Graeme Clark Collection
Permanent URI for this collection
Search Results
1 - 8 of 8
-
ItemInner ear implants(Dekker, 2004)The cochlear implant is an electronic device that brings useful hearing to severely to profoundly deaf people through multiple-channel electrical stimulation of the auditory nerves in the inner ear. This is required if their inner ears are so badly damaged by injury and disease, or so inadequately developed, that they cannot provide sufficient hearing for communication, even when the sound is amplified with a hearing aid. By stimulating the nerve directly with patterns of electrical pulses, the implant bypasses the normal function of the sense organ of hearing in the inner ear to partially reproduce the coding of sound. It consists of a wearable speech processor that picks up sound with a microphone, analyzes the signal, and then sends it by radio waves to the implanted receiver stimulator, which decodes the message and stimulates the electrode wires inserted into the inner ear.
-
Item
-
ItemControl strategies for neurons modeled by self-exciting point processes( 1996)Abstract not available due to copyright.
-
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.
-
ItemControl strategies for nerves modeled by self-exciting point processes( 1995)Cochlear implants electrically stimulate the auditory nerve with the aim of generating a perception of sound via an evoked neural response pattern. An electrically stimulated auditory nerve responds differently to an acoustically stimulated auditory nerve, and the surviving nerves of patients with a hearing loss may exhibit characteristics different from those of normal-hearing people. Thus, the cochlear implant evoked response pattern differs greatly from that of the normal hearing situation. One method of understanding such response patterns is to employ a mathematic model. If possible, the model should permit the determination of neural response differences between closely related sounds, and facilitate the design of stimuli that evoke desired neural response patterns. How should such a model be chosen?
-
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.
-
ItemChronic monopolar high rate simulation of the auditory nerve: physiological and histopathological effects(Kugler Publications, 2001)There is clinical interest in the development of high rate speech processing strategies, since there are indications that these might enhance speech perception due to an improved representation of the rapid variations in amplitude of speech. Significant improvement in speech perception using high rate stimulation has been demonstrated in cochlear implant recipients. However, it is important that the long-term safety of high rate stimulation is clearly established prior to its general clinical application. This is especially important, since acute animal studies have shown that high rate stimulation can induce a reduction in the excitability of the auditory nerve. This was also associated with an increase in both threshold and latency of the electrically evoked auditory brainstem response (EABR). However, while a chronic stimulation study indicated that monopolar electrical stimulation of the auditory nerve at rates of 1000 pulses per second (pps)/channel (three channels) had no adverse effects on the spiral ganglion cell density (SGCO),5 there is limited data concerning higher rates. In the present study, we evaluated the electrophysiological and histopathological effects of chronic monopolar electrical stimulation of the auditory nerve using considerably higher stimulus rates than have been used in previous studies.
-
ItemElectrical stimulation of the auditory nerve: stimulus induced reductions in neural excitability [Abstract]( 1987)Electrical stimulation of the auditory nerve elicits highly synchronised neural activity (Javel et al., in press). As the stimulus current is increased the neural response becomes highly deterministic with every current pulse eliciting a spike even at stimulus rates of 600-800 pulses per second (pps). Our previous acute experimental studies have shown that high stimulus rates (> 200 pps) and high stimulus currents (> 1.0 mA) can result in temporary and sometimes permanent reductions in the excitability of the auditory nerve (Shepherd and Clark, 1986). The present study was designed to examine the mechanisms underlying these stimulus induced reductions in excitability. These results will have implications for the maximum safe and effective stimulus rates that can be employed in cochlear implants.