Influence of the cochlear partition's flexibility on the macro mechanisms in the inner ear.

Recent studies have highlighted the anatomy of the cochlear partition (CP), revealing insights into the flexible nature of the osseous spiral lamina (OSL) and the existence of a flexible cochlear partition bridge (CPB) between the OSL and the basilar membrane (BM). However, most existing inner ear models treat the OSL as a rigid structure and ignore the CPB, neglecting their potential impact on intracochlear sound pressure and motion of the BM. In this paper, we investigate the effect of the CP's flexibility by including the OSL and CPB as either rigid or flexible structures in a numerical anatomical model of the human inner ear. Our findings demonstrate that the flexibility of the OSL and the presence of the CPB significantly affect cochlear macro mechanisms, including differential intracochlear sound pressure, resistive behavior in cochlear impedances, CP stiffness, and BM velocity. These results emphasize the importance of considering the flexibility of the entire CP to enhance our understanding of cochlear function and to accurately interpret experimental data on inner ear mechanics.

Research on sound quality prediction of vehicle interior noise using the human-ear physiological model.

In order to improve the prediction accuracy of the sound quality of vehicle interior noise, a novel sound quality prediction model was proposed based on the physiological response predicted metrics, i.e., loudness, sharpness, and roughness. First, a human-ear sound transmission model was constructed by combining the outer and middle ear finite element model with the cochlear transmission line model. This model converted external input noise into cochlear basilar membrane response. Second, the physiological perception models of loudness, sharpness, and roughness were constructed by transforming the basilar membrane response into sound perception related to neuronal firing. Finally, taking the calculated loudness, sharpness, and roughness of the physiological model and the subjective evaluation values of vehicle interior noise as the parameters, a sound quality prediction model was constructed by TabNet model. The results demonstrate that the loudness, sharpness, and roughness computed by the human-ear physiological model exhibit a stronger correlation with the subjective evaluation of sound quality annoyance compared to traditional psychoacoustic parameters. Furthermore, the average error percentage of sound quality prediction based on the physiological model is only 3.81%, which is lower than that based on traditional psychoacoustic parameters.

Frequency-Selective, Multi-Channel, Self-Powered Artificial Basilar Membrane Sensor with a Spiral Shape and 24 Critical Bands Inspired by the Human Cochlea.

A spiral-artificial basilar membrane (S-ABM) sensor is reported that mimics the basilar membrane (BM) of the human cochlea and can detect sound by separating it into 24 sensing channels based on the frequency band. For this, an analytical function is proposed to design the width of the BM so that the frequency bands are linearly located along the length of the BM. To fabricate the S-ABM sensor, a spiral-shaped polyimide film is used as a vibrating membrane, with maximum displacement at locations corresponding to specific frequency bands of sound, and attach piezoelectric sensor modules made of poly(vinylidene fluoride-trifluoroethylene) film on top of the polyimide film to measure the vibration amplitude at each channel location. As the result, the S-ABM sensor implements a characteristic frequency band of 96-12,821 Hz and 24-independent critical bands. Using real-time signals from discriminate channels, it is demonstrated that the sensor can rapidly identify the operational noises from equipment processes as well as vehicle sounds from environmental noises on the road. The sensor can be used in a variety of applications, including speech recognition, dangerous situation recognition, hearing aids, and cochlear implants, and more.

Asymmetric vibrations in the organ of Corti by outer hair cells measured from excised gerbil cochlea.

Pending questions regarding cochlear amplification and tuning are hinged upon the organ of Corti (OoC) active mechanics: how outer hair cells modulate OoC vibrations. Our knowledge regarding OoC mechanics has advanced over the past decade thanks to the application of tomographic vibrometry. However, recent data from live cochlea experiments often led to diverging interpretations due to complicated interaction between passive and active responses, lack of image resolution in vibrometry, and ambiguous measurement angles. We present motion measurements and analyses of the OoC sub-components at the close-to-true cross-section, measured from acutely excised gerbil cochleae. Specifically, we focused on the vibrating patterns of the reticular lamina, the outer pillar cell, and the basilar membrane because they form a structural frame encasing active outer hair cells. For passive transmission, the OoC frame serves as a rigid truss. In contrast, motile outer hair cells exploit their frame structures to deflect the upper compartment of the OoC while minimally disturbing its bottom side (basilar membrane). Such asymmetric OoC vibrations due to outer hair cell motility explain how recent observations deviate from the classical cochlear amplification theory.

