Sound-evoked vibrations along the tonotopic axis in the gerbil cochlea

Our exquisite sense of hearing involves micromechanical inner-ear processes that precede sound transduction by hair cells. These mechanics are however not well understood, especially in the apical regions of the cochlea that are essential for hearing the relatively low frequencies important for the intelligibility of speech. Recent observations in the high-frequency region of the cochlea's sensory organ of Corti (ooC) revealed unanticipated and complex motions, but low-frequency responses are not known. Here, we measured sound-evoked motions from the outer hair cell (OHC) region and the lateral compartment over an extended region in the apex of gerbil cochleae, and compared their phases to the basilar membrane (BM) response. By combining data from multiple tonotopic recording locations, low-frequency traveling waves that propagated along the BM with frequency dependent wavelengths between 1-3 mm were observed. In addition, we found a nonzero OHC BM phase difference which systematically depended on the angle between the OCT optical beam and the BM along its longitudinal axis. This result establishes that the OHC vibrations were predominantly in the longitudinal direction, orthogonal to the up-down motion of the BM. Such motion is unavoidable in the presence of traveling, surface (BM) waves, but has such a significant m a gnitude t hat i t m a sks t he detection of OHC vibrations within the cross-sectional plane. With this, interpreting relative ooC motions in terms of the cochlear micro-mechanisms underlying our ear's remarkable sensitivity and frequency selectivity is tentative because their directions are not a priori known and vibrations along the third cochlear dimension must be considered as an important contributor to the recorded response.