Optical coherence tomography reveals complex motion between the basilar membrane and organ of corti in the gerbil cochlea

Abstract

The mechanical basis of auditory transduction within the mammalian cochlea is not fully understood but relies on complex mechanical interactions between different structures within the Organ of Corti (OoC). We used spectral domain phase microscopy, a functional extension of optical coherence tomography (OCT), to measure the sound-evoked motion in response to “Zwuis” frequency complexes, in vivo, in the basal region of the gerbil cochlea. Because OCT allows for measurements from multiple axial positions along the optical path, we are able to simultaneously record vibration at different points within the OoC between the basilar membrane (BM) and tectorial membrane. Imaging through the round window membrane, we could record from several longitudinal locations along the cochlea and at each location, we measured from several radial positions. Both the BM and intra-OoC vibrations are tuned and show nonlinear amplification, or compressive growth, with the applied sound pressure level. Similar to recent reports (Ren et al, PNAS 2016; Ren and He, MoH 2017), we find that the motions of surfaces within the OoC can exhibit a greater degree of compressive nonlinearity (amplification) and physiological vulnerability than the motion of the BM. Additionally, some responses show sharp notches near the onset of the nonlinear regime.