Electrically evoked compound action potential (ECAP) of the cochlear nerve in 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 pulsatile electrical stimulation presented at high rates. The present study aimed to explore, in an animal model of cochlear implants, how the excitability of the cochlear nerve is affected by pulsatile electrical stimulation delivered at high rates, of up to 1,000-2,000 pulses per second (pps). Adult rats (n=23) were implanted with two or three stimulating electrodes in the left cochlea. In four of these rats, the left cochlea was deafened by local perfusion with 1 per cent or 4 per cent neomycin solutions prior to implantation. Pulsatile stimuli consisted of 20 micros electrical pulses, delivered in trains of 200 ms duration, separated by a pause of 200 ms. The pulse rates ranged from 100 to 2,000 pps (intra-train pulse rate). Electrically evoked compound action potentials (ECAPs) of the cochlear nerve were recorded either 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 above 300 pps, the amplitude of the ECAP to the individual successive pulses delivered in the train progressively decreased during the initial part of the train, corresponding to a short-term adaptation of the cochlear nerve. This effect progressively increased for pulse rates ranging from 300 to 2,000 pps. In addition, there was a phenomenon of long-term adaptation, as indicated by a decrease in the amplitude of the ECAP to the first pulse of the train, indicating that the pause of 200 ms between each train was not long enough 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.