Graeme Clark Collection
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ItemPercepts produced by electrical stimulation of the human cochlea [Abstract]( 1982)Electrical stimulation of the residual auditory nerve fibres in a postlingually deaf patient was effected by ten electrodes implanted 1.5 mm apart in the scala tympani. Biphasic current pulses with each phase fixed at 180 µs were used. Psychophysical results obtained by activating one electrode at a time showed the following characteristics: (a) loudness was found to increase with both current level and repetition rate; (b) pitch increased with repetition rate; (c) pitch and sharpness increased in the apical to basal direction in accordance with the tonotopic organisation of the cochlea; (d) dissimilarity measures obtained by triadic comparisons provided evidence that the sensations produced by repetition rate and electrode position are perceptually separable; (e) for short-duration stimuli the discrimination performance for electrode trajectories was much better than for repetition rate trajectories. For simultaneous activation of two electrodes, triadic comparisons showed that two perceptual components, one related to the more basal electrode and the other to the more apical one, could be discerned.
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ItemChronic electrical stimulation of the auditory nerve in cats( 1982)One requirement for the success of a cochlear hearing prosthesis is that long-term electrical stimulation must not have adverse effects on the residual spiral ganglion cell population. Electrochemically 'safe' stimulation regimes have been defined for the cortex (Brummer &Turner, 1977). However, few investigators have examined the effects of long-term intracochlear electrical stimulation. Walsh et al (1980), stimulating with current densities greater than the 'safe' limits defined by Brummer &Turner (1977), for periods of up to 800 hours at current levels of 4.0-8.0 mA, recorded slight local neural degeneration adjacent to the electrodes.
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ItemDistribution of electrically stimulated nerve fibres in the cat cochlea( 1982)An implant electrode array for a cochlear hearing prosthesis has been developed with mechanical properties which allow atraumatic implantation into the human scala tympani. It consists of small platinum electrode bands welded around a flexible silicon rubber tube (Clark et al, 1979). The present study examines the properites of this electrode in electrically simulating the auidtory nerve. The electrode was inserted through the round window for a distance of 5-6mm into the scala tympani of the cat. Brainstem evoked responses and those from the round window were recorded when stimulating with square biphasic current pulses (0.1 msec/phase). Since there was usually less than 10-20 dB hearing loss in the implanted ear, it was possible to selectively mask components of these responses with high-pass filtered noise. The noise masked the response component arising from fibres in the cochlear region corresponding to the noise band. Responses were recorded in the presence of noise with different cut-off frequencies F1, F2 therefore yielded a response band-limited to the region f1-2. In this way it was possible to measure the amount of electrically stimulated activity in a number of different frequency bands. This technique is identical to that of derived response audiometry using acoustic stimulation. The input-output characteristics of the cochlea to a variety of acoustic transients were measured to exclude the possibility of either electrophonic hearing or altered basilar membrane characteristics contaminating the results. The figure shows the distributions of excited fibres using an electrode with an extended ground system running longitudinally in the cochlea. They were measure as the amplitude of the band-limited responses. Results were similar for bipolar electrodes and these electrodes are thus equally suitable for our present cochlear implant prosthesis.