Broadband amplification of organ of Corti vibrations in the mouse cochlear apex

Mammalian hearing depends on the active amplification of sound-evoked waves as they travel along the basilar membrane within the cochlea. This amplification is mediated by the outer hair cells (OHCs), which generate force to enhance the vibrations of the surrounding structures. While OHCs at a given location only amplify basilar membrane motion for a narrow frequency range, recent measurements show that the amplification of motions deeper within the organ of Corti is much more broadband. However, the extent to which this broadband amplification influences the motions that are most relevant to inner hair cell stimulation -- i.e., at the organ's apical surface -- remains uncertain. Here, optical coherence tomography was used to demonstrate that OHCs nonlinearly amplify the motions near the top of the organ of Corti, including at the reticular lamina and tectorial membrane, over a wide frequency range in the mouse cochlear apex. Responses at all frequencies were physiologically vulnerable and grew compressively with stimulus level. Low-frequency responses also exhibited non-monotonic features that were due to interference between amplified motion and the underlying traveling wave. The data suggest that broadband amplification of motions at the top of the organ of Corti likely explains certain phenomena observed in auditory nerve responses.