Noninvasive in vivo imaging reveals differences between tectorial membrane and basilar membrane traveling waves in the mouse cochlea

Abstract

Sound is encoded within the auditory portion of the inner ear, the cochlea, after propagating down its length as a traveling wave. For over half a century, vibratory measurements to study cochlear traveling waves have been made using invasive approaches such as laser Doppler vibrometry. Although these studies have provided critical information regarding the nonlinear processes within the living cochlea that increase the amplitude of vibration and sharpen frequency tuning, the data have typically been limited to point measurements of basilar membrane vibration. In addition, opening the cochlea may alter its function and affect the findings. Here we describe volumetric optical coherence tomography vibrometry, a technique that overcomes these limitations by providing depthresolved displacement measurements at 200 kHz inside a 3D volume of tissue with picometer sensitivity. We studied the mouse cochlea by imaging noninvasively through the surrounding bone to measure sound-induced vibrations of the sensory structures in vivo, and report, to our knowledge, the first measures of tectorial membrane vibration within the unopened cochlea. We found that the tectorial membrane sustains traveling wave propagation. Compared with basilar membrane traveling waves, tectorial membrane traveling waves have larger dynamic ranges, sharper frequency tuning, and apically shifted positions of peak vibration. These findings explain discrepancies between previously published basilar membrane vibration and auditory nerve single unit data. Because the tectorial membrane directly overlies the inner hair cell stereociliary bundles, these data provide the most accurate characterization of the stimulus shaping the afferent auditory response available to date.hearing | cochlea | mechanics | vibrometry | auditory system

Two-Tone Suppression of Simultaneous Electrical and Mechanical Responses in the Cochlea

“…at frequencies around BF were observed in the guinea pig base (12), and similarly sized phase lags noted for TM re: BM motions in the mouse apex (13). A recent article by Lee et al (14), confirmed phase differences of fairly small magnitude at frequencies up to the BF.…”

“…These response notches are likely caused by motions within the cochlear partition that excite OHCs. Measurements using optical coherence tomography (OCT) have observed differential motion between intracochlear structures (e.g., (12)(13)(14)), and cochlear models show that these different motions could be due to coupled traveling waves on the tectorial membrane (TM) and BM (15)(16)(17). The OCT results, as well as two-scala pressure measurements in scala media and ST, indicate fairly tightly coupling between TM and BM traveling waves (18).…”

Finite-element model of the active organ of Corti

“…The dynamics of the cochlea is characterized by values of the Reynolds number well below one; furthermore, the organ of Corti moves by only a very small fraction of its size [4]. Therefore, the nonlinearities originating from large deformations, which are important in most classical engineering problems involving flows, can be neglected here.…”

Investigation of middle ear anatomy and function with combined video otoscopy-phase sensitive OCT

“…However, the functional characteristics of the middle ear structures cannot be analyzed with conventional OCT. A newer variant of OCT, called phase-sensitive OCT, has been used by some research groups both in animal models, such as mice and chinchillas, to measure the vibration of the middle ear TM and the ossicles [20], as well as to assess the functionality of the inner ear (e.g. the cochlea) [21]. The vibrational amplitudes of the malleus and the incus through the TM in cadaveric human ears have also been measured with phase sensitive when a 0.5 kHz sound stimulus was used [22].…”