Graeme Clark Collection
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ItemSpatial representation of the cochlea within the inferior colliculus of neonatally deafened kittens following chronic electrical stimulation of the auditory nerve [Abstract]( 1995)The orderly tonotopic representation of the cochlea is accurately reproduced within the central auditory system of normal hearing animals. Any degradation of this representation as a result of a neonatal hearing loss or chronic electrical stimulation during development could have important implications for the use of multichannel cochlear implants in young children. In the present study we have used 2-deoxyglucose autoradiography (2-00) to examine the topographic representation of the cochlea within the inferior colliculus (IC) of neonatally deafened kittens following periods of chronic intracochlear electrical stimulation.
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ItemElectrical stimulation of residual hearing in the implanted cochlea( 1995)The average profoundly deaf person using a cochlear implant can now understand more speech than some severely to profoundly deaf people who use a hearing aid. For this reason there will be an increasing need to consider implanting people with residual hearing. In many of these people there could be significant hearing in the operated ear, as a majority of severely to profoundly deaf people are likely to have a symmetrical hearing loss. When three frequency average hearing thresholds were measured on 219 pensioners from the Australian National Acoustic Laboratories (H. Dillon, unpublished findings), 64% had less than a 10-dB difference between thresholds in each ear.
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ItemCochlear implantation: osteoneogenesis, electrode-tissue impedance, and residual hearing( 1995)This study was undertaken to find out how new bone is produced in the implanted cochlea, and the effects of fibrous tissue and new bone growth on electrode-tissue impedance. This knowledge is essential, as bone and fibrous tissue in the cochlea could account for variations in patients' speech perception performance. The study was also carried out to examine the effects of implantation on residual hearing. This information is also important, as cochlear implant speech perception results in profoundly deaf people are now better on average than severely or profoundly deaf people obtain with a hearing aid. Consequently, more people will need to be considered for cochlear implantation in ears with some residual hearing. In this case we need to know to what extent residual hearing is affected by implantation. (From Introduction)
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ItemTemporal coding of frequency: neuron firing probabilities for acoustic and electric stimulation of the auditory nerve( 1995)A better understanding of the temporal coding of frequency, and its application to electrical stimulation of auditory nerve fibers, should lead to advances in cochlear implant speech processing. Past research studies have suggested that the intervals between nerve action potentials are important in the temporal coding of frequency. For sound frequencies up to approximately 500 Hz, the shortest or predominant intervals between the nerve action potentials are usually the same as the periods of the sound waves. The intervals between each nerve action potential can be plotted as an interval histogram. Although there is evidence that the intervals between spikes are important in the temporal coding of frequency, it is not known up to what frequency this applies. It is also not known whether the information transmitted along individual fibers or an ensemble of fibers is important, to what extent the coding of frequency is interrelated with the coding of intensity, the relative importance of temporal and place coding for different frequencies, and finally, how well electrical stimulation can simulate the temporal coding of sound.
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ItemA physiological investigation of chronic electrical stimulation with scala tympani electrodes in kittens( 1992)A physiological investigation of cochlear electrical stimulation was undertaken in six two-month-old kittens. The scala tympani electrodes were implanted and electrically stimulated using biphasic balanced electrical pulses' for periods of 1000-1500h in four ears. Four ears received implants for same period but without electrical stimulation. The other two ears served as normal control. The results indicated: 1) Chronic electrical stimulation of the cochlea within electrochemically safe limits did not influence the hearing of kittens and the normal delivery of impulses evoked by acoustic and electrical signals on the auditory brainstem pathway. 2) The wave shapes of EABRs were similar to those of ABRs. The aptitudes of EABRs showed a significant increase following chronic electrical stimulation, resulting in a leftward shift in the input/ output function. The absolute latencies and interwave latencies of waves II-III , III -IV and II -IV were significantly shorter than those of ABRs. These results imply that there was no adverse effect of chronic electrical stimulation on the maturing auditory systems of kittens using these electrical parameters and the mechanism of electrical hearing should be further studied.
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ItemExperimental study on extracochlear electric stimulation [Abstract]( 1992)The efficiency and feasibility of chronic extracochlear implantation and electric stimulation were studied in two adult cats and four 2-month kittens. The first electrode was placed on the round window by fixing the leadwire on the bridge of aditus between the middle ear and bulla cavity; the second electrode was placed on the surface of the tympanic promontory; the third was inserted into the temporal muscle out of the bulla and the forth fixed in transverse sinus with dental cement. ABRs and EABRs were recorded pre-and postoperatively and during electric stimulation.
