Graeme Clark Collection

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Author: Clark, Graeme × Author: Huang, Christie × Author: Shepherd, Robert × End date: 1998 × Start date: 1998 × Subject: otolaryngology ×

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    Direct current measurements in cochlear implants: an in vivo and in vitro study
    Huang, Christie Q. ; Carter, Paul M. ; Shepherd, Robert K. ; Seligman, Peter M. ; Tabor, Bruce ; Clark, Graeme M. ( 1998)
    Direct current (DC) was measured both in vivo and in vitro in cochlear implant electrodes with stimulation at moderate to high pulse rates in monopolar and bipolar modes. In vivo DC was approximately 2-3 times higher than that measured in vitro. In vivo DC levels were <100 nA even at very high rates, although DC levels increased as a function of stimulus rate and charge intensity. DC levels were lower: in the monopolar than in the bipolar stimulation condition. Stimulation with a monopolar capacitively coupled extracochlear electrode showed even lower DC levels in the intracochlear .electrodes. Our results indicated that the Nucleus electrode shorting system is able to maintain a low level of DC during very high rate stimulation for both monopolar and bipolar modes.
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    Changes in excitability of the auditory nerve following electrical stimulation using large surface area electrodes [Abstract]
    Huang, Christie Q. ; Shepherd, Robert K. ; Seligman, Peter M. ; Tabor, Bruce ; Clark, Graeme M. ( 1998)
    High rate intracochlear electrical stimulation at intensities well above clinical limits can induce significant reductions in the excitability of the auditory nerve. Such changes are primarily associated with stimulus induced neuronal activity, although direct current (DC) can also contribute. In the present study we examined the extent of stimulus induced change in auditory nerve excitability using large surface area platinum (Pt) electrodes (high-Q). These electrodes have an effective surface area 10-20 times larger than standard Pt electrodes, resulting in lower DC and charge density for a common stimulus. Twenty-three guinea pigs anaesthetized with ketamine (40 mg/kg i.p.) and xylazine (4 mg/kg i.p.), were bilaterally implanted with either high-Q or standard Pt electrodes, and unilaterally stimulated for two hours using a stimulus intensity of 0.34 μC/phase at stimulus rates of 200,400, or 1000 pulses/s (pps). Electrically evoked auditory brainstem responses (EABRs) were recorded before and periodically following the acute stimulation. No reduction in EABR amplitude was observed at 200 pps for both stimulating electrodes. However, EABRs were reduced significantly at 400 and 1000 pps. At 200 pps there was no significant difference (p>0.05 ANOVA) in the post-stimulus recovery of EABR amplitudes following stimulation with either high-Q or standard Pt electrodes. There was, however, significantly greater EABR recovery following stimulation with the high-Q electrode compared with the standard Pt electrode at 400 (p<0.05) and 1000 pps (p<0.05). These data indicate that large surface area high-Q electrodes can significantly reduce stimulus induced changes in auditory nerve excitability, and may therefore have important clinical application.