Responses of auditory nerve fibers to trains of clicks.

Responses of auditory nerve fibers to trains of clicks were recorded in ketamine anesthetized chinchillas. By varying the number of clicks and the interclick interval, this study examined whether "post-onset adaptation," described in psychoacoustic experiments on localization, occurred in auditory nerve fibers. The results showed that the number of action potentials recorded from a nerve fiber in response to a train of clicks was a power function of the number of clicks. For interclick intervals of 2 ms or greater the exponent of the power function was 0.5, and this exponent did not change over a 20-dB range of intensities. The timing of action potentials relative to the click stimuli was measured using synchronization coefficients. The coefficients increased with interclick interval, decreased with increasing intensity, and were greater for fibers with low rates of spontaneous activity than for high spontaneous fibers. Recovery functions showed that for interclick intervals of 2 ms or more, the responses to the second click were at least 70% of the response to the initial click. The recovery depended upon the number of clicks in the train. These findings indicate that auditory nerve fibers respond to high rates of stimulus presentation and do not display the adaptation observed in localization studies.