Electrically evoked compound action potential (ECAP) of the cochlear nervein response to pulsatile electrical stimulation of the cochlea in the rat:Effects of stimulation at high rates

Some cochlear implant patients achieve better speech recognition with pulsa tile electrical stimulation presented at high rates. The present study aime d to explore, in an animal model of cochlear implants, how the excitability of the cochlear nerve is affected by pulsatile electrical stimulation deli vered at high rates, of up to 1,000-2,000 pulses per second (pps). Adult ra ts (n=23) were implanted with two or three stimulating electrodes in the le ft cochlea. In four of these rats, the left cochlea was deafened by local p erfusion with 1 per cent or 3 per cent neomycin solutions prior to implanta tion. Pulsatile stimuli consisted of 20 mu s electrical pulses, delivered i n trains of 200 ms duration, separated by a pause of 200 ms. The pulse rate s ranged from 100 to 2,000 pps (intra-train pulse rate). Electrically evoke d compound action potentials (ECAPs) of the cochlear nerve were recorded ei ther intracochlearly or from epidural electrodes (extra-cochlearly). With increasing pulse rates, the average ECAP amplitude decreased, whereas the average ECAP latency and its variability (SD) increased. For rates abov e 300 pps, the amplitude of the ECAP to the individual successive pulses de livered in the train progressively decreased during the initial part of the train, corresponding to a short-term adaptation of the cochlear nerve. Thi s effect progressively increased for pulse rates ranging from 300 to 2,000 pps. In addition, there was a phenomenon of long-term adaptation, as indica ted by a decrease in the amplitude of the ECAP to the first pulse of the tr ain, indicating that the pause of 200 ms between each train was not long en ough for full recovery of the cochlear nerve. This long-term adaptation was progressively more pronounced for increasing pulse rates. To characterize further the recovery in excitability of the cochlear nerve, forward masking experiments were conducted, showing a decrease of the ECAP amplitude when the interval between the first pulse (masker) and the second pulse (probe) was shorter than 2 ms. This ECAP decrease was slow for intervals between 2 and 1 ms and then abrupt for shorter intervals. The observations described above were similar for extra- and intra-cochlear recordings and were little , if at all, affected by treatment of the cochlea with neomycin.