Microanatomy of the human tunnel of Corti structures and cochlear partition-tonotopic variations and transcellular signaling.

Auditory sensitivity and frequency resolution depend on the optimal transfer of sound-induced vibrations from the basilar membrane (BM) to the inner hair cells (IHCs), the principal auditory receptors. There remains a paucity of information on how this is accomplished along the frequency range in the human cochlea. Most of the current knowledge is derived either from animal experiments or human tissue processed after death, offering limited structural preservation and optical resolution. In our study, we analyzed the cytoarchitecture of the human cochlear partition at different frequency locations using high-resolution microscopy of uniquely preserved normal human tissue. The results may have clinical implications and increase our understanding of how frequency-dependent acoustic vibrations are carried to human IHCs. A 1-micron-thick plastic-embedded section (mid-modiolar) from a normal human cochlea uniquely preserved at lateral skull base surgery was analyzed using light and transmission electron microscopy (LM, TEM). Frequency locations were estimated using synchrotron radiation phase-contrast imaging (SR-PCI). Archival human tissue prepared for scanning electron microscopy (SEM) and super-resolution structured illumination microscopy (SR-SIM) were also used and compared in this study. Microscopy demonstrated great variations in the dimension and architecture of the human cochlear partition along the frequency range. Pillar cell geometry was closely regulated and depended on the reticular lamina slope and tympanic lip angle. A type II collagen-expressing lamina extended medially from the tympanic lip under the inner sulcus, here named "accessory basilar membrane." It was linked to the tympanic lip and inner pillar foot, and it may contribute to the overall compliance of the cochlear partition. Based on the findings, we speculate on the remarkable microanatomic inflections and geometric relationships which relay different sound-induced vibrations to the IHCs, including their relevance for the evolution of human speech reception and electric stimulation with auditory implants. The inner pillar transcellular microtubule/actin system's role of directly converting vibration energy to the IHC cuticular plate and ciliary bundle is highlighted.

Regional differences in cochlear nonlinearity across the basal organ of Corti of gerbil: Regional differences in cochlear nonlinearity.

Auditory sensation is based in nanoscale vibration of the sensory tissue of the cochlea, the organ of Corti complex (OCC). Motion within the OCC is now observable due to optical coherence tomography. In a previous study (Cooper et al., 2018), the region that includes the electro-motile outer hair cells (OHC) and Deiters cells (DC) was observed to move with larger amplitude than the basilar membrane (BM) and surrounding regions and was termed the "hotspot." In addition to this quantitative distinction, the hotspot moved qualitatively differently than the BM, in that its motion scaled nonlinearly with stimulus level at all frequencies, evincing sub-BF activity. Sub-BF activity enhances non-BF motion; thus the frequency tuning of the OHC/DC region was reduced relative to the BM. In this work we further explore the motion of the gerbil basal OCC and find that regions that lack significant sub-BF activity include the BM, the medial and lateral OCC, and the reticular lamina (RL) region. The observation that the RL region does not move actively sub-BF (already observed in Cho and Puria 2022), suggests that hair cell stereocilia are not exposed to sub-BF activity in the cochlear base. The observation that the lateral and RL regions move approximately linearly sub-BF indicates that linear forces dominate non-linear OHC-based forces on these components at sub-BF frequencies. A complex difference analysis was performed to reveal the internal motion of the OHC/DC region and showed that amplitude structure and phase shifts in the directly measured OHC/DC motion emerge due to the internal OHC/DC motion destructively interfering with BM motion.

Pathogenic mechanism analysis of cochlear key structural lesion and phonosensitive hearing loss.

Due to ethical issues and the very fine and complex structure of the cochlea, it is difficult to directly perform experimental measurement on the human cochlea. Therefore, the finite element method has become an effective and replaceable new research means. Accurate numerical analysis on human ear using finite element method can provide better understanding of sound transmission and can be used to assess the influence of diseases on hearing and to treat hearing loss. In this research, a three-dimensional (3D) finite element model (FEM) of the human ear of cochlea was presented to investigate the destruction of basilar membrane (BM), round window (RW) sclerosis and perilymph fistula, the key structures of the cochlea, and analyze the effects of these abnormal pathological states in the cochlea on cochlear hearing, resulting in the changes in cochlear sense structure biomechanical behavior and quantitative prediction of the degree and harm of the disorder to the decline of human hearing. Therefore, this paper can deepen reader's understanding of the cochlear biomechanical mechanism and provide a theoretical foundation for clinical otology.

