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

Mechanical properties of human tumour tissues and their implications for cancer development

The mechanical properties of cells and tissues help determine their architecture, composition and function. Alterations to these properties are associated with many diseases, including cancer. Tensional, compressive, adhesive, elastic and viscous properties of individual cells and multicellular tissues are mostly regulated by reorganization of the actomyosin and microtubule cytoskeletons and extracellular glycocalyx, which in turn drive many pathophysiological processes, including cancer progression. This Review provides an in-depth collection of quantitative data on diverse mechanical properties of living human cancer cells and tissues. Additionally, the implications of mechanical property changes for cancer development are discussed. An increased knowledge of the mechanical properties of the tumour microenvironment, as collected using biomechanical approaches capable of multi-timescale and multiparametric analyses, will provide a better understanding of the complex mechanical determinants of cancer organization and progression. This information can lead to a further understanding of resistance mechanisms to chemotherapies and immunotherapies and the metastatic cascade.

Similar Tuning of Distortion-Product Otoacoustic Emission Ratio Functions and Cochlear Vibrations in Mice

When elicited by two stimulus tones (at frequencies f1 and f2, f2 > f1), the amplitudes of specific distortion-product otoacoustic emission (DPOAE) components exhibit a characteristic bandpass shape as the ratio between f2 and f1 is varied. This bandpass shape has been attributed to various mechanisms including intracochlear resonance, suppression, and wave interference, and has been proposed to be related to cochlear frequency tuning. While human studies suggest modest correlations between psychophysical tuning and the tuning of DPOAE amplitude vs. f2/f1 ratio functions, a relationship between the latter and the tuning of cochlear mechanical responses has yet to be established. This was addressed here through direct comparisons of DPOAEs and cochlear vibrations in wild-type CBA/CaJ mice. DPOAEs were elicited using a fixed-f2, swept-f1 paradigm, and optical coherence tomography was used to measure displacements from cochlear locations with characteristic frequencies near f2. The tuning sharpness of 2f1-f2 DPOAE ratio functions was found to be remarkably similar to that of basilar membrane and/or tectorial membrane responses to single tones, with the tuning sharpness of all responses increasing similarly with decreasing stimulus level. This relationship was observed for f2 frequencies ranging from ~8 to 22 kHz. Intracochlear distortion products did not exhibit a bandpass shape as the f2/f1 ratio was varied, indicating that interference between distortion products traveling to the stapes may be responsible for the tuning of the DPOAE ratio function. While these findings suggest that DPOAE ratio functions could be used to noninvasively infer cochlear tuning, it remains to be determined whether this relationship holds in other species and for lower frequency regions.

Vibration modes of three-dimensional spiral cochlea covering the organ of Corti

So far, explaining the mechanism on active phonosensitive amplification in the cochlea is a major and difficult medical question. Among them, one of the key problems is that the motion pattern of the organ of Corti (OC) is still unknown. To this end, a multi-scale cochlear model including a three-dimensional spiral OC was established based on CT data and light source imaging experimental data, which complete combined the macroscopic and microscopic structure. On the basis of verifying the reliability of the model, acoustic-solid coupling calculation and modal analysis were performed on the model, and the vibration modes of basilar membrane (BM) and structures of the OC at different characteristic frequencies were discussed. The results show that tectorial membrane (TM) exhibits completely different vibration modes from BM at low frequencies, while the two movements gradually synchronize as the frequency increases. The amplitude position of OC's motion moves laterally with increasing frequency from Deiters' cells to Hensen's cells and then back to Deiters' cells. The OC exhibits longitudinal vibrations following BM when BM's displacement is large, while it manifests more as lateral movement of Deiters' cells when BM's displacement is small. This model can well simulate the motion process of BM and OC in the lymphatic fluid, which provides theoretical support and a numerical simulation computational platform to explore the interaction between macroscopic and microscopic tissue structures of the overall cochlea.

Multi-window approach enables two-fold improvement in OCT axial resolution with strong side-lobe suppression and improved phase sensitivity

A common processing approach for optical coherence tomography (OCT) uses a window function (e.g., Hann or rectangular window) for spectral shaping prior to calculating the Fourier transform. Here we build on a multi-window approach [Opt. Express8, 5267 (2017)10.1364/BOE.8.005267] that enables improved resolution while still suppressing side-lobe intensity. The shape of the window function defines the trade-off between main-lobe width (resolution) and side-lobe intensity. We have extended the approach to include the interferometric phase for phase-sensitive applications like vibrometry and Doppler OCT. Using the Hann window as a reference, we show that 11 Taylor windows are sufficient to achieve 50% improvement in axial resolution, -31 dB side-lobe intensity, and 20% improvement in phase sensitivity with low computational cost.

Linear mixed-effect modeling of organ of Corti vibratory tuning curves

Optical coherence tomography has become the most popular approach to experimental measures of sound-induced vibrations within the mammalian cochlea. Because it is relatively easy to use and works in the unopened cochlea, the measurement of vibratory tuning curves has become highly reliable, and averaging data from multiple animals in different experimental cohorts is now possible. Here I tested a modern statistical approach to compare cohorts for differences in the magnitude and phase of vibration. A linear mixed-effect approach with first, second, third, and fourth-order models to fit the data was tested. The third-order model best fit both the magnitude and phase data without having terms that did not contribute substantively to improving the R2 or the p-value for the independent variables. It identified a difference between cohorts of mice that were different and no difference between cohorts that should not be different. Thus, this approach provides a way to simply compare a full set of tuning curves between cohorts. While further analyses by the investigator will always be needed to study specific details related to the study hypothesis, this statistical technique provides a simple way for the cochlear physiologist to perform an initial assessment of whether the cohorts are same or different.

On the Tonotopy of the Low-Frequency Region of the Cochlea

It is generally assumed that frequency selectivity varies along the cochlea. For example, at the base of the cochlea, which is a region sensitive to high-frequency sounds, the best frequency of a cochlear location increases toward the most basal end, that is, near the stapes. Response phases also vary along cochlear locations. At any given frequency, there is a decrease in phase lag toward the stapes. This tonotopic arrangement in the cochlea was originally described by Georg von Békésy in a seminal series of experiments on human cadavers and has been confirmed in more recent works on live laboratory animals. Nonetheless, our knowledge of tonotopy at the apex of the cochlea remains incomplete in animals with low-frequency hearing, which is relevant to human speech. The results of our experiments on guinea pig, gerbil, and chinchilla cochleas, regardless of the sex of the animal, show that responses to sound differ at locations across the apex in a pattern consistent with previous studies of the base of the cochlea.SIGNIFICANCE STATEMENT Tonotopy is an important property of the auditory system that has been shown to exist in many auditory centers. In fact, most auditory implants work on the assumption of its existence by assigning different frequencies to different stimulating electrodes based on their location. At the level of the basilar membrane in the cochlea, a tonotopic arrangement implies that high-frequency stimuli evoke largest displacements at the base, near the ossicles, and low-frequency sounds have their greatest effects at the apex. Although tonotopy has been confirmed at the base of the cochlea on live animals at the apex of the cochlea, however, it has been less studied. Here, we show that a tonotopic arrangement does exist at the apex of the cochlea.

