Dynamic effects in the input/output relationship of auditory nerve

Cyclic histograms of the responses of single auditory ganglion cells in the guinea pig were recorded during stimulation with amplitude modulated tones. Modulation frequencies ranged from 10 Hz to 800 Hz. The response histograms, phase-locked to the modulation signals, were analysed for mean action potential rate and for the amplitude of the fundamental component at the modulation frequency. Expected values for the amplitude of the modulation responses were calculated using the variation of mean firing rate with intensity. The observed responses differed from the expected responses in several ways. First, the amplitudes of the modulation responses were larger than expected. Second, the stimulus intensities at which the observed modulation responses peaked was greater by about 7-10 dB than the expected intensity for maximum response. Third, both the magnitude of the response at a given intensity and the intensity at which the response peaked increased with modulation frequency. Fourth, the responses extended to higher stimulus intensities than expected. The observed modulation responses were compared with predictions from the Schroeder and Hall model of adaptation and were found to agree with good quantitative precision. These results suggest that the observed modulation responses are another manifestation of the very rapid (less than 20 ms) adaptation seen in the onset responses of nerve fibres [(1985) Hear. Res. 17, 1-12]. It is concluded that the static input-output responses of auditory nerve are not a good predictor of the dynamic responses to fluctuating stimuli.