Study on damage of the macrostructure of the cochlea under the impact load.

Due to the tiny and delicate structure of the cochlea, the auditory system is the most sensitive to explosion impact damage. After being damaged by the explosion impact wave, it usually causes long-term deafness, tinnitus, and other symptoms. To better understand the influence of impact load on the cochlea and basilar membrane (BM), a three-dimensional (3D) fluid-solid coupling finite element model was developed. This model accurately reflects the actual spatial spiral shape of the human cochlea, as well as the lymph environment and biological materials. Based on verifying the reliability of the model, the curve of impact load-amplitude response was obtained, and damage of impact load on the cochlea and the key macrostructure-BM was analyzed. The results indicate that impact wave at middle frequency has widest influence on the cochlea. Furthermore, impact loading causes tears in the BM and destroys the cochlear frequency selectivity.

Intracochlear overdrive: Characterizing nonlinear wave amplification in the mouse apex.

In this study, we explore nonlinear cochlear amplification by analyzing basilar membrane (BM) motion in the mouse apex. Through in vivo, postmortem, and mechanical suppression recordings, we estimate how the cochlear amplifier nonlinearly shapes the wavenumber of the BM traveling wave, specifically within a frequency range where the short-wave approximation holds. Our findings demonstrate that a straightforward mathematical model, depicting the cochlear amplifier as a wavenumber modifier with strength diminishing monotonically as BM displacement increases, effectively accounts for the various experimental observations. This empirically derived model is subsequently incorporated into a physics-based "overturned" framework of cochlear amplification [see Altoè, Dewey, Charaziak, Oghalai, and Shera (2022), J. Acoust. Soc. Am. 152, 2227-2239] and tested against additional experimental data. Our results demonstrate that the relationships established within the short-wave region remain valid over a much broader frequency range. Furthermore, the model, now exclusively calibrated to BM data, predicts the behavior of the opposing side of the cochlear partition, aligning well with recent experimental observations. The success in reproducing key features of the experimental data and the mathematical simplicity of the resulting model provide strong support for the "overturned" theory of cochlear amplification.

Otoacoustic emissions reveal the micromechanical role of organ-of-Corti cytoarchitecture in cochlear amplification.

The intricate, crystalline cytoarchitecture of the mammalian organ of Corti presumably plays an important role in cochlear amplification. As currently understood, the oblique, Y-shaped arrangement of the outer hair cells (OHCs) and phalangeal processes of the Deiters cells serves to create differential "push-pull" forces that drive the motion of the basilar membrane via the spatial feedforward and/or feedbackward of OHC forces. In concert with the cochlear traveling wave, the longitudinal separation between OHC sensing and forcing creates phase shifts that yield a form of negative damping, amplifying waves as they propagate. Unlike active forces that arise and act locally, push-pull forces are inherently directional-whereas forward-traveling waves are boosted, reverse-traveling waves are squelched. Despite their attractions, models based on push-pull amplification must contend with otoacoustic emissions (OAEs), whose existence implies that amplified energy escapes from the inner ear via mechanisms involving reverse traveling waves. We analyze hybrid local/push-pull models to determine the constraints that reflection-source OAEs place on the directionality of cochlear wave propagation. By implementing a special force-mixing control knob, we vary the mix of local and push-pull forces while leaving the forward-traveling wave unchanged. Consistency with stimulus-frequency OAEs requires that the active forces underlying cochlear wave amplification be primarily local in character, contradicting the prevailing view. By requiring that the oblique cytoarchitecture produce predominantly local forces, we reinterpret the functional role of the Y-shaped geometry, proposing that it serves not as a push-pull amplifier, but as a mechanical funnel that spatially integrates local OHC forces.

Single-cell transcriptomic profiling of the mouse cochlea: An atlas for targeted therapies.