Bandpass Shape of Distortion-Product Otoacoustic Emission Ratio Functions Reflects Cochlear Frequency Tuning in Normal-Hearing Mice

The frequency selectivity of the mammalian auditory system is critical for discriminating complex sounds like speech. This selectivity derives from the sharp tuning of the cochlea's mechanical response to sound, which is largely attributed to the amplification of cochlear vibrations by outer hair cells (OHCs). Due to its nonlinearity, the amplification process also leads to the generation of distortion products (DPs), some of which propagate out to the ear canal as DP otoacoustic emissions (DPOAEs). However, the insight that these signals provide about the tuned micro- and macro-mechanics underlying their generation remains unclear. Using optical coherence tomography to measure cochlear vibrations in mice, we show that the cochlea's frequency tuning is reflected in the bandpass shape that is observed in DPOAE amplitudes when the ratio of the two evoking stimulus frequencies is varied (here termed DPOAE "ratio functions"). The tuning sharpness of DPOAE ratio functions and cochlear vibrations co-varied with stimulus level, with a similar quantitative agreement in tuning sharpness observed for both apical and mid-cochlear locations. Measurement of intracochlear DPs revealed that the tuning of the DPOAE ratio functions was not caused by mechanisms that shape DPs locally near where they are generated. Instead, simple model simulations indicate that the bandpass shape is due to a more global wave interference phenomenon. It appears that the filtering of DPOAEs by wave interactions over an extended spatial region allows them to provide a window onto the frequency tuning of single cochlear locations.

The Remarkable Outer Hair Cell: Proceedings of a Symposium in Honour of W. E. Brownell

In 1985, Bill Brownell and colleagues published the remarkable observation that cochlear outer hair cells (OHCs) express voltage-driven mechanical motion: electromotility. They proposed OHC electromotility as the mechanism for the elusive "cochlear amplifier" required to explain the sensitivity of mammalian hearing. The finding and hypothesis stimulated an explosion of experiments that have transformed our understanding of cochlear mechanics and physiology, the evolution of hair cell structure and function, and audiology. Here, we bring together examples of current research that illustrate the continuing impact of the discovery of OHC electromotility.

A chemo-mechanical cochleostomy preserves hearing for the in vivo functional imaging of cochlear cells

In vivo and real-time multicellular imaging enables the decoding of sensory circuits and the tracking of systemic drug uptake. However, in vivo imaging of the auditory periphery remains technically challenging owing to the deep location, mechanosensitivity and fluid-filled, bone-encased nature of the cochlear structure. Existing methods that expose the cochlea invariably cause irreversible damage to auditory function, severely limiting the experimental measurements possible in living animals. Here we present an in vivo surgical protocol that permits the imaging of cochlear cells in hearing mice. Our protocol describes a ventro-lateral approach for preserving external and middle ear structures while performing surgery, the correct mouse positioning for imaging cochlear cells with effective sound transmission into the ear, the chemo-mechanical cochleostomy for creating the imaging window in the otic capsule bone that prevents intracochlear fluid leakage by maintaining an intact endosteum, and the release of intracochlear pressure that separates the endosteum from the otic capsule bone while creating an imaging window. The procedure thus preserves hearing thresholds. Individual inner and outer hair cells, supporting cells and nerve fibers can be visualized in vivo while hearing function is preserved. This approach may enable future original investigations, such as the real-time tracking of ototoxic drug transport into the cochleae. The technique may be applied to the monitoring of sound-evoked functional activity in multiple cochlear cells, in combination with optogenetic tools, and may help to improve cochlear implantation in humans. The cochleostomy takes ~1 h and requires experience in surgery.

Cochlear motion across the reticular lamina implies that it is not a stiff plate

Within the cochlea, the basilar membrane (BM) is coupled to the reticular lamina (RL) through three rows of piezo-like outer hair cells (OHCs) and supporting cells that endow mammals with sensitive hearing. Anatomical differences across OHC rows suggest differences in their motion. Using optical coherence tomography, we measured in vivo and postmortem displacements through the gerbil round-window membrane from approximately the 40-47 kHz best-frequency (BF) regions. Our high spatial resolution allowed measurements across the RL surface at the tops of the three rows of individual OHCs and their bottoms, and across the BM. RL motion varied radially; the third-row gain was more than 3 times greater than that of the first row near BF, whereas the OHC-bottom motions remained similar. This implies that the RL mosaic, comprised of OHC and phalangeal-process tops joined together by adhesion molecules, is much more flexible than the Deiters' cells connected to the OHCs at their bottom surfaces. Postmortem, the measured points moved together approximately in phase. These imply that in vivo, the RL does not move as a stiff plate hinging around the pillar-cell heads near the first row as has been assumed, but that its mosaic-like structure may instead bend and/or stretch.

The impact of targeted ablation of one row of outer hair cells and Deiters' cells on cochlear amplification

The mammalian cochlea contains three rows of outer hair cells (OHCs) that amplify the basilar membrane traveling wave with high gain and exquisite tuning. The pattern of OHC loss caused by typical methods of producing hearing loss in animal models (noise, ototoxic exposure, or aging) is variable and not consistent along the length of the cochlea. Thus, it is difficult to use these approaches to understand how forces from multiple OHCs summate to create normal cochlear amplification. Here, we selectively removed the third row of OHCs and Deiters' cells in adult mice and measured cochlear amplification. In the mature cochlear epithelia, expression of the Wnt target gene Lgr5 is restricted to the third row of Deiters' cells, the supporting cells directly underneath the OHCs. Diphtheria toxin administration to Lgr5DTR-EGFP/+ mice selectively ablated the third row of Deiters' cells and the third row of OHCs. Basilar membrane vibration in vivo demonstrated disproportionately lower reduction in cochlear amplification by about 13.5 dB. On a linear scale, this means that the 33% reduction in OHC number led to a 79% reduction in gain. Thus, these experimental data describe the impact of reducing the force of cochlear amplification by a specific amount. Furthermore, these data argue that because OHC forces progressively and sequentially amplify the traveling wave as it travels to its peak, the loss of even a relatively small number of OHCs, when evenly distributed longitudinally, will cause a substantial reduction in cochlear amplification.NEW & NOTEWORTHY Normal cochlear physiology involves force production from three rows of outer hair cells to amplify and tune the traveling wave. Here, we used a genetic approach to target and ablate the third row of outer hair cells in the mouse cochlea and found it reduced cochlear amplification by 79%. This means that the loss of even a relatively small number of OHCs, when evenly distributed, causes a substantial reduction in cochlear amplification.

