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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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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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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ItemThe pitch of amplitude-modulated electrical stimuli in cochlear implantees [Abstract]( 1993)The ability of cochlear implantees to detect amplitude modulation of pulsatile electrical stimulation, suggests that some speech feature information may be conveyed effectively by this means. For example, modulations at the fundamental frequency of speech may provide a voice pitch percept to implantees, particularly in speech processing strategies which generate constant-rate stimulation. The pitch evoked by sinusoidally modulated current pulse trains on a single electrodes has been studied. Modulation frequencies of 100, 150 and 200Hz, and carrier pulse rates varying from 200 to 1200Hz, were used. The results showed that the pitch of the stimulation was related to the modulation frequency, provided that either the carrier rate was a multiple of the modulation frequency, or the carrier rate was sufficiently high (at least four times the modulation frequency for the stimuli studied here). Furthermore, when the modulated stimuli were matched in pitch to non-modulated pulse trains, it was. found that the rate of the matched non-modulated stimuli was close to but somewhat higher than the modulation frequency. This difference depended on the carrier rate and varied among subjects.