of useful hearing is now possible in patients with bilateral acoustic neuromas by direct electrical stimulation of the cochlear nucleus. Our first experience with the Multichannel Auditory Brainstem Implant is reported. A forty four year old female with bilateral acoustic neuromas and a strong family history of Neurofibromatosis Type II presented with profound bilateral hearing impairment. Translabyrinthine removal of the right tumour was performed with placement of the Nucleus eight electrode Auditory Brainstem Implant. Intraoperative electrically evoked auditory brainstem response monitoring successfully confirmed placement over the cochlear nucleus. Postoperatively, auditory responses were obtained on stimulation of all electrodes with minimal non-auditory sensations. The patient now receives useful auditory sensations using the "SPEAK" speech processing strategy. Auditory brainstem Implantation should be considered for patients with Neurofibromatosis Type II in whom hearing preservation tumour removal is not possible.
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ItemChronic electrical stimulation of the auditory nerve at high stimulus rates: a physiological and histopathological study( 1997)A major factor associated with recent improvements in the clinical performance of cochlear implant patients has been the development of speech-processing strategies based on high stimulation rates. While these processing strategies show clear clinical advantage, we know little of their long-term safety implications. The present study was designed to evaluate the physiological and histopathological effects of long-term intracochlear electrical stimulation using these high rates. Thirteen normal-hearing adult cats were bilaterally implanted with scala tympani electrode arrays and unilaterally stimulated for periods of up to 2100 h using either two pairs of bipolar or three monopolar stimulating electrodes. Stimuli consisted of short duration (25-50 µs/phase) charge-balanced biphasic current pulses presented at 1000 pulses per second (pps) per channel for monopolar stimulation, and 2000 pps/channel for bipolar stimulation. The electrodes were shorted between current pulses to minimize any residual direct current, and the pulse trains were presented using a 50% duty cycle (500 ms on; 500 ms oft) in order to simulate speech. Both acoustic (ABR) and electrical (EABR) auditory brainstem responses were recorded periodically during the chronic stimulation program, All cochleas showed an increase in the click-evoked ABR threshold following implant surgery; however, recovery to near-normal levels occurred in approximately half of the stimulated cochleas 1 month post-operatively. The use of frequency-specific stimuli indicated that the most extensive hearing loss generally occurred in the high-frequency basal region of the cochlea (12 and 24 kHz) adjacent to the stimulating electrode. However, thresholds at lower frequencies (2, 4 and 8 kHz), appeared at near-normal levels despite long-term electrode implantation and electrical stimulation. Our longitudinal EABR results showed a statistically significant increase in threshold in nearly 40% of the chronically stimulated electrodes evaluated; however, the gradient of the EABR input/output (I/O) function (evoked potential response amplitude versus stimulus current) generally remained quite stable throughout the chronic stimulation period. Histopathological examination of the cochleas showed no statistically significant difference in ganglion cell densities between cochleas using monopolar and bipolar electrode configurations (P = 0.67), and no evidence of cochlear damage caused by high-rate electrical stimulation when compared with control cochleas. Indeed, there was no statistically significant relationship between spiral ganglion cell density and electrical stimulation (P = 0.459), or between the extent of loss of inner (IHC, P = 0.86) or outer (OHC, P=0.30) hair cells and electrical stimulation. Spiral ganglion cell loss was, however, influenced by the degree of inflammation (P=0.016) and electrode insertion trauma. These histopathological findings were consistent with the physiological data. Finally, electrode impedance, measured at completion of the chronic stimulation program, showed close correlation with the degree of tissue response adjacent to the electrode array. These results indicated that chronic intracochlear electrical stimulation, using carefully controlled charge-balanced biphasic current pulses at stimulus rates of up to 2000 pps/channel, does not appear to adversely affect residual auditory nerve elements or the cochlea in general. This study provides an important basis for the safe application of improved speech-processing strategies based on high-rate electrical stimulation.
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ItemAcoustic and electric forward-masking of the auditory nerve compound action potential: evidence for linearity of electro-mechanical transduction( 1997)We investigated electro-mechanical transduction within the cochlea by comparing masking of the auditory nerve compound action potential (CAP) by acoustical and electrical maskers. Forward-masking of the CAP reflects the response to the masker of the cochlear location tuned to the probe. Electrical stimulation was delivered through bipolar stimulating electrodes within the basal turn of the scala tympani. The growth of masking of high-frequency probes which excite cochlear locations close to the stimulating electrodes was similar for both acoustic and electrical maskers, suggesting a linear transduction of electrical energy to mechanical energy. Exposure to intense acoustic stimulation caused an equal loss of sensitivity to acoustic and electrical maskers. Masking of lower-frequency probes by electrical maskers increased rapidly with masker current, suggesting the direct electrical stimulation of neural elements. This masking was reduced by the administration of strychnine suggesting a contribution by the efferents towards masking of these low-frequency probes.
