Tianying Ren

Upon acoustic stimulation of the mammalian cochlea, a travelling wave will move in a base-to-apex direction, peaking close to the resonant place of the basilar membrane. A gradient of different resonances along the basilar membrane gives rise to frequency-place coding, or tonotopicity, throughout the majority of the cochlea. Per our previous findings, the organ of Corti in the guinea pig cochlear apex does not display tonotopicity. The behavior of the basilar membrane has not been explored throughout the cochlear apex, and so hypothetically tonotopicity is conserved at the level of this structure. Outer hair cells, which influence organ of Corti and basilar membrane mechanics, provide amplification throughout the cochlea. The mechanism for this process in the region close to the helicotrema, which is important for speech and music perception, is not fully described. Here, we describe the acoustically evoked vibrations measured using optical coherence tomography (OCT) at three locations within the guinea pig cochlear apex. We note a decline in the ratio of organ of Corti and basilar membrane gain with proximity to the helicotrema and show that distinct nonlinear behaviors are associated with each location. Phase differences between the cellular portion of the organ of Corti and basilar membrane were present and remained after furosemide treatment. We report that the frequency tuning of the apical basilar membrane is inconsistent with standard models. The analysis of nonlinearity indicates that the mechanisms governing sensory transduction, while still not elucidated, change rapidly within the apical turn—findings that are important for understanding how communication-relevant sounds are encoded by the cochlea.

Purpose: The basilar membrane (BM) motion evoked by single or two-tone stimuli shows nonlinearity largely confined to the region of the traveling wave peak(s) with a passive and linear response basal to the peak. For the same stimuli, nonlinear two-tone interactions in the ear canal pressure and cochlear microphonics appear to originate in a region that extends considerably basal to the peak of the BM traveling wave. Recent measurements from the reticular lamina region (RL) of the organ of Corti in the mouse apex exhibit active gain and broadly tuned two-tone suppression of the response to a lower-frequency probe stimulus that is not measured in the BM at the same location. These results suggest that suppressible active gain is evident in the RL region motion in the region basal to the characteristic frequency (CF) place of the probe tone. The purpose of this study is to explore the spatial extent of active gain and nonlinearity of the cochlear partition by measuring two-tone suppression in the RL region and BM responses to a probe two octaves lower in frequency than the CF of the recording location. Methods: In the current study in the basal turn of the Mongolian gerbil cochlea (15 animals), we used a sensitive custom-built scanning low-coherence heterodyne interferometer to measure two-tone interactions between a suppressor tone near the local CF (~ 20 kHz) and a probe tone two octaves below CF in the vibrations of the RL region and BM. Results: In sensitive animals, we demonstrate prominent two-tone suppression of the response in the motion of the RL region to a 5 kHz 40 dB SPL probe tone for suppressor tones near the ~ 20 kHz CF at levels as low as 40 dB SPL. Suppression of the probe response became more pronounced as the suppressor level was increased. Responses to the 40 dB SPL probe could not be measured in BM where responses were in the noise. When the probe level was raised to 60 dB SPL, such that the BM response was well above the system noise, we did not measure any change in BM vibrations for suppressor levels up to 70 dB SPL. Conclusions: Both active gain and nonlinearity in cochlear mechanics originate in the organ of Corti at a location that extends far basal to the place of the probe tone and not in the BM. Together with the previous reports by others, we conclude that the nonlinear acoustic interaction in the ear canal between a probe tone and tones much higher than the probe frequency is caused by the reduction (suppression) of mechanical gain actively generated by outer hair cells (OHC) in the organ of Corti. There was no indication that the suppressor generated a response at the probe frequency that was not already present in the response to the probe tone presented alone. Our results support the hypothesis that stimulus-frequency otoacoustic emissions (SFOAEs) measured in the ear canal sound pressure originate in a large region extending basal to the place of the probe tone. The BM does not appear to be the site of either generation or propagation of SFOAEs.

The prevailing theory of cochlear function states that outer hair cells amplify sound-induced vibration to improve hearing sensitivity and frequency specificity. Recent micromechanical measurements in the basal turn of gerbil cochleae through the round window have demonstrated that the reticular lamina vibration lags the basilar membrane vibration, and it is physiologically vulnerable not only at the best frequency but also at the low frequencies. These results suggest that outer hair cells from a broad cochlear region enhance hearing sensitivity through a global hydromechanical mechanism. However, the time difference between the reticular lamina and basilar membrane vibration has been thought to result from a systematic measurement error caused by the optical axis non-perpendicular to the cochlear partition. To address this concern, we measured the reticular lamina and basilar membrane vibrations in the transverse direction through an opening in the cochlear lateral wall in this study. Present results show that the phase difference between the reticular lamina and basilar membrane vibration decreases with frequency by ~ 180 degrees from low frequencies to the best frequency, consistent with those measured through the round window. Together with the round-window measurement, the low-coherence interferometry through the cochlear lateral wall demonstrates that the time difference between the reticular lamina and basilar membrane vibration results from the cochlear active processing rather than a measurement error.

