Auditory-nerve fiber responses to amplitude modulated tones and multi-tonal stimuli

Abstract

In normal-hearing ears, sound waves are amplified within the cochlea and a small fraction of the sound energy travels backward out into the ear canal, producing sounds known as "otoacoustic emissions" (OAE) that can be measured with a sensitive microphone. One class of OAE, called "stimulus-frequency-otoacoustic-emissions" (SFOAEs), has been hypothesized to be produced by a process known as "coherent reflection filtering" (CRF). The CRF theory provides a prediction between the SFOAE group delay and the group delays of tone responses on the basilar membrane within the cochlea. Using single and multiple-tone stimuli, we collected data from the firing patterns of single auditory-nerve-fibers (ANFs) from which basilar-membrane tone-response group delays can be calculated for both high and low best-frequency (BF) positions along the basilar membrane. These calculated basilar-membrane group delays were compared to published SFOAE group delays. Our results suggest that group delays calculated from the tip, the lower-frequency tail, or the above-BF region of ANF tuning curves do not match the CRF theory prediction. In obtaining the data to the test the CRF theory, we used two methods for obtaining ANF group delays at frequencies above BF: a previously published method and a simpler new method based on the same principle.

(cont.) Surprisingly, the two methods produced different results. Control measurements suggest that the previously published method does not do what it was expected to do.