Graeme Clark Collection
Permanent URI for this collection
Search Results
1 - 5 of 5
-
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.
-
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.
-
ItemSteady state evoked potentials to amplitude modulated tones [Abstract]( 1982)Evoked responses have been recorded from the scalp of normal human subjects to continuous sinusoidally-modulated amplitude-modulated tones. Phase locking the computer to the modulation envelope enabled an averaging technique to be used to improve the signal to noise ratio. The responses were found to be periodic with the same fundamental frequency as the modulation envelope. Fourier analysis was used to quantify the amplitude and phase of the first and second harmonic components of the responses. The variation of the response amplitude with SPL depended on the modulation and carrier frequency. At modulation frequencies of less than 20Hz and carrier frequencies of less than 1KHz, the amplitude of a response increases up to 50dBSPL but remains invariant at higher levels. At higher modulation frequencies, the response amplitude is uniform to 60-80dBSPL and then increases very rapidly, sometimes by as much as a factor of 20 for a 20dB increase in SPL. This is particularly true of high carrier frequencies. These amplitude growth functions can be explained in part by neural tuning curves. Phase locked responses ran be recorded down to 30dBSPL at most modulation rates and carrier frequencies. Estimates of latencies of these potentials were made by measuring the phase change of the first and second harmonic components whilst changing the modulation frequency. The latencies varied with modulation frequency, carrier frequency and SPL. Latencies of both first and second harmonic components fall into 14 discrete groups from 3msec up to 104msec, with the majority of responses having latencies of 9msec to 33msec. The origin of some of these responses is likely to be the auditory cortex.
-
ItemSingle versus multiple-channel electrical stimulation of the auditory nerve in speech processing for a totally deaf patient [Abstract]( 1982)Auditory neurophysiological studies have provided evidence that frequency is coded on both a place and time basis. Psychophysical studies on patients with a profound or total postlingual hearing loss have established that electrical stimulation of the auditory nerve can also convey pitch sensations that depend on toe place or site of auditory nerve stimulation. Pitch perception is also related to the rate of stimulation (Tong et al., 1980). The present study has been undertaken to determine whether a totally deaf patient can integrate both the place and time information from frequency coding in understanding speech signals. Audiological tests were performed to evaluate a totally deaf patient's perception of phonemes, words and sentences for single-channel stimulation that conveyed the fundamental frequency (FO) as rate of stimulation; and multiple-channel stimulation that presented the fundamental frequency as rate, and the second formant (F2) as place of stimulation. The results shown in the table indicate that multiplechannel stimulation provided significantly better scores than single-channel stimulation when using electrical stimulation alone. Furthermore, all scores were significantly better for multiple channel stimulation when combined with lip reading, except the MRT word test. This is an abstract of a paper from the Proceedings of the Australian Physiological and Pharmacological Society published by Australian Physiological and Pharmacological Society. This version is reproduced with the permission of the publisher.
-
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.