Graeme Clark Collection
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ItemElectrical stimulation of the auditory nerve: the coding of frequency, the perception of pitch and the development of cochlear implant speech processing strategies for profoundly deaf people( 1996)1. The development of speech processing strategies for multiple-channel cochlear implants has depended on encoding sound frequencies and intensities as temporal and spatial patterns of electrical stimulation of the auditory nerve fibres so that speech information of most importance for intelligibility could be transmitted. 2. Initial physiological studies showed that rate encoding of electrical stimulation above 200 pulses/s could not reproduce the normal response patterns in auditory neurons for acoustic stimulation in the speech frequency range above 200 Hz and suggested that place coding was appropriate for the higher frequencies. 3. Rate difference limens in the experimental animal were only similar to those for sound up to 200 Hz. 4. Rate difference limens in implant patients were similar to those obtained in the experimental animal. 5. Satisfactory rate discrimination could be made for durations of 50 and 100 ms, but not 25 ms. This made rate suitable for encoding longer duration suprasegmental speech information, but not segmental information, such as consonants. The rate of stimulation could also be perceived as pitch, discriminated at different electrode sites along the cochlea and discriminated for stimuli across electrodes. 6. Place pitch could be scaled according to the site of stimulation in the cochlea so that a frequency scale was preserved and it also had a different quality from rate pitch and was described as tonality. Place pitch could also be discriminated for the shorter durations (25 ms) required for identifying consonants. 8. As additional speech frequencies have been encoded as place of stimulation, the mean speech perception scores have continued to increase and are now better than the average scores that severely-profoundly deaf adults and children with some residual hearing obtain with a hearing aid.
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ItemCross-fiber interspike interval probability distribution in acoustic stimulation: a computer modeling study( 1995)Electrical stimulation strategies for cochlear implants may be improved by studying temporal frequency coding in single auditory fibers and across fibers in acoustic stimulation (Clark et al, this suppl, section 5). In single nerve fibers, phase locking between action potentials and the acoustic stimulus can only be maintained at frequencies below about 600 Hz. At these frequencies, the time interval between successive action potentials, called the interspike interval (lSI), is distributed around the period of the stimulus, and it can therefore be used to code frequency within single fibers. At higher frequencies, the phase locking of individual nerve fibers diminishes, but it may still be possible to retain phase-locking properties by combining the action potentials in an ensemble of nerve fibers. In an ensemble of fibers, the lSI in each nerve is affected by factors such as the spectral shape of the stimulus, the characteristic frequency, and the firing characteristics of the nerve. The lSI between the fibers, however, is further affected by the propagation or phase delay of the traveling wave. It is therefore uncertain how these factors would affect frequency coding across fibers. It is possible that the propagation delay between the fibers may lower the phase locking in an ensemble of nerves -because the probability that the majority of nerves in an ensemble will fire simultaneously may be low. It is also possible that the combined firing statistics of the fibers in an ensemble may result in a higher degree of synchrony such that the predominant intervals in an ensemble are preserved over a wider frequency range than in a single fiber. Are these accurate postulations of the physical system? In a future electrical stimulation strategy that incorporates temporal frequency coding, is it necessary to mimic the spatial-temporal delay in the firing patterns caused by the traveling wave? These are important questions that need to be studied and answered. (From Introduction)
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ItemPsychophysics of electrical stimulation of the auditory nerve: implications for coding of sound and speech processing for cochlear implants [Keynote address]( 1994)Psychophysical studies on electrical stimulation of the auditory nerve have contributed to our understanding of the coding of sound and speech signals. Those studies have also helped establish speech processing strategies for multiple-electrode cochlear implant patients. The first studies were on temporal coding of frequency and pitch perception to help determine whether a single or multiple electrode implant would be preferable for the coding of speech frequencies. Temporal frequency coding was initially studied in the experimental animal by measuring difference limens for frequency of stimulus rate. The results showed that rate coding occurs for low frequencies up to 200 or even 600 pulses per second. It was concluded that higher speech frequencies cannot be conveyed by variations in stimulus rate but require multiple-electrode stimulation. These studies in experimental animals were essentially confirmed in the human.
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ItemAdvances in cochlear implant speech processing [Abstract]( 1997)Our early research emphasized there was a restriction on the amount of speech and other acoustic information that could be transmitted to the nervous system by electrical stimulation of the auditory nerve. It also showed the need to use multiple-channel stimulation, and minimize channel interaction. As a result our research over the last 30 years has been directed towards optimizing the information presented to the auditory nervous system. This has involved extracting the energy of the first and second formants (FO/F2-WSP II; FO/FI/F2-WSP III; Multipeak-MSP) as well as the outputs of high band pass fixed filters (Multipeak - MSP) and coding these outputs as cochlear place of stimulation. The voicing frequency was coded as rate of stimulation. Our most recent speech processing strategy (SPEAK) extracts a specified number of .maximal outputs from a series of band pass filters, rather than selecting the peaks of energy which was the case with the other strategies. The voltages from the maximal outputs are used to stimulate appropriate electrodes on a place coding basis. The stimuli are presented at a constant stimulus rate to reduce channel interaction. Voicing is conveyed as amplitude variations.