Unloading outer hair cell bundles in vivo does not yield evidence of spontaneous oscillations in the mouse cochlea

Along with outer hair cell (OHC) somatic electromotility as the actuator of cochlear amplification, active hair bundle motility may be a complementary mechanism in the mammalian auditory system. Here, we searched the mouse cochlea for the presence of spontaneous bundle oscillations that have been observed in non-mammalian ears. In those systems, removal of the overlying membrane is necessary for spontaneous bundle oscillations to manifest. Thus, we used a genetic mouse model with a C1509G (cysteine-to-glycine) point mutation in the Tecta gene where the tectorial (TM) is lifted away from the OHC bundles, allowing us to explore whether unloaded bundles spontaneously oscillate. We used VOCTV in vivo to detect OHC length changes due to electromotility as a proxy for the spontaneous opening and closing of the mechanoelectrical transduction (MET) channels associated with bundle oscillation. In wild type mice with the TM attached to OHC bundles, we did find peaks in vibratory magnitude spectra. Such peaks were not observed in the mutants where the TM is detached from the OHC bundles. Statistical analysis of the time signals indicates that these peaks do not signify active oscillations. Rather, they are filtered responses of the sensitive wild type cochlea to weak background noise. We therefore conclude that, to the limits of our system (∼30 pm), there is no spontaneous mechanical activity that manifests as oscillations in OHC electromotility within the mouse cochlea, arguing that unloaded OHC bundles do not oscillate in vivo. This article is part of the Special Issue Outer hair cell Edited by Joseph Santos-Sacchi and Kumar Navaratnam.

Role of Oxidative Stress and Antioxidants in Acquired Inner Ear Disorders

Oxygen metabolism in the mitochondria is essential for biological activity, and reactive oxygen species (ROS) are produced simultaneously in the cell. Once an imbalance between ROS production and degradation (oxidative stress) occurs, cells are damaged. Sensory organs, especially those for hearing, are constantly exposed during daily life. Therefore, almost all mammalian species are liable to hearing loss depending on their environment. In the auditory pathway, hair cells, spiral ganglion cells, and the stria vascularis, where mitochondria are abundant, are the main targets of ROS. Excessive generation of ROS in auditory sensory organs is widely known to cause sensorineural hearing loss, and mitochondria-targeted antioxidants are candidates for treatment. This review focuses on the relationship between acquired hearing loss and antioxidant use to provide an overview of novel antioxidants, namely medicines, supplemental nutrients, and natural foods, based on clinical, animal, and cultured-cell studies.

Interplay between traveling wave propagation and amplification at the apex of the mouse cochlea

Sounds entering the mammalian ear produce waves that travel from the base to the apex of the cochlea. An electromechanical active process amplifies traveling wave motions and enables sound processing over a broad range of frequencies and intensities. The cochlear amplifier requires combining the global traveling wave with the local cellular processes that change along the length of the cochlea given the gradual changes in hair cell and supporting cell anatomy and physiology. Thus, we measured basilar membrane (BM) traveling waves in vivo along the apical turn of the mouse cochlea using volumetric optical coherence tomography and vibrometry. We found that there was a gradual reduction in key features of the active process toward the apex. For example, the gain decreased from 23 to 19 dB and tuning sharpness decreased from 2.5 to 1.4. Furthermore, we measured the frequency and intensity dependence of traveling wave properties. The phase velocity was larger than the group velocity, and both quantities gradually decrease from the base to the apex denoting a strong dispersion characteristic near the helicotrema. Moreover, we found that the spatial wavelength along the BM was highly level dependent in vivo, such that increasing the sound intensity from 30 to 90 dB sound pressure level increased the wavelength from 504 to 874 μm, a factor of 1.73. We hypothesize that this wavelength variation with sound intensity gives rise to an increase of the fluid-loaded mass on the BM and tunes its local resonance frequency. Together, these data demonstrate a strong interplay between the traveling wave propagation and amplification along the length of the cochlea.

Thyroid tumor ratio: Improving the assessment of the impact of size in pediatric thyroid cancer

BACKGROUND

The impact of thyroid nodule size is less useful in children who have smaller thyroid volumes than in adults. We investigate using a novel thyroid tumor ratio measurement in children with thyroid cancer.

METHODS

Patient and pathologic characteristics were investigated via Student's t-test in a univariate analysis for any correlation with the log-transformed tumor ratio, followed by a multivariate linear regression.

RESULTS

Of 75 patients with malignancy and tumor ratio information, mean ratio decreased with increasing age (p = 0.04). Out of several clinical factors, patients with lymph node metastases and those treated with postoperative radioactive iodine had significantly higher mean tumor ratios on multivariate analysis (p = 0.04 for both factors).

CONCLUSIONS

Our study is the first to describe thyroid tumor volume in pediatric thyroid cancer and shows that increased tumor ratio was associated with indicators of more advanced disease such as lymph node metastases and use of radioactive iodine.

Signatures of cochlear processing in neuronal coding of auditory information

The vertebrate ear is endowed with remarkable perceptual capabilities. The faintest sounds produce vibrations of magnitudes comparable to those generated by thermal noise and can nonetheless be detected through efficient amplification of small acoustic stimuli. Two mechanisms have been proposed to underlie such sound amplification in the mammalian cochlea: somatic electromotility and active hair-bundle motility. These biomechanical mechanisms may work in concert to tune auditory sensitivity. In addition to amplitude sensitivity, the hearing system shows exceptional frequency discrimination allowing mammals to distinguish complex sounds with great accuracy. For instance, although the wide hearing range of humans encompasses frequencies from 20 Hz to 20 kHz, our frequency resolution extends to one-thirtieth of the interval between successive keys on a piano. In this article, we review the different cochlear mechanisms underlying sound encoding in the auditory system, with a particular focus on the frequency decomposition of sounds. The relation between peak frequency of activation and location along the cochlea - known as tonotopy - arises from multiple gradients in biophysical properties of the sensory epithelium. Tonotopic mapping represents a major organizational principle both in the peripheral hearing system and in higher processing levels and permits the spectral decomposition of complex tones. The ribbon synapses connecting sensory hair cells to auditory afferents and the downstream spiral ganglion neurons are also tuned to process periodic stimuli according to their preferred frequency. Though sensory hair cells and neurons necessarily filter signals beyond a few kHz, many animals can hear well beyond this range. We finally describe how the cochlear structure shapes the neural code for further processing in order to send meaningful information to the brain. Both the phase-locked response of auditory nerve fibers and tonotopy are key to decode sound frequency information and place specific constraints on the downstream neuronal network.