Functional molecular characterization of the cochlea has mainly been driven by the deciphering of the genetic architecture of sensorineural deafness. As a result, the search for curative treatments, which are sorely lacking in the hearing field, has become a potentially achievable objective, particularly via cochlear gene and cell therapies. To this end, a complete inventory of cochlear cell types, with an in-depth characterization of their gene expression profiles right up to their final differentiation, is indispensable. We therefore generated a single-cell transcriptomic atlas of the mouse cochlea based on an analysis of more than 120,000 cells on postnatal day 8 (P8), during the prehearing period, P12, corresponding to hearing onset, and P20, when cochlear maturation is almost complete. By combining whole-cell and nuclear transcript analyses with extensive in situ RNA hybridization assays, we characterized the transcriptomic signatures covering nearly all cochlear cell types and developed cell type-specific markers. Three cell types were discovered; two of them contribute to the modiolus which houses the primary auditory neurons and blood vessels, and the third one consists in cells lining the scala vestibuli. The results also shed light on the molecular basis of the tonotopic gradient of the biophysical characteristics of the basilar membrane that critically underlies cochlear passive sound frequency analysis. Finally, overlooked expression of deafness genes in several cochlear cell types was also unveiled. This atlas paves the way for the deciphering of the gene regulatory networks controlling cochlear cell differentiation and maturation, essential for the development of effective targeted treatments.

Hearing as adaptive cascaded envelope interpolation.

The human auditory system is designed to capture and encode sounds from our surroundings and conspecifics. However, the precise mechanisms by which it adaptively extracts the most important spectro-temporal information from sounds are still not fully understood. Previous auditory models have explained sound encoding at the cochlear level using static filter banks, but this vision is incompatible with the nonlinear and adaptive properties of the auditory system. Here we propose an approach that considers the cochlear processes as envelope interpolations inspired by cochlear physiology. It unifies linear and nonlinear adaptive behaviors into a single comprehensive framework that provides a data-driven understanding of auditory coding. It allows simulating a broad range of psychophysical phenomena from virtual pitches and combination tones to consonance and dissonance of harmonic sounds. It further predicts the properties of the cochlear filters such as frequency selectivity. Here we propose a possible link between the parameters of the model and the density of hair cells on the basilar membrane. Cascaded Envelope Interpolation may lead to improvements in sound processing for hearing aids by providing a non-linear, data-driven, way to preprocessing of acoustic signals consistent with peripheral processes.

Estimating cochlear impulse responses using frequency sweeps.

Cochlear mechanics tends to be studied using single-location measurements of intracochlear vibrations in response to acoustical stimuli. Such measurements, due to their invasiveness and often the instability of the animal preparation, are difficult to accomplish and, thus, ideally require stimulus paradigms that are time efficient, flexible, and result in high resolution transfer functions. Here, a swept-sine method is adapted for recordings of basilar membrane impulse responses in mice. The frequency of the stimulus was exponentially swept from low to high (upward) or high to low (downward) at varying rates (from slow to fast) and intensities. The cochlear response to the swept-sine was then convolved with the time-reversed stimulus waveform to obtain first and higher order impulse responses. Slow sweeps of either direction produce cochlear first to third order transfer functions equivalent to those measured with pure tones. Fast upward sweeps, on the other hand, generate impulse responses that typically ring longer, as observed in responses obtained using clicks. The ringing of impulse response in mice was of relatively small amplitude and did not affect the magnitude spectra. It is concluded that swept-sine methods offer flexible and time-efficient alternatives to other approaches for recording cochlear impulse responses.

Effects of basilar-membrane lesions on dynamic responses of the middle ear.

BACKGROUND: Numerical simulations can reflect the changes in physiological properties caused by various factors in the cochlea. AIMS/OBJECTIVE: To analyze the influence of lesions of the basilar membrane (BM) on the dynamic response of the middle ear. METHOD: Based on healthy human ear CT scan images, use PATRAN software to build a three-dimensional finite element model of the human ear, then apply NASTRAN software to conduct analysis of solid-fluid coupled frequency response. The influence of lesions in the BM on the dynamic response of the middle ear is simulated through the method of numerical simulation. RESULT: Through comparing experimental data and the frequency-response curve of displacement of BM and stapes, the validity of the model in this paper was verified. CONCLUSION: Regarding sclerosis in BM, the most obvious decline of displacement and velocity exists in the range of 800-10,000Hz and 800-2000Hz frequency, respectively. The higher degree of sclerosis, the more obvious decline becomes. The maximal decline of hearing can reach from 6.2 dB to 9.1 dB. Regarding added mass in BM, the most obvious decline of displacement exists in the range of 600-1000Hz frequency, and the maximal decline of hearing can reach 4.0 dB. There is no obvious decline in velocity.

Visualizing Collagen Fibrils in the Cochlea's Tectorial and Basilar Membranes Using a Fluorescently Labeled Collagen-Binding Protein Fragment.