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ItemHistological and physiological effects of the central auditory prosthesis: surface versus penetrating electrodes( 1997)Unavailable due to copyright.
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ItemElectrical stimulation of the auditory nerve with a cochlear implant and the temporal coding of sound frequencies: a brief review( 1997)There is considerable evidence that the brain translates (encodes) the frequency of a sound into both place of excitation (place encoding), and the pattern of intervals between action potentials (temporal encoding). Furthermore, temporal encoding is now thought to be due to a temporal as well as spatial pattern of action potentials in a small group of neurons. This pattern needs to be reproduced with a cochlear implant for improved speech processing. Our recent research has also demonstrated that the timing of excitatory postsynaptic potentials seen with intracellular recordings from brain cells, rather than extracellularly recorded action potentials, correlates better with the frequency of sound. These excitatory postsynaptic potentials are likely to be the link between the patterns of action potentials arriving at nerve cells and the biomolecular activity in the cell. This response also needs to be replicated with improved speech processing strategies.
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ItemIntracellular responses of anteroventral cochlear nucleus neurones to intracochlear electrical stimulation in the rat [Abstract]( 1996)The anterior division of the ventral cochlear nucleus (AVCN) is the first relay station of the auditory pathway. Currently little is known about the intracellular physiological responses of neurones in the AVCN to electrical stimulation of the cochlea. We investigated the effect of cochlear electrical stimulation in the rat AVCN using in vivo intracellular recordings. Male rats were anaesthetised with urethane (1.3g/kg i.p), placed in a stereotaxic frame, the crania and dura removed and the cochlear nucleus exposed.
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ItemElectrophonically driven single unit responses of the anteroventral cochlear nucleus in cat [Abstract]( 1996)Electrical stimulation of the cochlea results in both direct and electrophonic excitation of auditory nerve fibres. It has been proposed that electrophonic stimulation results from the creation of a mechanical disturbance on the basilar membrane which has properties similar those resulting from acoustic stimuli. Auditory nerve compound action potential (CAP) forward masking studies1 show the level of frequency specific electrophonic stimulation is highly correlated with the spectral energy of the electrical stimulus waveform. The level of spectral energy in pulsatile biphasic electrical stimuli decreases toward low frequencies suggesting the level of electrophonic stimulation will be diminished in the low frequency region of the cochlea.
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ItemControl strategies for neurons modeled by self-exciting point processes( 1996)Abstract not available due to copyright.
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ItemReduction in excitability of the auditory nerve in guinea pigs following acute high rate electrical stimulation [Abstract]( 1996)Electrical stimulation of neural tissue involves the transfer of charge to tissue via electrodes. Safe charge transfer can be achieved using biphasic current pulses designed to reduce the generation of direct current (DC) or the production of electrochemical products. However, neural stimulators must also use capacitors in series with electrodes, or electrode shorting between current pulses, to further minimize DC due to electrode polarization.
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ItemElectrical stimulation of the auditory nerve: the coding of frequency, the perception of pitch and the development of cochlear implant speech processing strategies for profoundly deaf people( 1996)1. The development of speech processing strategies for multiple-channel cochlear implants has depended on encoding sound frequencies and intensities as temporal and spatial patterns of electrical stimulation of the auditory nerve fibres so that speech information of most importance for intelligibility could be transmitted. 2. Initial physiological studies showed that rate encoding of electrical stimulation above 200 pulses/s could not reproduce the normal response patterns in auditory neurons for acoustic stimulation in the speech frequency range above 200 Hz and suggested that place coding was appropriate for the higher frequencies. 3. Rate difference limens in the experimental animal were only similar to those for sound up to 200 Hz. 4. Rate difference limens in implant patients were similar to those obtained in the experimental animal. 5. Satisfactory rate discrimination could be made for durations of 50 and 100 ms, but not 25 ms. This made rate suitable for encoding longer duration suprasegmental speech information, but not segmental information, such as consonants. The rate of stimulation could also be perceived as pitch, discriminated at different electrode sites along the cochlea and discriminated for stimuli across electrodes. 6. Place pitch could be scaled according to the site of stimulation in the cochlea so that a frequency scale was preserved and it also had a different quality from rate pitch and was described as tonality. Place pitch could also be discriminated for the shorter durations (25 ms) required for identifying consonants. 8. As additional speech frequencies have been encoded as place of stimulation, the mean speech perception scores have continued to increase and are now better than the average scores that severely-profoundly deaf adults and children with some residual hearing obtain with a hearing aid.