EFFECT OF HIGH-FREQUENCY ELECTRICAL-STIMULATION OF THE AUDITORY-NERVEIN AN ANIMAL-MODEL OF COCHLEAR IMPLANTS

Hypothesis: Electrical stimulation of the cochlea at high rates induce s significant adaptation of the auditory nerve. Background: A new deve lopment of cochlear implants is the use of speech processors deliverin g electrical pulses on the implanted electrodes at high rates, such as 1,000 pulses per second (pps) and above. Such a stimulation mode allo ws subjects with cochlear implants to reach excellent understanding of speech. Methods: Long Evans-rats received implantation of stimulating electrodes in the left cochlea. Two hundred-millisecond trains of sho rt (20 mu s) monophasic pulses were delivered in 50% duty cycle at 500 mu A above threshold. The pulse rate in the train was increased from 100 pps to 1,500 pps. Electrically evoked auditory brainstem responses (EABR) were recorded. The amplitude of the compound action potential of the auditory nerve to each single pulse in the train was measured a s the first vertex positive wave (WAVE I) of the EABR. Results: At 100 and 200 pps, WAVE I amplitudes to each pulse were large and remained stable throughout the pulse train. For increasing pulse rates, WAVE I amplitudes progressively decreased during the first 40 to 50 ms of the train and reached 80% at 300 pps to 15% at 1,500 pps of the maximal a mplitude observed for the first pulse in the train. Conclusions: The d ecrease of the WAVE I amplitude in response to high-rate pulsatile sti mulation reflects an adaptation of the auditory nerve due, at least in part, to the refractory period of auditory nerve fibers.