PURPOSE: A probe that binds to unfixed collagen fibrils was used to image the shapes and fibrous properties of the TM and BM. The probe (CNA35) is derived from the bacterial adhesion protein CNA. We present confocal images of hydrated gerbil TM, BM, and other cochlear structures stained with fluorescently labeled CNA35. A primary purpose of this article is to describe the use of the CNA35 collagen probe in the cochlea. METHODS: Recombinant poly-histidine-tagged CNA35 was expressed in Escherichia coli, purified by cobalt-affinity chromatography, fluorescence labeled, and further purified by gel filtration chromatography. Cochleae from freshly harvested gerbil bullae were irrigated with and then incubated in CNA35 for periods ranging from 2 h - overnight. The cochleae were fixed, decalcified, and dissected. Isolated cochlear turns were imaged by confocal microscopy. RESULTS: The CNA35 probe stained the BM and TM, and volumetric imaging revealed the shape of these structures and the collagen fibrils within them. The limbal zone of the TM stained intensely. In samples from the cochlear base, intense staining was detected on the side of the TM that faces hair cells. In the BM pectinate zone, staining was intense at the upper and lower boundaries. The BM arcuate zone was characterized by a prominent longitudinal collagenous structure. The spiral ligament, limbus and lamina stained for collagen, and within the spiral limbus the habenula perforata were outlined with intense staining. CONCLUSION: The CNA35 probe provides a unique and useful view of collagenous structures in the cochlea.

Broad nonlinearity in reticular lamina vibrations requires compliant organ of Corti structures.

In the mammalian cochlea, each longitudinal position of the basilar membrane (BM) has a nonlinear vibratory response in a limited frequency range around the location-dependent frequency of maximum response, known as the best frequency (BF). This nonlinear response arises from the electromechanical feedback from outer hair cells (OHCs). However, recent in vivo measurements have demonstrated that the mechanical response of other organ of Corti (OoC) structures, such as the reticular lamina (RL), and the electrical response of OHCs (measured in the local cochlear microphonic [LCM]) are nonlinear even at frequencies significantly below BF. In this work, a physiologically motivated model of the gerbil cochlea is used to demonstrate that the source of this discrepancy between the frequency range of the BM, RL, and LCM nonlinearities is greater compliance in the structures at the top of the OHCs. The predicted responses of the BM, RL, and LCM to pure tone and two-tone stimuli are shown to be in line with experimental evidence. Simulations then demonstrate that the sub-BF nonlinearity in the RL requires the structures at the top of the OHCs to be significantly more compliant than the BM. This same condition is also necessary for "optimal" gain near BF, i.e., high amplification that is in line with the experiment. This demonstrates that the conditions for OHCs to operate optimally at BF inevitably yield nonlinearity of the RL response over a broad frequency range.

Study of fatigue damage to the cochlea.

In order to explore the hearing loss resulting from exposure to continuous or intermittent loud noise. A three-dimensional liquid-solid coupling finite element model of spiral cochlea was established. The reliability of the model was verified, and the stress and amplitude of the basilar membrane of the pivotal structure in cochlea were analyzed. The results show that under the action of the same high-pressure sound, the preferential fatigue area of the cochlear high-frequency area mainly causes fatigue in the cochlear. The safer area is a sound pressure level below 70 dB, while one above 90 dB accelerates damage to the ear.

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.

Organ of Corti vibrations are dominated by longitudinal motion in vivo.

Recent observations of sound-evoked vibrations of the cochlea's sensory organ of Corti (ooC) using optical coherence tomography (OCT) have revealed unanticipated and complex motions. Interpreting these results in terms of the micromechanical inner-ear processes that precede hair-cell transduction is not trivial since OCT only measures a projection of the true motion, which may include transverse and longitudinal displacements. We measure ooC motions at multiple OCT beam angles relative to the longitudinal axis of the basilar membrane (BM) by using the cochlea's natural curvature and find that the relative phase between outer hair cells (OHC) and BM varies with this angle. This includes a relatively abrupt phase reversal where OHC lead (lag) the BM by ~0.25 cycles for negative (positive) beam angles, respectively. We interpret these results as evidence for significant longitudinal motion within the ooC, which should be considered when interpreting (relative) ooC vibrations in terms of inner-ear sound processing.

The reticular lamina and basilar membrane vibrations in the transverse direction in the basal turn of the living gerbil cochlea.

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.