Interaction between response modulation and adaptation in the auditory nerve

Abstract

Dynamic response properties of single auditory-nerve fibers and the corresponding modulation following response (MFR) at the round window were obtained in anesthetized Mongolian gerbils. Stimuli consisted of adapting tones followed, after a silent interval, by probe tones. Both tones were amplitude-modulated by sinusoids, and the modulating waveforms were applied first in one phase, then 180 deg out of phase. Responses to the two phases of modulation were combined in order to generate sum and difference waveforms. For the single-unit responses, this allowed computation of both adapting firing rates (from the sum) and response modulation (from the difference) as function of time, while only the latter were obtained for the MFR. When the intensity of the adapting and probe tones were varied, it was observed that the single-unit response modulation varied in proportion to the underlying firing rate for both adapting and probe responses. In addition, the amount of adaptation varied in proportion to the steady-state firing rate, in agreement with previous results. A similar proportionality was observed for the MFR, consistent with the hypothesis that the MFR provides a convenient indirect measure of the ongoing firing rate during adaptation of auditory nerve-fiber responses. [Work sponsored by NIH and NSF.]