Cochlear Fluid Spaces and Structures of the Gerbil High-Frequency Region Measured Using Optical Coherence Tomography (OCT)

Since it has been difficult to directly observe the morphology of the living cochlea, our ability to infer the mechanical functioning of the living ear has been limited. Nearly all our knowledge about cochlear morphology comes from postmortem tissue that was fixed and processed using procedures that possibly distort the structures and fluid spaces of the organ of Corti. In this study, optical coherence tomography was employed to obtain volumetric images of the high-frequency hook region of the gerbil cochlea, as viewed through the round window, with far better resolution capability than had been possible before. The anatomical structures and fluid spaces of the organ of Corti were segmented and quantified in vivo and over a 90-min postmortem period. We find that the arcuate-zone and pectinate-zone widths change very little postmortem. The volume of the scala tympani between the round-window membrane and basilar membrane and the volume of the inner spiral sulcus decrease in the first 60-min postmortem. While textbook drawings of the mammalian organ of Corti and cortilymph prominently depict the tunnel of Corti, the outer tunnel is typically missing. This is likely because textbook drawings are typically made from images obtained by histological methods. Here, we show that the outer tunnel is nearly twice as big as the tunnel of Corti or the space of Nuel. This larger outer tunnel fluid space could have a substantial, little-appreciated effect on cochlear micromechanics. We speculate that the outer tunnel forms a resonant structure that may affect reticular-lamina motion.

Vector of motion measurements in the living cochlea using a 3D OCT vibrometry system

Optical coherence tomography (OCT) has become an important tool for measuring the vibratory response of the living cochlea. It stands alone in its capacity to measure the intricate motion of the hearing organ through the surrounding otic capsule bone. Nevertheless, as an extension of phase-sensitive OCT, it is only capable of measuring motion along the optical axis. Hence, measurements are 1-D. To overcome this limitation and provide a measure of the 3-D vector of motion in the cochlea, we developed an OCT system with three sample arms in a single interferometer. Taking advantage of the long coherence length of our swept laser, we depth (frequency) encode the three channels. An algorithm to depth decode and coregister the three channels is followed by a coordinate transformation that takes the vibrational data from the experimental coordinate system to Cartesian or spherical polar coordinates. The system was validated using a piezo as a known vibrating element that could be positioned at various angles. The angular measurement on the piezo was shown to have an RMSE of ≤ 0.30° (5.2 mrad) with a standard deviation of the amplitude of ≤ 120 pm. Finally, we demonstrate the system for in vivo imaging by measuring the vector of motion over a volume image in the apex of the mouse cochlea.

Intrinsic mechanical sensitivity of mammalian auditory neurons as a contributor to sound-driven neural activity

Mechanosensation – by which mechanical stimuli are converted into a neuronal signal – is the basis for the sensory systems of hearing, balance, and touch. Mechanosensation is unmatched in speed and its diverse range of sensitivities, reaching its highest temporal limits with the sense of hearing; however, hair cells (HCs) and the auditory nerve (AN) serve as obligatory bottlenecks for sounds to engage the brain. Like other sensory neurons, auditory neurons use the canonical pathway for neurotransmission and millisecond-duration action potentials (APs). How the auditory system utilizes the relatively slow transmission mechanisms to achieve ultrafast speed, and high audio-frequency hearing remains an enigma. Here, we address this paradox and report that the mouse, and chinchilla, AN are mechanically sensitive, and minute mechanical displacement profoundly affects its response properties. Sound-mimicking sinusoidal mechanical and electrical current stimuli affect phase-locked responses. In a phase-dependent manner, the two stimuli can also evoke suppressive responses. We propose that mechanical sensitivity interacts with synaptic responses to shape responses in the AN, including frequency tuning and temporal phase locking. Combining neurotransmission and mechanical sensation to control spike patterns gives the mammalian AN a secondary receptor role, an emerging theme in primary neuronal functions.

In vivo real-time imaging reveals megalin as the aminoglycoside gentamicin transporter into cochlea whose inhibition is otoprotective

Aminoglycosides (AGs) are commonly used antibiotics that cause deafness through the irreversible loss of cochlear sensory hair cells (HCs). How AGs enter the cochlea and then target HCs remains unresolved. Here, we performed time-lapse multicellular imaging of cochlea in live adult hearing mice via a chemo-mechanical cochleostomy. The in vivo tracking revealed that systemically administered Texas Red-labeled gentamicin (GTTR) enters the cochlea via the stria vascularis and then HCs selectively. GTTR uptake into HCs was completely abolished in transmembrane channel-like protein 1 (TMC1) knockout mice, indicating mechanotransducer channel-dependent AG uptake. Blockage of megalin, the candidate AG transporter in the stria vascularis, by binding competitor cilastatin prevented GTTR accumulation in HCs. Furthermore, cilastatin treatment markedly reduced AG-induced HC degeneration and hearing loss in vivo. Together, our in vivo real-time tracking of megalin-dependent AG transport across the blood-labyrinth barrier identifies new therapeutic targets for preventing AG-induced ototoxicity.

Endolymphatic Hydrops is a Marker of Synaptopathy Following Traumatic Noise Exposure

After acoustic trauma, there can be loss of synaptic connections between inner hair cells and auditory neurons in the cochlea, which may lead to hearing abnormalities including speech-in-noise difficulties, tinnitus, and hyperacusis. We have previously studied mice with blast-induced cochlear synaptopathy and found that they also developed a build-up of endolymph, termed endolymphatic hydrops. In this study, we used optical coherence tomography to measure endolymph volume in live CBA/CaJ mice exposed to various noise intensities. We quantified the number of synaptic ribbons and postsynaptic densities under the inner hair cells 1 week after noise exposure to determine if they correlated with acute changes in endolymph volume measured in the hours after the noise exposure. After 2 h of noise at an intensity of 95 dB SPL or below, both endolymph volume and synaptic counts remained normal. After exposure to 2 h of 100 dB SPL noise, mice developed endolymphatic hydrops and had reduced synaptic counts in the basal and middle regions of the cochlea. Furthermore, round-window application of hypertonic saline reduced the degree of endolymphatic hydrops that developed after 100 dB SPL noise exposure and partially prevented the reduction in synaptic counts in the cochlear base. Taken together, these results indicate that endolymphatic hydrops correlates with noise-induced cochlear synaptopathy, suggesting that these two pathologic findings have a common mechanistic basis.

