Outer hair cell somatic electromotility in vivo and power transfer to the organ of Corti

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.

In Vivo Optogenetics Reveals Control of Cochlear Electromechanical Responses by Supporting Cells

Cochlear sensitivity, essential for communication and exploiting the acoustic environment, results from sensory-motor outer hair cells (OHCs) operating in a structural scaffold of supporting cells and extracellular cortilymph within the organ of Corti (OoC). Cochlear sensitivity control is hypothesized to involve interaction between the OHCs and OoC supporting cells (e.g., Deiters' cells [DCs] and outer pillar cells [OPCs]), but this has never been established in vivo Here, we conditionally expressed channelrhodopsins (ChR2) specifically in male and female mouse DCs and OPCs. Illumination of the OoC activated the nonselective ChR2 cation conductance and depolarized DCs when measured in vivo and in isolated OoC. Measurements of sound-induced cochlear mechanical and electrical responses revealed that OoC illumination suppressed the normal functions of OoC supporting cells transiently and reversibly. OoC illumination blocked normally occurring continuous minor adjustments of tone-evoked basilar membrane displacements over their entire dynamic range and OHC voltage responses to tones at levels and frequencies subject to cochlear amplification. OoC illumination altered the OHC mechanoelectrical transduction conductance operating point, which reversed the asymmetry of OHC voltage responses to high level tones. OoC illumination accelerated recovery from temporary loud sound-induced acoustic desensitization. We concluded that DCs and OPCs are involved in both the control of cochlear responses (which are essential for normal hearing) and the recovery from temporary acoustic desensitization. This is the first direct in vivo evidence for the interdependency of the structural, mechanical, and electrochemical arrangements of OHCs and OoC supporting cells that together provide fine control of cochlear responses.SIGNIFICANCE STATEMENT A striking feature of the mammalian cochlear sensory epithelium, the organ of Corti, is the cellular architecture and supporting cell arrangement that provides a structural scaffold for the sensory-motor outer hair cells. The role of the supporting cell scaffold, however, has never been elucidated in vivo, although in vitro and modeling studies indicate the scaffold is involved in exchange of forces between the outer hair cells and the organ of Corti. We used in vivo techniques, including optogenetics, that do not disrupt arrangements between the outer hair cells and supporting cells, but selectively, transiently, and reversibly interfere with supporting cell normal function. We revealed the supporting cells provide continuous adjustment of cochlear sensitivity, which is instrumental in normal hearing.

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.

A flexible anatomical set of mechanical models for the organ of Corti

We build a flexible platform to study the mechanical operation of the organ of Corti (OoC) in the transduction of basilar membrane (BM) vibrations to oscillations of an inner hair cell bundle (IHB). The anatomical components that we consider are the outer hair cells (OHCs), the outer hair cell bundles, Deiters cells, Hensen cells, the IHB and various sections of the reticular lamina. In each of the components we apply Newton's equations of motion. The components are coupled to each other and are further coupled to the endolymph fluid motion in the subtectorial gap. This allows us to obtain the forces acting on the IHB, and thus study its motion as a function of the parameters of the different components. Some of the components include a nonlinear mechanical response. We find that slight bending of the apical ends of the OHCs can have a significant impact on the passage of motion from the BM to the IHB, including critical oscillator behaviour. In particular, our model implies that the components of the OoC could cooperate to enhance frequency selectivity, amplitude compression and signal to noise ratio in the passage from the BM to the IHB. Since the model is modular, it is easy to modify the assumptions and parameters for each component.

Electrophysiologic Characteristics of Various Degrees of Selective Outer Hair Cell Loss in Rats

HYPOTHESIS

The characteristics of auditory brainstem response (ABR), electrocochleogram (ECochG), and distortion product otoacoustic emissions (DPOAE) of different degrees of selective outer hair cells (OHCs) loss may be helpful for clinicians to evaluate the pathogeny, diagnosis, and rehabilitation of individuals' hearing loss.

