Graeme Clark Collection
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ItemResponses of cat auditory nerve fibers to biphasic electrical current pulses( 1987)Discharge patterns of single auditory nerve fibers were recorded from normal-hearing cats implanted with a I2-band intracochlear electrode array. Stimuli were biphasic current pulses of specifiable width, amplitude, and rate. Acoustic tuning curves were obtained to determine the cochlear positions of the fibers. Response latencies to electrical stimuli formed two groups. Short latency (0.3 to 0.7 ms) responses were attributed to direct activation of spiral ganglion neurons. At high stirnulus intensities, these often exhibited abrupt shifts toward even shorter latencies. Long latency (> 1.5 ms) responses were probably caused by electrophonic activation of functional hair cells. Response thresholds to electrical stimuli depended on a fiber's proximity to the stimulating electrodes, and they did not depend on a fiber's acoustic response threshold or spontaneous discharge rate. High intensity (> 1.5 mA) stimuli could excite fibers over a wide range of characteristic frequencies, even for the narrowest (0.45 mm) electrode separations. Response threshold was an exponentially decreasing function of pulse width for widths up to 300µs/phase. Fiber discharges were highly phase-locked at all suprathreshold intensities, and saturation discharge rates usually equaled stimulus pulse rates for rates up to at least 800 pulses/s. Dynamic ranges were small (I to 6 dB), increased with pulse rate, and were uncorrelated with electrical response threshold. Within the dynamic range, shapes of poststimulus time and interspike interval histograms resembled those obtained in response to acoustic stimuli. Depolarization block caused fiber activity to cease in 2 to 5 seconds for sustained stimuli presented at high (> 600 pulses/s) pulse rates and intensities.
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ItemMiddle ear infection postimplantation: response of the round window membrane to Streptococcus pyogenes( 1987)The seal of the implanted round window membrane to resist Streptococcus pyogenes invasion from the middle ear was investigated in 12 cats. Results showed that the implanted round window membrane is able to form a barrier for S pyogenes starting 1 week postimplantation. Under normal conditions S pyogenes did not pass through the round window membrane, nor through the gap that existed between the membrane and the prosthesis. Mechanical disruption of the round window seal, however, and severe inflammatory response to S pyogenes caused the infection to extend into the inner ear.
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ItemA multiple-electrode intracochlear implant for children( 1987)A multiple-electrode intracochlear implant that provides 21 stimulus channels has been designed for use in young children. It is smaller than the adult version and has magnets to facilitate the attachment of the headset. It has been implanted in two children aged 5 and 10 years. The two children both lost hearing in their third year, when they were still learning language. Following implantation, it was possible to determine threshold and comfortable listening levels for each electrode pair. This was facilitated in the younger child by prior training in scaling visual and electrotactile stimuli. Both children are regular users of the implant, and a training and assessment program has been commenced.
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ItemPneumococcal middle ear infection and cochlear implantation( 1987)A limited study for the experimental induction of pneumococcal otitis media is presented. It is a useful model to study the effects of otitis media in the implanted and nonimplanted cochlea of the cat. Pneumococcal otitis media caused minor pathological changes in two nonimplanted cochleas and more widespread changes together with significant loss of neural elements in two implanted cochleas. However, the small number of animals used in this study did not allow us to distinguish between the effects of electrode insertion trauma, infection, or the combination of both.
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ItemThe histopathology of the human temporal bone following cochlear implantation in a patient: a summary( 1987)The macroscopic and microscopic evaluation of the implanted and unimplanted temporal bones in a patient who had a cochlear prosthesis for 27 months prior to his death from cardiac disease has shown that the device is biocompatible, and does not lead to any significant adverse effects. The cause of deafness was meningitis.