Although auditory harmonic distortion has been demonstrated psychophysically in humans and electrophysiologically in experimental animals, the cellular origin of the mechanical harmonic distortion remains unclear. To demonstrate the outer hair cell-generated harmonics within the organ of Corti, we measured sub-nanometer vibrations of the reticular lamina from the apical ends of the outer hair cells in living gerbil cochleae using a custom-built heterodyne low-coherence interferometer. The harmonics in the reticular lamina vibration are significantly larger and have broader spectra and shorter latencies than those in the basilar membrane vibration. The latency of the second harmonic is significantly greater than that of the fundamental at low stimulus frequencies. These data indicate that the mechanical harmonics are generated by the outer hair cells over a broad cochlear region and propagate from the generation sites to their own best-frequency locations.

It has been demonstrated that isolated auditory sensory cells, outer hair cells, can generate distortion products at low frequencies. It remains unknown, however, whether or not motile outer hair cells are able to generate two-tone distortion at high frequencies in living cochleae under the mechanical loads caused by surounding tissues and fluids. By measuring sub-nanometer vibration directly from the apical ends of outer hair cells using a custom-built heterodyne low-coherence interferometer, here we show outer hair cell-generated two-tone distortion in reticular lamina motion in the living cochlea. Reticular-lamina distortion is significantly greater and occurs at a broader frequency range than that of the basilar membrane. Contrary to expectations, our results indicate that motile outer hair cells are capable of generating two-tone distortion in vivo not only at the locations tuned to primary tones but also at a broad region basal to these locations.

Auditory sensory outer hair cells are thought to amplify sound-induced basilar membrane vibration through a feedback mechanism to enhance hearing sensitivity. For optimal amplification, the outer hair cell-generated force must act on the basilar membrane at an appropriate time at every cycle. However, the temporal relationship between the outer hair cell-driven reticular lamina vibration and the basilar membrane vibration remains unclear. By measuring sub-nanometer vibrations directly from outer hair cells using a custom-built heterodyne low-coherence interferometer, we demonstrate in living gerbil cochleae that the reticular lamina vibration occurs after, not before, the basilar membrane vibration. Both tone- and click-induced responses indicate that the reticular lamina and basilar membrane vibrate in opposite directions at the cochlear base and they oscillate in phase near the best-frequency location. Our results suggest that outer hair cells enhance hearing sensitivity through a global hydromechanical mechanism, rather than through a local mechanical feedback as commonly supposed.

Low-coherence interferometry in living cochleae has provided valuable information for understanding cochlear micromechanics. A recent measurement of the reticular lamina and basilar membrane vibrations in mouse cochleae, however, is inconsistent with data collected from guinea pig cochleae. To determine whether a species difference accounts for the observed difference, a custom-built heterodyne low-coherence interferometer was used to measure reticular lamina and basilar membrane vibrations at the basal turn of sensitive gerbil and mouse cochleae. For the gerbil and mouse, both the reticular lamina and basilar membrane vibrations show sharp tuning and nonlinear compressive growth near the best frequency. The magnitude of the reticular lamina vibration is significantly greater than that of the basilar membrane vibration not only near the best frequency, but also at low frequencies. The phase of the reticular lamina vibration leads the basilar membrane phase by up to 180-degrees at low frequencies, and this phase lead decreases with frequency, approaching zero near the best frequency. The best frequency of the reticular lamina and basilar membrane vibrations at the cochlear basal turn in mice is significantly higher than that in gerbils. Besides this difference, cochlear micromechanical responses in the gerbil are similar to those in the mouse. Thus, the current results indicate that gerbil and mouse cochleae detect and process sounds likely through a similar micromechanical mechanism.

Significance: The remarkable sensitivity of mammalian hearing depends on auditory sensory outer hair cells, yet how these cells enhance the hearing sensitivity remains unclear. By measuring subnanometer vibrations directly from motile outer hair cells in living inner ear, we demonstrate that outer hair cells do not directly amplify the basilar membrane vibration by local feedback as commonly expected; instead, they actively vibrate the reticular lamina over a broad frequency range. The outer hair cell-driven reticular lamina vibration interacts with the basilar membrane traveling wave through the cochlear fluid, resulting in maximal vibrations at the best-frequency location, consequently enhancing hearing sensitivity. This finding advances our knowledge of mammalian hearing and may lead to strategies for restoring hearing in patients.

It is generally believed that the remarkable sensitivity and frequency selectivity of mammalian hearing depend on outer hair cell-generated force, which amplifies sound-induced vibrations inside the cochlea. This ‘reverse transduction’ force production has never been demonstrated experimentally, however, in the living ear. Here by directly measuring microstructure vibrations inside the cochlear partition using a custom-built interferometer, we demonstrate that electrical stimulation can evoke both fast broadband and slow sharply tuned responses of the reticular lamina, but only a slow tuned response of the basilar membrane. Our results indicate that outer hair cells can generate sufficient force to drive the reticular lamina over all audible frequencies in living cochleae. Contrary to expectations, the cellular force causes a travelling wave rather than an immediate local vibration of the basilar membrane; this travelling wave vibrates in phase with the reticular lamina at the best frequency, and results in maximal vibration at the apical ends of outer hair cells.