BACKGROUND

How many OHCs are necessary to maintain cochlear amplifier function remains unknown. The electrophysiologic characteristics may indicate different degrees of OHCs loss.

METHODS

Electrophysiological characteristics were tested using 8-kHz pure-tone stimulus and OHCs counted specifically in the region of the cochlea corresponding to 8-kHz. Rat models of selective OHCs loss were established by injecting kanamycin (KM) at various dosages, and the region of 8-kHz was obtained by 8-kHz pure-tone exposure.

RESULTS

The ABR thresholds were affected slightly with OHCs loss < 30%, and were increased dramatically with OHCs loss ranging from 30 to 70%, but the thresholds did not increase further when OHCs loss exceeded 70%. As OHCs loss increased, the compound action potential (CAP) amplitude decreased. The CAP amplitude and OHCs loss were negatively correlated. Moreover, the summating potential (SP)/action potential (AP) increased as OHCs loss increased. DPOAE and cochlear microphonics (CM) exhibited reduced amplitudes when OHCs loss < 30%.

CONCLUSIONS

Electrophysiologic characteristics may indicate different degrees of OHCs loss. While OHCs loss > 70%, the cochlear amplification may lose completely, but it is difficult to detect OHCs loss < 30%, because the ABR or DPOAE may reveal "normal" at this level. Moreover, the decreased CAP amplitude or increased SP/AP may be indicators for OHCs loss.

The effect of AP-2δ on transcription of the Prestin gene in HEI-OC1 cells upon oxidative stress

Background

The study aimed to investigate the effect of oxidative stress on Prestin expression, and explore the transcription factors (TFs) that are involved in regulating the expression of Prestin in House Ear Institute-Organ of Corti 1 (HEI-OC1) cells upon oxidative stress.

Methods

Quantitative real-time polymerase chain reaction (qRT-PCR) and Western blot were used to detect the expression level of Prestin. Reverse chromatin immunoprecipitation (reverse ChIP) assay was performed to identify proteins that could bind to the Prestin gene. Small interfering RNA (siRNA) and chromatin immunoprecipitation (ChIP) experiments were used to further verify the results. HEI-OC1 cells were incubated with four different concentrations of tert-butyl hydroperoxide (t-BHP) for 24 h or 48 h to construct the oxidative stress model.

Results

Oxidative stress induced Prestin increase at the mRNA level but with a concomitant decrease at the protein level. TF activating enhancer binding protein-2δ (AP-2δ) screened by reverse ChIP assay was demonstrated to bind to transcriptional start site 1441 of the Prestin promoter region and negatively regulate the expression of Prestin by siRNA and ChIP experiments. Furthermore, AP-2δ was down-regulated under oxidative stress.

Conclusions

In conclusion, oxidative stress inhibits the expression of Prestin protein, and the transcription mechanism is triggered to compensate for the loss of Prestin protein. AP-2δ is one of the important TFs that suppresses transcription of the Prestin gene, and AP-2δ suppression further boosted Prestin mRNA activation under oxidative stress.

Electronic supplementary material

The online version of this article (10.1186/s11658-019-0170-0) contains supplementary material, which is available to authorized users.

Power Dissipation in the Cochlea Can Enhance Frequency Selectivity

The cochlear cavity is filled with viscous fluids, and it is partitioned by a viscoelastic structure called the organ of Corti complex. Acoustic energy propagates toward the apex of the cochlea through vibrations of the organ of Corti complex. The dimensions of the vibrating structures range from a few hundred (e.g., the basilar membrane) to a few micrometers (e.g., the stereocilia bundle). Vibrations of microstructures in viscous fluid are subjected to energy dissipation. Because the viscous dissipation is considered to be detrimental to the function of hearing-sound amplification and frequency tuning-the cochlea uses cellular actuators to overcome the dissipation. Compared to extensive investigations on the cellular actuators, the dissipating mechanisms have not been given appropriate attention, and there is little consensus on damping models. For example, many theoretical studies use an inviscid fluid approximation and lump the viscous effect to viscous damping components. Others neglect viscous dissipation in the organ of Corti but consider fluid viscosity. We have developed a computational model of the cochlea that incorporates viscous fluid dynamics, organ of Corti microstructural mechanics, and electrophysiology of the outer hair cells. The model is validated by comparing with existing measurements, such as the viscoelastic response of the tectorial membrane, and the cochlear input impedance. Using the model, we investigated how dissipation components in the cochlea affect its function. We found that the majority of acoustic energy dissipation of the cochlea occurs within the organ of Corti complex, not in the scalar fluids. Our model suggests that an appropriate dissipation can enhance the tuning quality by reducing the spread of energy provided by the outer hair cells' somatic motility.

