High‐intensity “notches” in responses of auditory nerve fibers

Abstract

In cat auditory nerve fibers, pure‐tone input‐output functions for both discharge rate and synchronization can be nonmonotonic. That is, there can exist an intensity range over which the response saturates, then declines, then increases again at higher intensities. In our experiments, this intensity “notch” usually began at 70–100 dB SPL and persisted for the next 10–50 dB, with 30 dB being a typical range. The upper leg of the rate‐intensity function had a steeper slope and saturated at a higher rate than the lower leg. The depth of the “notch” could vary from only a slight inflection in response to a depression down to spontaneous level. We have observed “notches” in all regions of fibers' response areas. The intensity range and magnitude of the “notch” varied both within and across fibers. “Notches” at low stimulus frequencies were usually accompanied by rapid shifts in mean discharge phase angle. The cumulative phase shift was not consistent, ranging from less than 90° to more than 200°. Reversals in the direction of phase change were common. “Notches” for tones below 600 Hz often displayed bimodal period histograms, but the two peaks usually existed only over a narrow range of intensities. [Work supported by NIH.]