Nanomechanics of the subtectorial space caused by electromechanics of cochlear outer hair cells

Sound-induced length changes in outer hair cell stereocilia

Hearing relies on mechanical stimulation of stereocilia bundles on the sensory cells of the inner ear. When sound hits the ear, these stereocilia pivot about a neck-like taper near their base. More than three decades of research have established that sideways deflection of stereocilia is essential for converting mechanical stimuli into electrical signals. Here we show that mammalian outer hair cell stereocilia not only move sideways but also change length during sound stimulation. Currents that enter stereocilia through mechanically sensitive ion channels control the magnitude of both length changes and bundle deflections in a reciprocal manner: the smaller the length change, the larger is the bundle deflection. Thus, the transduction current is important for maintaining the resting mechanical properties of stereocilia. Hair cell stimulation is most effective when bundles are in a state that ensures minimal length change.

A mouse model for human deafness DFNB22 reveals that hearing impairment is due to a loss of inner hair cell stimulation

The gene causative for the human nonsyndromic recessive form of deafness DFNB22 encodes otoancorin, a 120-kDa inner ear-specific protein that is expressed on the surface of the spiral limbus in the cochlea. Gene targeting in ES cells was used to create an EGFP knock-in, otoancorin KO (Otoa(EGFP/EGFP)) mouse. In the Otoa(EGFP/EGFP) mouse, the tectorial membrane (TM), a ribbon-like strip of ECM that is normally anchored by one edge to the spiral limbus and lies over the organ of Corti, retains its general form, and remains in close proximity to the organ of Corti, but is detached from the limbal surface. Measurements of cochlear microphonic potentials, distortion product otoacoustic emissions, and basilar membrane motion indicate that the TM remains functionally attached to the electromotile, sensorimotor outer hair cells of the organ of Corti, and that the amplification and frequency tuning of the basilar membrane responses to sounds are almost normal. The compound action potential masker tuning curves, a measure of the tuning of the sensory inner hair cells, are also sharply tuned, but the thresholds of the compound action potentials, a measure of inner hair cell sensitivity, are significantly elevated. These results indicate that the hearing loss in patients with Otoa mutations is caused by a defect in inner hair cell stimulation, and reveal the limbal attachment of the TM plays a critical role in this process.

Instrumentation for studies of cochlear mechanics: from von Békésy forward

Georg von Békésy designed the instruments needed for his research. He also created physical models of the cochlea allowing him to manipulate the parameters (such as volume elasticity) that could be involved in controlling traveling waves. This review is about the specific devices that he used to study the motion of the basilar membrane thus allowing the analysis that lead to his Nobel Prize Award. The review moves forward in time mentioning the subsequent use of von Békésy's methods and later technologies important for motion studies of the organ of Corti. Some of the seminal findings and the controversies of cochlear mechanics are mentioned in relation to the technical developments.

How are inner hair cells stimulated? Evidence for multiple mechanical drives

Recent studies indicate that the gap over outer hair cells (OHCs) between the reticular lamina (RL) and the tectorial membrane (TM) varies cyclically during low-frequency sounds. Variation in the RL-TM gap produces radial fluid flow in the gap that can drive inner hair cell (IHC) stereocilia. Analysis of RL-TM gap changes reveals three IHC drives in addition to classic SHEAR. For upward basilar-membrane (BM) motion, IHC stereocilia are deflected in the excitatory direction by SHEAR and OHC-MOTILITY, but in the inhibitory direction by TM-PUSH and CILIA-SLANT. Upward BM motion causes OHC somatic contraction which tilts the RL, compresses the RL-TM gap over IHCs and expands the RL-TM gap over OHCs, thereby producing an outward (away from the IHCs) radial fluid flow which is the OHC-MOTILITY drive. For upward BM motion, the force that moves the TM upward also compresses the RL-TM gap over OHCs causing inward radial flow past IHCs which is the TM-PUSH drive. Motions that produce large tilting of OHC stereocilia squeeze the supra-OHC RL-TM gap and caused inward radial flow past IHCs which is the CILIA-SLANT drive. Combinations of these drives explain: (1) the reversal at high sound levels of auditory nerve (AN) initial peak (ANIP) responses to clicks, and medial olivocochlear (MOC) inhibition of ANIP responses below, but not above, the ANIP reversal, (2) dips and phase reversals in AN responses to tones in cats and chinchillas, (3) hypersensitivity and phase reversals in tuning-curve tails after OHC ablation, and (4) MOC inhibition of tail-frequency AN responses. The OHC-MOTILITY drive provides another mechanism, in addition to BM motion amplification, that uses active processes to enhance the output of the cochlea. The ability of these IHC drives to explain previously anomalous data provides strong, although indirect, evidence that these drives are significant and presents a new view of how the cochlea works at frequencies below 3 kHz.

Progress in cochlear physiology after Békésy

In the fifty years since Békésy was awarded the Nobel Prize, cochlear physiology has blossomed. Many topics that are now current are things Békésy could not have imagined. In this review we start by describing progress in understanding the origin of cochlear gross potentials, particularly the cochlear microphonic, an area in which Békésy had extensive experience. We then review progress in areas of cochlear physiology that were mostly unknown to Békésy, including: (1) stereocilia mechano-electrical transduction, force production, and response amplification, (2) outer hair cell (OHC) somatic motility and its molecular basis in prestin, (3) cochlear amplification and related micromechanics, including the evidence that prestin is the main motor for cochlear amplification, (4) the influence of the tectorial membrane, (5) cochlear micromechanics and the mechanical drives to inner hair cell stereocilia, (6) otoacoustic emissions, and (7) olivocochlear efferents and their influence on cochlear physiology. We then return to a subject that Békésy knew well: cochlear fluids and standing currents, as well as our present understanding of energy dependence on the lateral wall of the cochlea. Finally, we touch on cochlear pathologies including noise damage and aging, with an emphasis on where the field might go in the future.

