Graeme Clark Collection

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Author: Shepherd, Robert × End date: 1999 × Start date: 1990 × Type: Conference Item × Subject: cochlear implants × Subject: electrical stimulation ×

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    Electrical stimulus induced changes in excitability of the auditory nerve
    Huang, C. O. ; Shepherd, Robert K. ; Seligman, P. M. ; Clark, Graeme M. ( 1997)
    High rate electrical stimulation of the auditory nerve using stimulus intensities well above the clinical limits can induce a significant reduction in the excitability of the auditory nerve as measured by a decrement in the amplitude of the electrically evoked auditory brainstem response (EABR). Two potential mechanisms may be associated with this stimulus induced reduction in activity: 1) stimulus induced prolonged neuronal hyperactivity; and 2) the generation of adverse electrochemical productions from the electrode surface. The purpose of the present study was to assess the extent to which adverse electrochemical damage contributes to the stimulus induced reduction in auditory nerve excitability. Twenty-six adult guinea pigs anaesthetized with ketamine (40 mg/kg i.p.) and xylazine (4 mglkg i.p.), were bilaterally implanted and unilaterally stimulated for two hours using a stimulus intensity of two or four times EABR threshold. Stimulus rates of 200, 400, or 1000 pulses/s (pps) were delivered via a standard platinum scala tympani electrode or large surface area ("high Q") platinum electrode.
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    Electrical stimulation of the auditory nerve: comparison of half-band with full-band scala tympani bipolar electrodes
    Xu, Shi-Ang ; McAnally, Ken I. ; Xu, J. ; Shepherd, R. K. ; Clark, Graeme M. ( 1993)
    The Melbourne/Cochlear auditory prosthesis uses an intracochlear electrode array containing 22 circumferential full-band electrodes mounted on a Silastic carrier. It could be hypothesized that half-band electrodes, oriented towards the modiolus, would produce lower stimulus thresholds than conventional full-band electrodes. This hypothesis is based on the assumption that, compared with full-band electrodes, half-band electrodes would produce an electrical field in which a greater proportion of the current would excite a defined group of neurons. In order to verify this hypothesis we recorded electrically evoked auditory brainstem responses (EABRs) for both full- and half-band electrodes inserted in the scala tympani of deafened cats. EABR thresholds for half-band electrodes oriented towards the modiolus were not significantly different from thresholds evoked using full-band electrodes (p>0.05, paired t-test), whereas thresholds evoked using half-band electrodes oriented towards the outer scala wall were significantly higher (p<0.01) than either the modiolar half-band or the full-band electrodes. These physiological results suggest that the electrical field generated within the auditory nerve by modiolar oriented half-band electrodes does not differ significantly from that produced by full-band electrodes. On the basis of these results, together with the fact that half-band electrodes would have higher current densities and electrode impedances, and would require careful orientation during implantation, we consider that there is no benefit in incorporating half-band electrodes in the design of scala tympani electrode arrays.