Age-related degradation of tectorial membrane dynamics with loss of CEACAM16

Studies of genetic disorders of sensorineural hearing loss have been instrumental in delineating mechanisms that underlie the remarkable sensitivity and selectivity that are hallmarks of mammalian hearing. For example, genetic modifications of TECTA and TECTB, which are principal proteins that comprise the tectorial membrane (TM), have been shown to alter auditory thresholds and frequency tuning in ways that can be understood in terms of changes in the mechanical properties of the TM. Here, we investigate effects of genetic modification targeting CEACAM16, a third important TM protein. Loss of CEACAM16 has been recently shown to lead to progressive reductions in sensitivity. Whereas age-related hearing losses have previously been linked to changes in sensory receptor cells, the role of the TM in progressive hearing loss is largely unknown. Here, we show that TM stiffness and viscosity are significantly reduced in adult mice that lack functional CEACAM16 relative to age-matched wild-type controls. By contrast, these same mechanical properties of TMs from juvenile mice that lack functional CEACAM16 are more similar to those of wild-type mice. Thus, changes in hearing phenotype align with changes in TM material properties and can be understood in terms of the same TM wave properties that were previously used to characterize modifications of TECTA and TECTB. These results demonstrate that CEACAM16 is essential for maintaining TM mechanical and wave properties, which in turn are necessary for sustaining the remarkable sensitivity and selectivity of mammalian hearing with increasing age.

Cochlear outer hair cell electromotility enhances organ of Corti motion on a cycle-by-cycle basis at high frequencies in vivo

Mammalian hearing depends on an amplification process involving prestin, a voltage-sensitive motor protein that enables cochlear outer hair cells (OHCs) to change length and generate force. However, it has been questioned whether this prestin-based somatic electromotility can operate fast enough in vivo to amplify cochlear vibrations at the high frequencies that mammals hear. In this study, we measured sound-evoked vibrations from within the living mouse cochlea and found that the top and bottom of the OHCs move in opposite directions at frequencies exceeding 20 kHz, consistent with fast somatic length changes. These motions are physiologically vulnerable, depend on prestin, and dominate the cochlea's vibratory response to high-frequency sound. This dominance was observed despite mechanisms that clearly low-pass filter the in vivo electromotile response. Low-pass filtering therefore does not critically limit the OHC's ability to move the organ of Corti on a cycle-by-cycle basis. Our data argue that electromotility serves as the primary high-frequency amplifying mechanism within the mammalian cochlea.

Rectifying and sluggish: Outer hair cells as regulators rather than amplifiers

In the cochlea, mechano-electrical transduction is preceded by dynamic range compression. Outer hair cells (OHCs) and their voltage dependent length changes, known as electromotility, play a central role in this compression process, but the exact mechanisms are poorly understood. Here we review old and new experimental findings and show that (1) just audible high-frequency tones evoke an ∼1-microvolt AC receptor potential in basal OHCs; (2) any mechanical amplification of soft high-frequency tones by OHC motility would have an adverse effect on their audibility; (3) having a higher basolateral K+ conductance, while increasing the OHC corner frequency, does not boost the magnitude of the high-frequency AC receptor potential; (4) OHC receptor currents display a substantial rectified (DC) component; (5) mechanical DC responses (baseline shifts) to acoustic stimuli, while insignificant on the basilar membrane, can be comparable in magnitude to AC responses when recorded in the organ of Corti, both in the apex and the base. In the basal turn, the DC component may even exceed the AC component, lending support to Dallos' suggestion that both apical and basal OHCs display a significant degree of rectification. We further show that (6) low-intensity cochlear traveling waves, by virtue of their abrupt transition from fast to slow propagation, are well suited to transport high-frequency energy with minimal losses (∼2-dB loss for 16-kHz tones in the gerbil); (7) a 90-dB, 16-kHz tone, if transmitted without loss to its tonotopic place, would evoke a destructive displacement amplitude of 564 nm. We interpret these findings in a framework in which local dissipation is regulated by OHC motility.

Fast time-domain solution of a nonlinear three-dimensional cochlear model using the fast Fourier transform

A fast numerical time-domain solution of a nonlinear three-dimensional (3D) cochlear model is proposed. In dynamical systems, a time-domain solution can determine nonlinear responses, and the human faculty of hearing depends on nonlinear behaviors of the microscopically structured organs of the cochlea. Thus, time-domain 3D modeling can help explain hearing. The matrix product, an n² operation, is a central part of the time-domain solution procedure in cochlear models. To solve the cochlear model faster, the fast Fourier transform (FFT), an n log n operation, is used to replace the matrix product. Numerical simulation results verified the similarity of the matrix product and the FFT under coarse grid settings. Furthermore, applying the FFT reduced the computation time by a factor of up to 100 owing to the computational complexity of the proposed approach being reduced from n² to n log n. Additionally, the proposed method successfully computed 3D models under moderate and fine grid settings that were unsolvable using the matrix product. The 3D cochlear model exhibited nonlinear responses for pure tones and clicks under various gain distributions in a time-domain simulation. Thus, the FFT-based method provides fast numerical solutions and supports the development of 3D models for cochlear mechanics.

Fluid Jet Stimulation of Auditory Hair Bundles Reveal Spatial Non-uniformities and Two Viscoelastic-Like Mechanisms

Hair cell mechanosensitivity resides in the sensory hair bundle, an apical protrusion of actin-filled stereocilia arranged in a staircase pattern. Hair bundle deflection activates mechano-electric transduction (MET) ion channels located near the tops of the shorter rows of stereocilia. The elicited macroscopic current is shaped by the hair bundle motion so that the mode of stimulation greatly influences the cell’s output. We present data quantifying the displacement of the whole outer hair cell bundle using high-speed imaging when stimulated with a fluid jet. We find a spatially non-uniform stimulation that results in splaying, where the hair bundle expands apart. Based on modeling, the splaying is predominantly due to fluid dynamics with a small contribution from hair bundle architecture. Additionally, in response to stimulation, the hair bundle exhibited a rapid motion followed by a slower motion in the same direction (creep) that is described by a double exponential process. The creep is consistent with originating from a linear passive system that can be modeled using two viscoelastic processes. These viscoelastic mechanisms are integral to describing the mechanics of the mammalian hair bundle.

In situ motions of individual inner-hair-cell stereocilia from stapes stimulation in adult mice

In vertebrate hearing organs, mechanical vibrations are converted to ionic currents through mechanoelectrical-transduction (MET) channels. Concerted stereocilia motion produces an ensemble MET current driving the hair-cell receptor potential. Mammalian cochleae are unique in that the tuning of sensory cells is determined by their mechanical environment and the mode of hair-bundle stimulation that their environment creates. However, little is known about the in situ intra-hair-bundle motions of stereocilia relative to one another, or to their environment. In this study, high-speed imaging allowed the stereocilium and cell-body motions of inner hair cells to be monitored in an ex vivo organ of Corti (OoC) mouse preparation. We have found that the OoC rotates about the base of the inner pillar cell, the hair bundle rotates about its base and lags behind the motion of the apical surface of the cell, and the individual stereocilia move semi-independently within a given hair bundle.

