Supra-Characteristic-Frequency Response in Gerbil Auditory Nerve Frequency Tuning Curves

Sound arriving at the ear causes the vibration of the sensory tissues, including the basilar membrane (BM), inside the cochlea and, in turn, leads to inner hair cell excitation and auditory nerve fiber (ANF) responses. The goal of this study is to better understand the mechanics of inner hair cell excitation which leads to hearing. BM motion and ANF tuning are generally very similar, but the ANF had appeared to be unresponsive to a plateau mode of BM motion that occurs at frequencies above an ANF's characteristic frequency (CF). We recorded ANF responses from the gerbil, concentrating on this supra-CF region. We observed a supra-CF plateau in ANF responses at high stimulus level, indicating that the plateau mode of BM motion can be excitatory. Quantitative aspects of our findings suggest that the differential longitudinal motion that occurs within the traveling wave but not the plateau mode increases the sensitivity of inner hair cell excitation. The main findings of this study include: The detection of the plateau threshold within the supra-CF region of the ANF tuning curve. A larger BM motion was necessary for an ANF to reach a threshold response within the plateau region than the traveling wave region, based on the previous lack of ANF plateau threshold detection and a comparison to the BM plateau levels in the literature. Stimuli used in this study, even though unnaturally high in level, advanced our understanding of cochlear mechanics. However, at high sound pressure levels used, the middle ear generated subharmonic distortions that could produce confounding effects in the plateau responses. Hence, we also characterized the subharmonics and were able to rule out the possibility that they were solely responsible for the plateau responses we observed.