Graeme Clark Collection
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ItemTraining place pitch perception in cochlear implant users [Abstract]( 1997)The study has aimed at determining whether the ability to use place coded vowel formant information could be enhanced with analytical vowel training in a group of -congenitally deafened patients, who showed limited speech perception skills after cochlear implant experience ranging from 1y8m to 6y11m. It has investigated whether improvements in vowel perception after training can carry over to word recognition. A further objective was to see whether poorer vowel perception was associated with poorer electrode position difference limens. Three children, one adolescent and one young adult were assessed with synthesized versions of the words /hid, head, had, hud, hod, hood/ and a natural version of these words as well as with a closed-set monosyllabic word task. The change in performance after 10 training sessions was compared to the change in performance during a non-training period. Four of the five patients showed a significant gain in synthetic vowel perception post-training on at least one assessment, but only two patients showed gains across a number of tests post-training. For one of these 2 children improvements in vowel perception generalized to word perception. Patients’ electrode limens ranged from 1 to 3 electrodes except for 1 adolescent whose minimal progress post-training could be partly explained by poorer apical electrode discrimination. The findings are discussed with reference to a number of factors, including the notion of a "critical period" for neural plasticity.
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ItemChronic electrical stimulation of the auditory nerve at high rates: I. Effect on residual hearing [Abstract]( 1996)In addition to direct excitation of auditory nerve fibres, cochlear implant patients with small amounts of residual hearing may receive important additional auditory cues via electrophonic activation of hair cells 1. Before incorporating electrophonic hearing into speech processing strategies, the extent of hair cell survival following cochlear implantation must first be determined. We have recently demonstrated widespread survival of hair cells apical to electrode arrays implanted for periods of up to three years, the present report describes the effects of chronic electrical stimulation on hair cell survival.
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ItemChronic electrical stimulation of the auditory nerve at high rates: II. Cochlear pathophysiology [Abstract]( 1996)A major factor in the improved performance of cochlear implant patients has been the use of high stimulus rate speech processing strategies. While these strategies show clear clinical advantage, we know little of their long-term safety. Indeed, recent studies have indicated that high stimulus rates at intensities above clinical limits, can result in neural damage as a result of prolonged neuronal hyperactivity. The present study was designed to evaluate the effects of chronic electrical stimulation of the auditory nerve at high rates, using intensities within clinical limits.
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ItemIntrinsic connections of the rat cochlear nucleus [Abstract]( 1996)In mammals three subdivisions of the cochlear nucleus can be distinguished: the dorsal (DCN), the posterior (PVCN) and the anteroventral (AVCN) cochlear nucleus (CN) I. The intrinsic connections between and within these areas have not been well defined. Wickesberg et al.2 revealed that projections from DCN to AVCN in the mouse are frequency specific and tonotopic. In contrast Synder and Leake in the cat revealed projections from AVCN to PVCN and DCN but only modest projections from PVCN and DCN to AVCN with no frequency specificity observed. These previous studies utilized the retrograde tracer horseradish peroxidase. We investigated this apparent contradiction further, using neurobiotin, a retrograde and anterograde tracer, to examine connections within the cochlear nucleus of the rat, with emphasis on the AVCN subdivision.
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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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ItemAcute study on the neuronal excitability of the cochlear nucleus of guinea pig following electrical stimulation [Abstract]( 1996)To help deaf patients who cannot benefit from the cochlear implant due to interruption of the auditory nerve, a central auditory prosthesis has been developed to directly stimulate the cochlear nucleus in the brainstem. To examine the safety of this prosthesis guinea pig cochlear nuclei were stimulated acutely with bipolar surface electrodes using charge balanced biphasic current pulses at rates of 250, 500 or 1000 Hz and charge intensities of 1.8, 2.8, 3.5 or 7.1 μC/phase cm[to the power of]-2. The electrically evoked auditory brainstem response (EABR) was used to monitor neuronal excitability of the cochlear nuclei following this acute six hours electrical stimulation.
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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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ItemComparison and alternate designs for peri-modiolar electrode arrays: insertion trauma and position [Abstract]( 1997)While it has been shown that the straight but flexible banded electrode array can be safely inserted into the scala tympani of the human cochlea, histological studies have revealed that the array lies along the outer walll.2. Since a profound total hearing loss is generally associated with a moderate to complete degeneration of the spiral ganglion peripheral process, these electrodes lie some distance from their target neural population -the spiral ganglion soma -located within Rosenthal's canal.
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ItemThe use of click-ABR and steady state evoked potentials for hearing assessment in young cochlear implant candidates [Abstract]( 1997)The accurate assessment of hearing thresholds in prospective cochlear implant candidates is essential. As the minimum age of implantation has reduced, audiologists have been faced with the complicated task of obtaining precise audiometric information in children whose immaturity may severely restrict the assessment process. Clearly for these young candidates, there is a place for a reliable, objective measure of residual hearing in the pre-operative test battery. This paper examines the degree of accuracy with which the click-ABR and the steady-state evoked potential (SSEP) techniques can provide estimates of hearing level in subjects with several profound hearing loss.