Modulation of Glucose Takeup by Glucose Transport on the Isolated OHCs

Glucose is a fundamental source of energy for mammalian cells; however, whether glucose is taken up through the lateral walls of cochlear outer hair cells (OHCs) is unknown. The OHC lateral wall is complex, composed of a plasma membrane, cortical lattice, and subsurface cisternae. This study assessed the uptake of glucose by OHCs using live-cell microscopy and examined the distribution of glucose transporter isoforms by immunohistochemistry. We found that glucose transporter-4 was mostly expressed on the lateral wall of OHCs. Glucose crossed the lateral walls of OHCs via glucose transporters-4 mainly, and this process could be modulated. These results suggest that the lateral walls are involved in modulating energy transport into OHCs.

Negative membrane capacitance of outer hair cells: electromechanical coupling near resonance

Outer hair cells in the cochlea have a unique motility in their cell body based on mechanoelectric coupling, with which voltage changes generated by stimuli at their hair bundles drive the cell body and, in turn, it has been assumed, amplifies the signal. In vitro experiments show that the movement of the charges of the motile element significantly increases the membrane capacitance, contributing to the attenuation of the driving voltage. That is indeed the case in the absence of mechanical load. Here it is predicted, however, that the movement of motile charges creates negative capacitance near the condition of mechanical resonance, such as those in the cochlea, enhancing energy output.

Two passive mechanical conditions modulate power generation by the outer hair cells

In the mammalian cochlea, small vibrations of the sensory epithelium are amplified due to active electro-mechanical feedback of the outer hair cells. The level of amplification is greater in the base than in the apex of the cochlea. Theoretical studies have used longitudinally varying active feedback properties to reproduce the location-dependent amplification. The active feedback force has been considered to be proportional to the basilar membrane displacement or velocity. An underlying assumption was that organ of Corti mechanics are governed by rigid body kinematics. However, recent progress in vibration measurement techniques reveals that organ of Corti mechanics are too complicated to be fully represented with rigid body kinematics. In this study, two components of the active feedback are considered explicitly-organ of Corti mechanics, and outer hair cell electro-mechanics. Physiological properties for the outer hair cells were incorporated, such as the active force gain, mechano-transduction properties, and membrane RC time constant. Instead of a kinematical model, a fully deformable 3D finite element model was used. We show that the organ of Corti mechanics dictate the longitudinal trend of cochlear amplification. Specifically, our results suggest that two mechanical conditions are responsible for location-dependent cochlear amplification. First, the phase of the outer hair cell's somatic force with respect to its elongation rate varies along the cochlear length. Second, the local stiffness of the organ of Corti complex felt by individual outer hair cells varies along the cochlear length. We describe how these two mechanical conditions result in greater amplification toward the base of the cochlea.

Amplification mode differs along the length of the mouse cochlea as revealed by connexin 26 deletion from specific gap junctions

The sharp frequency tuning and exquisite sensitivity of the mammalian cochlea is due to active forces delivered by outer hair cells (OHCs) to the cochlear partition. Force transmission is mediated and modulated by specialized cells, including Deiters’ cells (DCs) and pillar cells (PCs), coupled by gap-junctions composed of connexin 26 (Cx26) and Cx30. We created a mouse with conditional Cx26 knock-out (Cx26 cKO) in DCs and PCs that did not influence sensory transduction, receptor-current-driving-voltage, low-mid-frequency distortion-product-otoacoustic-emissions (DPOAEs), and passive basilar membrane (BM) responses. However, the Cx26 cKO desensitizes mid-high-frequency DPOAEs and active BM responses and sensitizes low-mid-frequency neural excitation. This functional segregation may indicate that the flexible, apical turn cochlear partition facilitates transfer of OHC displacements (isotonic forces) for cochlear amplification and neural excitation. DC and PC Cx26 expression is essential for cochlear amplification in the stiff basal turn, possibly through maintaining cochlear partition mechanical impedance, thereby ensuring effective transfer of OHC isometric forces.