Time-domain analysis of a three-dimensional numerical model of the human spiral cochlea at medium intensity

For the processing and detection of speech and music, the human cochlea has an exquisite sensitivity and selectivity of frequency and a dynamic range. How the cochlea performs these remarkable functions has fascinated auditory scientists for decades. Because it is not possible to measure sound-induced vibrations within the cochlea in a living human being, mathematical modeling has played an important role in cochlear mechanics. For this study, a three-dimensional human cochlear model with a fluid‒structure coupling was constructed. Time-domain analysis was performed to calculate the displacement, velocity, and stress of the basilar membrane (BM) and osseous spiral lamina (OSL) at different times in response to a pure tone stimulus. The model reproduced the traveling-wave motion of the BM. The model also showed that the cochlea's spiral shape can induce asymmetrical mechanical behavior of the BM and cause cochlear fluid to move in a radial direction; this may contribute to human sound perception. The cochlea's spiral shape not only enhances a low-frequency vibration of the BM but also changes the maximization of the positions of vibration. Therefore, the spiral's characteristics play a key role in the cochlea's frequency selectivity for low-frequency sounds. And this suggests that the OSL can react to sound as quickly as the BM. Furthermore, the basal region of the BM tends to have more stress than its other regions, and this may explain the clinical observation that human sensorineural hearing loss often occurs at high frequencies.

In vivo functional imaging of the human middle ear with a hand-held optical coherence tomography device

We describe an optical coherence tomography and vibrometry system designed for portable hand-held usage in the otology clinic on awake patients. The system provides clinically relevant point-of-care morphological imaging with 14-44 µm resolution and functional vibratory measures with sub-nanometer sensitivity. We evaluated various new approaches for extracting functional information including a multi-tone stimulus, a continuous chirp stimulus, and alternating air and bone stimulus. We also explored the vibratory response over an area of the tympanic membrane (TM) and generated TM thickness maps. Our results suggest that the system can provide real-time in vivo imaging and vibrometry of the ear and could prove useful for investigating otologic pathology in the clinic setting.

In Vivo Cochlear imaging provides a tool to study endolymphatic hydrops

Exposure to noise trauma, such as that from improvised explosive devices, can lead to sensorineural hearing loss and a reduced quality of life. In order to elucidate the mechanisms underlying noise-induced hearing loss, we have adapted optical coherence tomography (OCT) for real-time cochlear visualization in live mice after blast exposure. We demonstrated that endolymphatic hydrops develops following blast injury, and that this phenomenon may be associated with glutamate excitotoxicity and cochlear synaptopathy. Additionally, osmotic stabilization of endolymphatic hydrops partially rescues cochlear synapses after blast trauma. OCT is thus a valuable research tool for investigating the mechanisms underlying acoustic trauma and dynamic changes in endolymph volume. It may also help with the diagnosis and treatment of human hearing loss and/or vertigo in the near future.

Mechanisms in cochlear hair cell mechano-electrical transduction for acquisition of sound frequency and intensity

Sound signals are acquired and digitized in the cochlea by the hair cells that further transmit the coded information to the central auditory pathways. Any defect in hair cell function may induce problems in the auditory system and hearing-based brain function. In the past 2 decades, our understanding of auditory transduction has been substantially deepened because of advances in molecular, structural, and functional studies. Results from these experiments can be perfectly embedded in the previously established profile from anatomical, histological, genetic, and biophysical research. This review aims to summarize the progress on the molecular and cellular mechanisms of the mechano-electrical transduction (MET) channel in the cochlear hair cells, which is involved in the acquisition of sound frequency and intensity-the two major parameters of an acoustic cue. We also discuss recent studies on TMC1, the molecule likely to form the MET channel pore.

Rapid optical tomographic vibrometry using a swept multi-gigahertz comb

We propose a rapid tomographic vibrometer technique using an optical comb to measure internal vibrations, transient phenomena, and tomographic distributions in biological tissue and microelectromechanical system devices at high frequencies. This method allows phase-sensitive tomographic measurement in the depth direction at a multi-MHz scan rate using a frequency-modulated broadband electrooptic multi-GHz supercontinuum comb. The frequency spacing was swept instantaneously in time and axisymmetrically about the center wavelength via a dual-drive Mach-Zehnder modulator driven by a variable radio frequency signal. This unique sweeping method permits direct measurement of fringe-free interferometric amplitude and phase with arbitrarily changeable measurement range and scan rate. Therefore, a compressive measurement can be made in only the depth region where the vibration exists, reducing the number of measurement points. In a proof-of-principle experiment, the interferometric amplitude and phase were investigated for in-phase and quadrature phase-shifted interferograms obtained by a polarization demodulator. Tomographic transient displacement measurements were performed using a 0.12 mm thick glass film and piezo-electric transducer oscillating at 10-100 kHz with scan rates in the range 1-20 MHz. The depth resolution and precision of the vibrometer were estimated to be approximately 25 µm and 1.0 nm, respectively.

Cochlear supporting cells require GAS2 for cytoskeletal architecture and hearing

In mammals, sound is detected by mechanosensory hair cells that are activated in response to vibrations at frequency-dependent positions along the cochlear duct. We demonstrate that inner ear supporting cells provide a structural framework for transmitting sound energy through the cochlear partition. Humans and mice with mutations in GAS2, encoding a cytoskeletal regulatory protein, exhibit hearing loss due to disorganization and destabilization of microtubule bundles in pillar and Deiters' cells, two types of inner ear supporting cells with unique cytoskeletal specializations. Failure to maintain microtubule bundle integrity reduced supporting cell stiffness, which in turn altered cochlear micromechanics in Gas2 mutants. Vibratory responses to sound were measured in cochleae from live mice, revealing defects in the propagation and amplification of the traveling wave in Gas2 mutants. We propose that the microtubule bundling activity of GAS2 imparts supporting cells with mechanical properties for transmitting sound energy through the cochlea.

Quantitative spectroscopic comparison of the optical properties of mouse cochlea microstructures using optical coherence tomography at 1.06 µm and 1.3 µm wavelengths

Currently, the cochlear implantation procedure mainly relies on using a hand lens or surgical microscope, where the success rate and surgery time strongly depend on the surgeon's experience. Therefore, a real-time image guidance tool may facilitate the implantation procedure. In this study, we performed a systematic and quantitative analysis on the optical characterization of ex vivo mouse cochlear samples using two swept-source optical coherence tomography (OCT) systems operating at the 1.06-µm and 1.3-µm wavelengths. The analysis results demonstrated that the 1.06-µm OCT imaging system performed better than the 1.3-µm OCT imaging system in terms of the image contrast between the cochlear conduits and the neighboring cochlear bony wall structure. However, the 1.3-µm OCT imaging system allowed for greater imaging depth of the cochlear samples because of decreased tissue scattering. In addition, we have investigated the feasibility of identifying the electrode of the cochlear implant within the ex vivo cochlear sample with the 1.06-µm OCT imaging. The study results demonstrated the potential of developing an image guidance tool for the cochlea implantation procedure as well as other otorhinolaryngology applications.

