Organ of Corti vibration within the intact gerbil cochlea measured by volumetric optical coherence tomography and vibrometry

Abstract

There is indirect evidence that the mammalian cochlea in the low-frequency apical and the more commonly studied high-frequency basal regions function in fundamentally different ways. Here, we directly tested this hypothesis by measuring sound-induced vibrations of the organ of Corti (OoC) at three turns of the gerbil cochlea using volumetric optical coherence tomography vibrometry (VOCTV), an approach that permits noninvasive imaging through the bone. In the apical turn, there was little frequency selectivity, and the displacement-vs.-frequency curves had low-pass filter characteristics with a corner frequency of ~0.5–0.9 kHz. The vibratory magnitudes increased compressively with increasing stimulus intensity at all frequencies. In the middle turn, responses were similar except for a slight peak in the response at ~2.5 kHz. The gain was ~50 dB at the peak and 30–40 dB at lower frequencies. In the basal turn, responses were sharply tuned and compressively nonlinear, consistent with observations in the literature. These data demonstrated that there is a transition of the mechanical response of the OoC along the length of the cochlea such that frequency tuning is sharper in the base than in the apex. Because the responses are fundamentally different, it is not appropriate to simply frequency shift vibratory data measured at one cochlear location to predict the cochlear responses at other locations. Furthermore, this means that the number of hair cells stimulated by sound is larger for low-frequency stimuli and smaller for high-frequency stimuli for the same intensity level. Thus the mechanisms of central processing of sounds must vary with frequency. NEW & NOTEWORTHY A volumetric optical coherence tomography and vibrometry system was used to probe cochlear mechanics within the intact gerbil cochlea. We found a gradual transition of the mechanical response of the organ of Corti along the length of the cochlea such that tuning at the base is dramatically sharper than that at the apex. These data help to explain discrepancies in the literature regarding how the cochlea processes low-frequency sounds.

Cochlear partition anatomy and motion in humans differ from the classic view of mammals

“…The longitudinally carried TM radial motion may be more important than TM resonances, but in vivo TM motion is poorly understood and is likely different from the classic view. In addition, recent experiments show that the reticular lamina moves much more than the BM, and there is differential motion between structures at the top of the OoC, such as rotation of the reticular lamina (2)(3)(4)(5)(6)(7)(8)46). All of the above indicate that the classic view in Fig.…”

“…Considering these observations, the overall pattern of OSL and bridge motion we have seen in the base is likely present throughout the human cochlea. This does not rule out there being differences in cochlear motions between base and apex as has been found in laboratory animals (5,6,8).…”

“…The classic view has been that BM motion is translated into a shearing motion at the top of the OoC between the reticular lamina and the tectorial membrane (TM), and this shearing deflects hair cell stereocilia (1). Recent studies that were able to measure motion near the reticular lamina, TM, and hair cells have shown that OoC motion is more complex (2)(3)(4)(5)(6)(7)(8). Despite the lack of a full detailed understanding of cochlear micromechanics, the belief persists that the basic pattern of CP motion is universal across mammalian species because BM and OoC anatomy are similar across mammals (9).…”

Two-tone distortion in reticular lamina vibration of the living cochlea

“…Third, it is well documented that only sensitive living cochleae can amplify sounds [19][20][21][22][23] and generate distortion product otoacoustic emissions [24][25][26] . The basilar membrane distortion 8,9,27,28 and the reticular lamina nonlinearity [29][30][31][32][33][34][35][36] vanish upon the death of the animals before the outer hair cells can be isolated. Thus, whether the outer hair cells can generate distortion products at high frequencies in living cochleae remains unknown.…”

Overtone focusing in biphonic Tuvan throat singing

“…Fourth, it is well known that harmonic components of perceived pitch transition from resolved to unresolved around 1–2 kHz ( Plack et al, 2006) . Lastly, advances in cochlear imaging show that the micromechanical vibrations of the cochlea are different between base and apex [e.g., Dong et al (2018) ], with large differences in tuning sharpness. While recent modeling efforts have made progress in reconciling these differences in apical and basal sound processing for mammals ( Sasmal and Grosh, 2019) , it has been suggested that the human cochlea may be different in many regards from the canonical viewpoint of a relatively generic cochlear structure/physiology/function across mammals ( Joris et al, 2011 ; Raufer et al, 2019) .…”