Energy Output from a Single Outer Hair Cell

Electromotility of outer hair cells (OHCs) has been extensively studied with in vitro experiments because of its physiological significance to the cochlear amplifier, which provides the exquisite sensitivity and frequency selectivity of the mammalian ear. However, these studies have been performed largely under load-free conditions or with static load, while these cells function in vivo in a dynamic environment, receiving electrical energy to enhance mechanical oscillation in the inner ear. This gap leaves uncertainties in addressing a key issue, how much mechanical energy an OHC provides. This study is an attempt of bridging the gap by introducing a simple one-dimensional model for electromotility of OHC in a dynamic environment. This model incorporates a feedback loop involving the receptor potential and the mechanical load on OHC, and leads to an analytical expression for the membrane capacitance, which explicitly describes the dependence on the elastic load, viscous drag, and the mass. The derived equation of motion was examined in a mass-less model system with realistic parameter values for OHC. It was found that viscous drag is more effective than elastic load in enhancing the receptor potential that drives the cell. For this reason, it is expected that OHCs are more effective in counteracting viscous drag than providing elastic energy to the system.

Optogenetic Control of Mouse Outer Hair Cells

Normal hearing in mammals depends on sound amplification by outer hair cells (OHCs) presumably by their somatic motility and force production. However, the role of OHC force production in cochlear amplification and frequency tuning are not yet fully understood. Currently, available OHC manipulation techniques for physiological or clinical studies are limited by their invasive nature, lack of precision, and poor temporal-spatial resolution. To overcome these limitations, we explored an optogenetic approach based on channelrhodopsin 2 (ChR-2), a direct light-activated nonselective cation channel originally discovered in Chlamydomonas reinhardtii. Three approaches were compared: 1) adeno-associated virus-mediated in utero transfer of the ChR-2 gene into the developing murine otocyst, 2) expression of ChR-2(H134R) in an auditory cell line (HEI-OC1), and 3) expression of ChR-2 in the OHCs of a mouse line carrying a ChR-2 conditional allele. Whole cell recording showed that blue light (470 nm) elicited the typical nonselective cation current of ChR-2 with reversal potential around zero in both mouse OHCs and HEI-OC1 cells and generated depolarization in both cell types. In addition, pulsed light stimulation (10 Hz) elicited a 1:1 repetitive depolarization and ChR-2 currents in mouse OHCs and HEI-OC1 cells, respectively. The time constant of depolarization in OHCs, 1.45 ms, is 10 times faster than HEI-OC1 cells, which allowed light stimulation up to rates of 10/s to elicit corresponding membrane potential changes. Our study demonstrates that ChR-2 can successfully be expressed in mouse OHCs and HEI-OC1 cells and that these present a typical light-sensitive current and depolarization. However, the amount of ChR-2 current induced in our in vivo experiments was insufficient to result in measurable cochlear effects.

Cochlear Outer-Hair-Cell Power Generation and Viscous Fluid Loss

Since the discovery of otoacoustic emissions and outer hair cell (OHC) motility, the fundamental question of whether the cochlea produces mechanical power remains controversial. In the present work, direct calculations are performed on power loss due to fluid viscosity and power generated by the OHCs. A three-dimensional box model of the mouse cochlea is used with a feed-forward/feed-backward approximation representing the organ of Corti cytoarchitecture. The model is fit to in vivo basilar membrane motion with one free parameter for the OHCs. The calculations predict that the total power output from the three rows of OHCs can be over three orders of magnitude greater than the acoustic input power at 10 dB sound pressure level (SPL). While previous work shows that the power gain, or the negative damping, diminishes with intensity, we show explicitly based on our model that OHC power output increases and saturates with SPL. The total OHC power output is about 2 pW at 80 dB SPL, with a maximum of about 10 fW per OHC.