Cochlear mechanics: new insights from vibrometry and Optical Coherence Tomography

The cochlea is a complex biological machine that transduces sound-induced mechanical vibrations to neural signals. Hair cells within the sensory tissue of the cochlea transduce vibrations into electrical signals, and exert electromechanical feedback that enhances the passive frequency separation provided by the cochlea's traveling wave mechanics; this enhancement is termed cochlear amplification. The vibration of the sensory tissue has been studied with many techniques, and the current state of the art is optical coherence tomography (OCT). The OCT technique allows for motion of intra-organ structures to be measured in vivo at many layers within the sensory tissue, at several angles and in previously under-explored species. OCT-based observations are already impacting our understanding of hair cell excitation and cochlear amplification.

Does the "Reticular Lamina Nonlinearity" Contribute to the Basal DPOAE Source?

The spatial extent of the cochlear region that actually contributes to the DPOAE signal measured in the ear canal may be evaluated experimentally using interference tones or computed numerically using nonlinear cochlear models. A nonlinear transmission-line cochlear model is used in this study to evaluate whether the recently reported nonlinear behavior of the reticular lamina (RL) over a wide basal region may be associated with generation of a significant distortion product otoacoustic emission (DPOAE) component. A two-degrees-of-freedom 1-D nonlinear model was used as discussed by Sisto et al. (2019), in which each local element consists of two coupled oscillators, roughly representing the basilar membrane (BM) and the RL. In this model, the RL shows a strongly nonlinear response over a wide region basal to the characteristic place, whereas the BM response is linear outside the narrow peak region. Such a model may be considered as that using the minimal number of degrees of freedom necessary to separately predict the motion of the BM and RL, while preserving important cochlear symmetries, such as the zero-crossing invariance of the impulse response. In the numerical simulations, the RL nonlinearity generates indeed a large intracochlear distortion product source, extended down to very basal cochlear regions. Nevertheless, due to the weak and indirect coupling between the RL motion and the differential fluid pressure in the basal part of the traveling wave path, no significant contribution from this mechanism is predicted by the model to the generation of the DPOAE signal that is eventually measured in the ear canal.

The interplay of organ-of-Corti vibrational modes, not tectorial- membrane resonance, sets outer-hair-cell stereocilia phase to produce cochlear amplification

The mechanical motions that deflect outer-hair-cell (OHC) stereocilia and the resulting effects of OHC motility are reviewed, concentrating on high-frequency cochlear regions. It has been proposed that a tectorial-membrane (TM) resonance makes the phase of OHC stereocilia motion be appropriate to produce cochlear amplification, i.e. so that the OHC force that pushes the basilar membrane (BM) is in the same direction as BM velocity. Evidence for and against the TM-resonance hypothesis are considered, including new cochlear-motion measurements using optical coherence tomography, and it is concluded that there is no such TM resonance. The evidence points to there being an advance in the phase of reticular lamina (RL) radial motion at a frequency approximately ½ octave below the BM characteristic frequency, and that this is the main source of the phase difference between the TM and RL radial motions that produces cochlear amplification. It appears that the change in phase of RL radial motion comes about because of a transition between different organ-of-Corti (OoC) vibrational modes that changes RL motion relative to BM and TM motion. The origins and consequences of the large phase change of RL radial motion relative to BM motion are considered; differences in the reported patterns of these changes may be due to different viewing angles. Detailed motion data and new models are needed to better specify the vibrational patterns of the OoC modes and the role of the various OoC structures in producing the modes and the mode transition.

Optical coherence tomography: current and future clinical applications in otology

PURPOSE OF REVIEW

This article reviews literature on the use of optical coherence tomography (OCT) in otology and provides the reader with a timely update on its current clinical and research applications. The discussion focuses on the principles of OCT, the use of the technology for the diagnosis of middle ear disease and for the delineation of in-vivo cochlear microarchitecture and function.

RECENT FINDINGS

Recent advances in OCT include the measurement of structural and vibratory properties of the tympanic membrane, ossicles and inner ear in healthy and diseased states. Accurate, noninvasive diagnosis of middle ear disease, such as otosclerosis and acute otitis media using OCT, has been validated in clinical studies, whereas inner ear OCT imaging remains at the preclinical stage. The development of recent microscopic, otoscopic and endoscopic systems to address clinical and research problems is reviewed.

SUMMARY

OCT is a real-time, noninvasive, nonionizing, point-of-care imaging modality capable of imaging ear structures in vivo. Although current clinical systems are mainly focused on middle ear imaging, OCT has also been shown to have the ability to identify inner ear disease, an exciting possibility that will become increasingly relevant with the advent of targeted inner ear therapies.

Methylene blue-filled biodegradable polymer particles as a contrast agent for optical coherence tomography

Optical coherence tomography (OCT) images largely lack molecular information or molecular contrast. We address that issue here, reporting on the development of biodegradable micro and nano-spheres loaded with methylene blue (MB) as molecular contrast agents for OCT. MB is a constituent of FDA approved therapies and widely used as a dye in off-label clinical applications. The sequestration of MB within the polymer reduced toxicity and improved signal strength by drastically reducing the production of singlet oxygen and leuco-MB. The former leads to tissue damage and the latter to reduced image contrast. The spheres are also strongly scattering which improves molecular contrast signal localization and enhances signal strength. We demonstrate that these contrast agents may be imaged using both pump-probe OCT and photothermal OCT, using a 830 nm frequency domain OCT system and a 1.3 µm swept source OCT system. We also show that these contrast agents may be functionalized and targeted to specific receptors, e.g. the VCAM receptor known to be overexpressed in inflammation.

Interactions between Passive and Active Vibrations in the Organ of Corti In Vitro

High sensitivity and selectivity of hearing require an active cochlea. The cochlear sensory epithelium, the organ of Corti, vibrates because of external and internal excitations. The external stimulation is acoustic pressures mediated by the scala fluids, whereas the internal excitation is generated by a type of sensory receptor cells (the outer hair cells) in response to the acoustic vibrations. The outer hair cells are cellular actuators that are responsible for cochlear amplification. The organ of Corti is highly structured for transmitting vibrations originating from acoustic pressure and active outer hair cell force to the inner hair cells that synapse on afferent nerves. Understanding how the organ of Corti vibrates because of acoustic pressure and outer hair cell force is critical for explaining cochlear function. In this study, cochleae were freshly isolated from young gerbils. The organ of Corti in the excised cochlea was subjected to mechanical and electrical stimulation that are analogous to acoustic and cellular stimulation in the natural cochlea. Organ of Corti vibrations, including those of individual outer hair cells, were measured using optical coherence tomography. Respective vibration patterns due to mechanical and electrical stimulation were characterized. Interactions between the two vibration patterns were investigated by applying the two forms of stimulation simultaneously. Our results show that the interactions could be either constructive or destructive, which implies that the outer hair cells can either amplify or reduce vibrations in the organ of Corti. We discuss a potential consequence of the two interaction modes for cochlear frequency tuning.

