ADAPTATION IN THE COMPOUND ACTION-POTENTIAL RESPONSE OF THE GUINEA-PIG 8TH NERVE TO ELECTRIC-STIMULATION

An experimental study, carried out in guinea pigs, was designed to inv estigate whether forward masking measured psychophysically in 3M-House cochlear implant users might have a correlate in VIIIth nerve activit y. The study was based on electrically evoked VIIIth nerve compound ac tion potentials (ECAPs), using a masking paradigm comparable to the on e used in the psychophysical study. Trains of 50 maskers with inter-ma sker-intervals of 509 ms appeared to induce a long-term fatigue effect that could influence the recovery from adaptation measurements. Fatig ue stabilized within about 1 to 3 min when masker trains were repeated with intervening silent intervals of 10.5 s. The change in amplitude of probe-evoked ECAPs with increasing masker-probe delays was determin ed within the steady fatigue state. The recovery-from-adaptation funct ions obtained from these measurements resembled the forward masking fu nctions found in 3M-House cochlear implant users. No correlate of psyc hophysical backward masking was found at the VIIIth nerve level. To ex amine whether hair cells were involved in fatigue and recovery from ad aptation, the measurements described above were carried out in intact cochleas and in cochleas without hair cells. Results were essentially the same in the different preparations. The results suggest that proce sses at the level of the VIIIth nerve could, at least partly, account for forward masking found in 3M-House cochlear implant users. Backward masking must be attributed to mechanisms located centrally to the VII Ith nerve.