Power dissipation in the subtectorial space of the mammalian cochlea is modulated by inner hair cell stereocilia

The stereocilia bundle is the mechano-transduction apparatus of the inner ear. In the mammalian cochlea, the stereocilia bundles are situated in the subtectorial space (STS)--a micrometer-thick space between two flat surfaces vibrating relative to each other. Because microstructures vibrating in fluid are subject to high-viscous friction, previous studies considered the STS as the primary place of energy dissipation in the cochlea. Although there have been extensive studies on how metabolic energy is used to compensate the dissipation, much less attention has been paid to the mechanism of energy dissipation. Using a computational model, we investigated the power dissipation in the STS. The model simulates fluid flow around the inner hair cell (IHC) stereocilia bundle. The power dissipation in the STS because of the presence IHC stereocilia increased as the stimulating frequency decreased. Along the axis of the stimulating frequency, there were two asymptotic values of power dissipation. At high frequencies, the power dissipation was determined by the shear friction between the two flat surfaces of the STS. At low frequencies, the power dissipation was dominated by the viscous friction around the IHC stereocilia bundle--the IHC stereocilia increased the STS power dissipation by 50- to 100-fold. There exists a characteristic frequency for STS power dissipation, CFSTS, defined as the frequency where power dissipation drops to one-half of the low frequency value. The IHC stereocilia stiffness and the gap size between the IHC stereocilia and the tectorial membrane determine the characteristic frequency. In addition to the generally assumed shear flow, nonshear STS flow patterns were simulated. Different flow patterns have little effect on the CFSTS. When the mechano-transduction of the IHC was tuned near the vibrating frequency, the active motility of the IHC stereocilia bundle reduced the power dissipation in the STS.

Consequences of Location-Dependent Organ of Corti Micro-Mechanics

The cochlea performs frequency analysis and amplification of sounds. The graded stiffness of the basilar membrane along the cochlear length underlies the frequency-location relationship of the mammalian cochlea. The somatic motility of outer hair cell is central for cochlear amplification. Despite two to three orders of magnitude change in the basilar membrane stiffness, the force capacity of the outer hair cell’s somatic motility, is nearly invariant over the cochlear length. It is puzzling how actuators with a constant force capacity can operate under such a wide stiffness range. We hypothesize that the organ of Corti sets the mechanical conditions so that the outer hair cell’s somatic motility effectively interacts with the media of traveling waves—the basilar membrane and the tectorial membrane. To test this hypothesis, a computational model of the gerbil cochlea was developed that incorporates organ of Corti structural mechanics, cochlear fluid dynamics, and hair cell electro-physiology. The model simulations showed that the micro-mechanical responses of the organ of Corti are different along the cochlear length. For example, the top surface of the organ of Corti vibrated more than the bottom surface at the basal (high frequency) location, but the amplitude ratio was reversed at the apical (low frequency) location. Unlike the basilar membrane stiffness varying by a factor of 1700 along the cochlear length, the stiffness of the organ of Corti complex felt by the outer hair cell remained between 1.5 and 0.4 times the outer hair cell stiffness. The Y-shaped structure in the organ of Corti formed by outer hair cell, Deiters cell and its phalange was the primary determinant of the elastic reactance imposed on the outer hair cells. The stiffness and geometry of the Deiters cell and its phalange affected cochlear amplification differently depending on the location.