Periodicity Pitch Perception

This study presents a computational model to reproduce the biological dynamics of "listening to music." A biologically plausible model of periodicity pitch detection is proposed and simulated. Periodicity pitch is computed across a range of the auditory spectrum. Periodicity pitch is detected from subsets of activated auditory nerve fibers (ANFs). These activate connected model octopus cells, which trigger model neurons detecting onsets and offsets; thence model interval-tuned neurons are innervated at the right interval times; and finally, a set of common interval-detecting neurons indicate pitch. Octopus cells rhythmically spike with the pitch periodicity of the sound. Batteries of interval-tuned neurons stopwatch-like measure the inter-spike intervals of the octopus cells by coding interval durations as first spike latencies (FSLs). The FSL-triggered spikes synchronously coincide through a monolayer spiking neural network at the corresponding receiver pitch neurons.

Characterisation of the static offset in the travelling wave in the cochlear basal turn

In mammals, audition is triggered by travelling waves that are evoked by acoustic stimuli in the cochlear partition, a structure containing sensory hair cells and a basilar membrane. When the cochlea is stimulated by a pure tone of low frequency, a static offset occurs in the vibration in the apical turn. In the high-frequency region at the cochlear base, multi-tone stimuli induce a quadratic distortion product in the vibrations that suggests the presence of an offset. However, vibrations below 100 Hz, including a static offset, have not been directly measured there. We therefore constructed an interferometer for detecting motion at low frequencies including 0 Hz. We applied the interferometer to record vibrations from the cochlear base of guinea pigs in response to pure tones. When the animals were exposed to sound at an intensity of 70 dB or higher, we recorded a static offset of the sinusoidally vibrating cochlear partition by more than 1 nm towards the scala vestibuli. The offset’s magnitude grew monotonically as the stimuli intensified. When stimulus frequency was varied, the response peaked around the best frequency, the frequency that maximised the vibration amplitude at threshold sound pressure. These characteristics are consistent with those found in the low-frequency region and are therefore likely common across the cochlea. The offset diminished markedly when the somatic motility of mechanosensitive outer hair cells, the force-generating machinery that amplifies the sinusoidal vibrations, was pharmacologically blocked. Therefore, the partition offset appears to be linked to the electromotile contraction of outer hair cells.

Nonlinear cochlear mechanics without direct vibration-amplification feedback

Recent in vivo recordings from the mammalian cochlea indicate that although the motion of the basilar membrane appears actively amplified and nonlinear only at frequencies relatively close to the peak of the response, the internal motions of the organ of Corti display these same features over a much wider range of frequencies. These experimental findings are not easily explained by the textbook view of cochlear mechanics, in which cochlear amplification is controlled by the motion of the basilar membrane (BM) in a tight, closed-loop feedback configuration. This study shows that a simple phenomenological model of the cochlea inspired by the work of Zweig [J. Acoust. Soc. Am. 138, 1102 (2015)] can account for recent data in mouse and gerbil. In this model, the active forces are regulated indirectly, through the effect of BM motion on the pressure field across the cochlear partition, rather than via direct coupling between active-force generation and BM vibration. The absence of strong vibration-amplification feedback in the cochlea also provides a compelling explanation for the observed intensity invariance of fine time structure in the BM response to acoustic clicks.

Diverse Mechanisms of Sound Frequency Discrimination in the Vertebrate Cochlea

Discrimination of different sound frequencies is pivotal to recognizing and localizing friend and foe. Here, I review the various hair cell-tuning mechanisms used among vertebrates. Electrical resonance, filtering of the receptor potential by voltage-dependent ion channels, is ubiquitous in all non-mammals, but has an upper limit of ~1 kHz. The frequency range is extended by mechanical resonance of the hair bundles in frogs and lizards, but may need active hair-bundle motion to achieve sharp tuning up to 5 kHz. Tuning in mammals uses somatic motility of outer hair cells, underpinned by the membrane protein prestin, to expand the frequency range. The bird cochlea may also use prestin at high frequencies, but hair cells <1 kHz show electrical resonance.

Noise and sensitivity in optical coherence tomography based vibrometry

There is growing interest in using the exquisite phase sensitivity of optical coherence tomography (OCT) to measure the vibratory response in organ systems such as the middle and inner ear. Using frequency domain analysis, it is possible to achieve picometer sensitivity to vibration over a wide frequency band. Here we explore the limits of the frequency domain vibratory sensitivity due to additive noise and consider the implication of phase noise statistics on the estimation of vibratory amplitude and phase. Noise statistics are derived in both the Rayleigh (s/n = 0) and Normal distribution (s/n > 3) limits. These theoretical findings are explored using simulation and verified with experiments using a swept-laser system and a piezo electric element. A metric for sensitivity is proposed based on the 98% confidence interval for the Rayleigh distribution.

Imaging hair cells through laser-ablated cochlear bone

We report an innovative technique for the visualization of cells through an overlying scattering medium by combining femtosecond laser bone ablation and two-photon excitation fluorescence (TPEF) microscopy. We demonstrate the technique by imaging hair cells in an intact mouse cochlea ex vivo. Intracochlear imaging is important for the assessment of hearing disorders. However, the small size of the cochlea and its encasement in the densest bone in the body present challenging obstacles, preventing the visualization of the intracochlear microanatomy using standard clinical imaging modalities. The controlled laser ablation reduces the optical scattering of the cochlear bone while the TPEF allows visualization of individual cells behind the bone. We implemented optical coherence tomography (OCT) simultaneously with the laser ablation to enhance the precision of the ablation and prevent inadvertent damage to the cells behind the bone.

The release of surface-anchored α-tectorin, an apical extracellular matrix protein, mediates tectorial membrane organization

The tectorial membrane (TM) is an apical extracellular matrix (ECM) that hovers over the cochlear sensory epithelium and plays an essential role in auditory transduction. The TM forms facing the luminal endolymph-filled space and exhibits complex ultrastructure. Contrary to the current extracellular assembly model, which posits that secreted collagen fibrils and ECM components self-arrange in the extracellular space, we show that surface tethering of α-tectorin (TECTA) via a glycosylphosphatidylinositol anchor is essential to prevent diffusion of secreted TM components. In the absence of surface-tethered TECTA, collagen fibrils aggregate randomly and fail to recruit TM glycoproteins. Conversely, conversion of TECTA into a transmembrane form results in a layer of collagens on the epithelial surface that fails to form a multilayered structure. We propose a three-dimensional printing model for TM morphogenesis: A new layer of ECM is printed on the cell surface concomitant with the release of a preestablished layer to generate the multilayered TM.