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ItemDecrement in auditory nerve function following acute high rate stimulation in guinea pigs [Abstract]( 1995)Cochlear implants have been shown to successfully provide profoundly deaf patients with auditory cues for speech discrimination. Psychophysical studies suggested that speech processing strategies based on stimulus rates of up to 1000 pulses per second (pps) may lead to an improvement in speech perception, due to a better representation of the rapid variations in the amplitude of speech. However, "neural fatigue" has been known to occur following brief periods of electrical stimulation at rates high enough to ensure that stimuli occur within the neurons relative refractory period, and has been shown to depend on stimulus duration and rate of the evoked neural activity. Prolonged electrical stimulation at these high stimulus rates could, therefore, have an adverse effect on the neurons metabolism and result in cellular energy depletion.
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ItemThe auditory cortex and auditory deprivation: experience with cochlear implants in the congenitally deaf [Abstract]( 1995)The primary auditory cortex (AI) exhibits a topographic representation of the organ of Corti in normal hearing animals. Plasticity studies have shown that this orderly representation of frequency can be modified following a restricted hearing loss or by behavioural trainingl,2. Little is known, however, of the effects of a profound hearing loss on AI, although a number of early studies have suggested an enhancement of activity from other modalities3. Knowledge of the functional status of the central auditory pathway in the profoundly deaf, and the ability of these structures to undergo reorganization particularly following long periods of auditory deprivation - are important issues for the clinical management of cochlear implant patients. In this paper we review our recent clinical and experimental experience with cochlear implants in the congenitally deaf.
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ItemOnset of ototoxicity in the cat is related to the onset of auditory function [Abstract]( 1993)Cats are altricious mammals, they are born deaf and undergo rapid maturation of the auditory system within the first three weeks of life. In the present report we describe the use of ototoxic drugs to deafen kittens early in their auditory development as part of a separate study that required deaf animal models without prior auditory experience. Nine healthy kittens were deafened by the co-adminislr3tion of kanamycin (300 mg/kg) and ethacrynic acid (25 mg/kg) at a dose level sufficient to profoundly deafen adult cal, (I). Their ages varied from six to 16 days after birth (DAB). All animals made an uneventful recovery and were returned to their mothers within three hours of the procedure. At 26 DAB each animal was anaesthetized with ketamine (20 mg/kg) and xylazine (3.8 mg/kg), and auditory brainstem responses to both click and tone pip stimuli were recorded. Two kittens deafened at 6 DAB exhibited a 60 - 70 dB hearing loss at high frequencies (24, 12 & 8 kHz), while for frequencies at and below 4 kHz thresholds were similar to those in control animals. In contrast, the remaining seven animals, deafened at 8 DAB or older, exhibited a profound hearing loss (i.e. no response to a click-evoked ABR at an intensity of 92 dB p.e. SPL). While these findings require confirmation from additional animals, they imply that the onset of ototoxicity is related to the onset of auditory function, as both single unit and autoradiographic studies have indicated that the onset of auditory function in the kitten is first observed at 7 DAB (2 & 3). Moreover, maturation of the mammalian cochlea occurs initially in the base and proceeds in an apical direction (4). Such a maturation profile may explain the high frequency hearing loss observed in the two 6 DAB animals. While the mechanism responsible for these age dependent changes in ototoxic susceptibility remains unknown, it is possible that the immature hair cell has not developed the metabolically active transport system thought to be responsible for the uptake of aminoglycosides in mature hair cells (5).
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ItemCochlear implantation in young children: studies on head growth, leadwire design and electrode fixation in the monkey model [Abstract]( 1992)For the safety of cochlear implantation in children under two, the implant assembly must not adversely effect the tissue of compromise head growth. Furthermore, growth changes and tissue responses should not impair functioning of the device. Dummy receiver-stimulators, interconnect plugs and leadwire-lengthening systems have been implanted for periods of 40 months in the young monkey to most effectively model the implantation of the young human child. The results show that implanting a receiver-stimulator package has no effect on skull growth or brain tissue under the package. The system for fixing the electrode at the fossa includes proved effective. There was marked osteoneogenesis in the mastoid cavity and this also resulted in fixation of the leadwire outside the cochlea. This study provides evidence for the safety of cochlear implantation in young children.