Non-uniform distribution of outer hair cell transmembrane potential induced by extracellular electric field

Intracochlear electric fields arising out of sound-induced receptor currents, silent currents, or electrical current injected into the cochlea induce transmembrane potential along the outer hair cell (OHC) but its distribution along the cells is unknown. In this study, we investigated the distribution of OHC transmembrane potential induced along the cell perimeter and its sensitivity to the direction of the extracellular electric field (EEF) on isolated OHCs at a low frequency using the fast voltage-sensitive dye ANNINE-6plus. We calibrated the potentiometric sensitivity of the dye by applying known voltage steps to cells by simultaneous whole-cell voltage clamp. The OHC transmembrane potential induced by the EEF is shown to be highly nonuniform along the cell perimeter and strongly dependent on the direction of the electrical field. Unlike in many other cells, the EEF induces a field-direction-dependent intracellular potential in the cylindrical OHC. We predict that without this induced intracellular potential, EEF would not generate somatic electromotility in OHCs. In conjunction with the known heterogeneity of OHC membrane microdomains, voltage-gated ion channels, charge, and capacitance, the EEF-induced nonuniform transmembrane potential measured in this study suggests that the EEF would impact the cochlear amplification and electropermeability of molecules across the cell.

Effects of cochlear loading on the motility of active outer hair cells

Outer hair cells (OHCs) power the amplification of sound-induced vibrations in the mammalian inner ear through an active process that involves hair-bundle motility and somatic motility. It is unclear, though, how either mechanism can be effective at high frequencies, especially when OHCs are mechanically loaded by other structures in the cochlea. We address this issue by developing a model of an active OHC on the basis of observations from isolated cells, then we use the model to predict the response of an active OHC in the intact cochlea. We find that active hair-bundle motility amplifies the receptor potential that drives somatic motility. Inertial loading of a hair bundle by the tectorial membrane reduces the bundle's reactive load, allowing the OHC's active motility to influence the motion of the cochlear partition. The system exhibits enhanced sensitivity and tuning only when it operates near a dynamical instability, a Hopf bifurcation. This analysis clarifies the roles of cochlear structures and shows how the two mechanisms of motility function synergistically to create the cochlear amplifier. The results suggest that somatic motility evolved to enhance a preexisting amplifier based on active hair-bundle motility, thus allowing mammals to hear high-frequency sounds.

Microdomains shift and rotate in the lateral wall of cochlear outer hair cells

Outer hair cell (OHC) electromotility, a response consisting of reversible changes in cell length and diameter induced by electrical stimulation, confers remarkable sensitivity and frequency resolution to the mammalian inner ear. Looking for a better understanding of this mechanism, we labeled isolated guinea pig OHCs with microspheres and, using high-speed video recording, investigated their movements at the apical, mid, and basal regions of osmotically and electrically stimulated cells. After hypoosmotic challenge, OHCs shortened and their diameter increased, with microspheres moving always toward the central plane; iso-osmolarity returned OHCs to their original shape and microspheres to their original positions. Under electrical stimulation, microspheres exhibited robust movements, with their displacement vectors changing in direction from random to parallel to the longitudinal axis of the cells with peak reorientation speeds of up to 6 rad/s and returning to random after 5 min without stimulation. Alterations in plasma-membrane cholesterol levels as well as cytoskeleton integrity affected microsphere responses. We concluded that microspheres attach to different molecular microdomains, and these microdomains are able to shift and rotate in the plane of the OHC lateral wall with a dynamics tightly regulated by membrane lipid composition and the cortical cytoskeleton.

Half-octave shift in mammalian hearing is an epiphenomenon of the cochlear amplifier

The cochlear amplifier is a hypothesized positive feedback process responsible for our exquisite hearing sensitivity. Experimental evidence for or against the positive feedback hypothesis is still lacking. Here we apply linear control theory to determine the open-loop gain and the closed-loop sensitivity of the cochlear amplifier from available measurements of basilar membrane vibration in sensitive mammalian cochleae. We show that the frequency of peak closed-loop sensitivity is independent of the stimulus level and close to the characteristic frequency. This implies that the half-octave shift in mammalian hearing is an epiphenomenon of the cochlear amplifier. The open-loop gain is consistent with positive feedback and suggests that the high-frequency cut-off of the outer hair cell transmembrane potential in vivo may be necessary for cochlear amplification.