Sensitivity of auditory-nerve fibers to changes in intensity: a dichotomy between decrements and increments.

Adaptation of auditory-nerve responses was investigated by applying increments and decrements in intensity to an ongoing tonal background. The change in firing rate produced by a change in intensity was obtained as a function of the time delay from the onset of the background to the onset of the change in intensity. The initial change in firing rate was measured using both small (1 ms) and large (10 ms) time intervals in order to evaluate properties of rapid and short-term adaptation, respectively. Consistent with previous results, the incremental and decremental responses measured with large windows were independent of time delay and the amount of prior adaptation. A similar additivity was observed for the incremental response measured with a small time window. In contrast, the decremental response measured with a small window decreased with increasing time delay and in proportion to the decrease in firing rate produced by the background. A similar decrease was observed in the response modulation produced by sinusoidal amplitude modulation. It was concluded that sensitivity to decrements in intensity decreases during adaptation, so that this response component does not reflect the additivity inherent in other aspects of adaptation.