Population study of non‐monotonic response‐intensity curves in auditory nerve fibers

Abstract

For pure‐tone signals, responses of many auditory nerve fibers in cats are nonmonotonic functions of intensity. In the experiments described here, we examined responses across the fiber population to tones at 200 Hz, 500 Hz, and 1000 Hz. Below 70–90 dB SPL, responses to 200 and 500 Hz generated unimodal period histograms. Above 70–90 dB SPL, period histograms were bimodal. This was not the result of an externally generated harmonic distortion product. At yet higher intensities, period histograms became unimodal, but the average discharge phase angle changed. Typically, the second peak which emerged in the period histograms at high levels dominated the temporal response and the original peak disappeared. Only unimodal period histograms were seen at 1000 Hz; however, discharge phase shifts still occurred at high intensities. The intensity corresponding to the decline of response amplitude was independent of fiber CF and fiber sensitivity at the stimulus frequency. These findings indicate that the mechanism producing the nonmonotonic input‐output functions is not confined to a restricted cochlear region. Further, cochlear mechanics is likely to be the source of the nonmonotonicities, perhaps producing standing waves in the cochlea.