ADAPTATION AND RECOVERY FROM ADAPTATION IN SINGLE FIBER RESPONSES OF THE CAT AUDITORY-NERVE

This study examined the time course of adaptation and recovery from adaptation of single auditory-nerve fiber responses. The conditions studied were: (1) adaptation response using low level, 800 Hz or characteristic frequency (CF) stimuli; and (2) onset recovery and whole tone response recovery of a probe tone following a masker of equal frequency with variable silent intervals between the masker offset and probe onset. Single unit responses to 290 ms long, 800 Hz or CF tones presented at 10-30 dB SL were recorded from the auditory nerve of the cat. Adaptation properties were determined and fit to the equation: A(t(p)) = Y(r)e(-t(p)/tau-Rr) + Y(s)e(-t(p)/tau-Rs) + A(ss). Recovery from adaptation was determined by recording the response of a probe tone following a 100-ms masker tone equal in frequency to the probe, and with amplitudes ranging from 20- to 30-dB relative to the probe amplitude. Both the onset recovery and the whole tone recovery were determined for the single unit responses. The onset data were analyzed and fit to either the equation: A(DELTA-xt,tp) = A(ss) - Y(r)e(-tp/tau-Rr) - Y(s)e(-DELTA-t/tau-Rs) or A(DELTA-t,tp) = A(ss) - Y(r)e(-DELTA-t/tau-R). The whole tone response showed two distinctive time patterns that could be fit to either an adaptation equation or to the two-time-constant recovery equation, depending on the relative amplitude of the masker and the length of the silent interval between masker offset and probe onset. The results of this study indicate that single fiber time constants are comparable to those measured in previous studies using the auditory-nerve neurophonic (ANN). Likewise, the pattern of recovery of the whole tone response for single fiber responses is comparable to the ANN. Possible sites and mechanisms for adaptation and recovery from adaptation taking into account recent data from electrical stimulation studies and receptor channel morphology and kinetics are discussed.