Transmission of cochlear distortion products as slow waves: a comparison of experimental and model data

There is a long-lasting question of how distortion products (DPs) arising from nonlinear amplification processes in the cochlea are transmitted from their generation sites to the stapes. Two hypotheses have been proposed: (1) the slow-wave hypothesis whereby transmission is via the transverse pressure difference across the cochlear partition and (2) the fast-wave hypothesis proposing transmission via longitudinal compression waves. Ren with co-workers have addressed this topic experimentally by measuring the spatial vibration pattern of the basilar membrane (BM) in response to two tones of frequency f(1) and f(2). They interpreted the observed negative phase slopes of the stationary BM vibrations at the cubic distortion frequency f(DP) = 2f(1) - f(2) as evidence for the fast-wave hypothesis. Here, using a physically based model, it is shown that their phase data is actually in accordance with the slow-wave hypothesis. The analysis is based on a frequency-domain formulation of the two-dimensional motion equation of a nonlinear hydrodynamic cochlea model. Application of the analysis to their experimental data suggests that the measurement sites of negative phase slope were located at or apical to the DP generation sites. Therefore, current experimental and theoretical evidence supports the slow-wave hypothesis. Nevertheless, the analysis does not allow rejection of the fast-wave hypothesis.

Vibration responses of the organ of Corti and the tectorial membrane to electrical stimulation

Coupling of somatic electromechanical force from the outer hair cells (OHCs) into the organ of Corti is investigated by measuring transverse vibration patterns of the organ of Cori and tectorial membrane (TM) in response to intracochlear electrical stimulation. Measurement places at the organ of Corti extend from the inner sulcus cells to Hensen's cells and at the lower (and upper) surface of the TM from the inner sulcus to the OHC region. These locations are in the neighborhood of where electromechanical force is coupled into (1) the mechanoelectrical transducers of the stereocilia and (2) fluids of the organ of Corti. Experiments are conducted in the first, second, and third cochlear turns of an in vitro preparation of the adult guinea pig cochlea. Vibration measurements are made at functionally relevant stimulus frequencies (0.48-68 kHz) and response amplitudes (<15 nm). The experiments provide phase relations between the different structures, which, dependent on frequency range and longitudinal cochlear position, include in-phase transverse motions of the TM, counterphasic transverse motions between the inner hair cell and OHCs, as well as traveling-wave motion of Hensen's cells in the radial direction. Mechanics of sound processing in the cochlea are discussed based on these phase relationships.

Auditory mechanics of the tectorial membrane and the cochlear spiral

PURPOSE OF REVIEW

This review is timely and relevant because new experimental and theoretical findings suggest that cochlear mechanics from the nanoscale to the macroscale are affected by the mechanical properties of the tectorial membrane and the cochlea's spiral shape.

RECENT FINDINGS

Main tectorial membrane themes addressed in this review are composition and morphology, nanoscale mechanical interactions with the outer hair cell bundle, macroscale longitudinal coupling, fluid interaction with inner hair cell bundles, and macroscale dynamics and waves. Main cochlear spiral themes are macroscale, low-frequency energy focusing and microscale organ of Corti shear gain.

SUMMARY

Recent experimental and theoretical findings reveal exquisite sensitivity of cochlear mechanical performance to tectorial membrane structural organization, mechanics, and its positioning with respect to hair bundles. The cochlear spiral geometry is a major determinant of low-frequency hearing. These findings suggest a number of important research directions.

Coupling active hair bundle mechanics, fast adaptation, and somatic motility in a cochlear model

One of the central questions in the biophysics of the mammalian cochlea is determining the contributions of the two active processes, prestin-based somatic motility and hair bundle (HB) motility, to cochlear amplification. HB force generation is linked to fast adaptation of the transduction current via a calcium-dependent process and somatic force generation is driven by the depolarization caused by the transduction current. In this article, we construct a global mechanical-electrical-acoustical mathematical model of the cochlea based on a three-dimensional fluid representation. The global cochlear model is coupled to linearizations of nonlinear somatic motility and HB activity as well as to the micromechanics of the passive structural and electrical elements of the cochlea. We find that the active HB force alone is not sufficient to power high frequency cochlear amplification. However, somatic motility can overcome resistor-capacitor filtering by the basolateral membrane and deliver sufficient mechanical energy for amplification at basal locations. The results suggest a new theory for high frequency active cochlear mechanics, in which fast adaptation controls the transduction channel sensitivity and thereby the magnitude of the energy delivered by somatic motility.

The endocochlear potential alters cochlear micromechanics

Acoustic stimulation gates mechanically sensitive ion channels in cochlear sensory hair cells. Even in the absence of sound, a fraction of these channels remains open, forming a conductance between hair cells and the adjacent fluid space, scala media. Restoring the lost endogenous polarization of scala media in an in vitro preparation of the whole cochlea depolarizes the hair cell soma. Using both digital laser interferometry and time-resolved confocal imaging, we show that this causes a structural refinement within the organ of Corti that is dependent on the somatic electromotility of the outer hair cells (OHCs). Specifically, the inner part of the reticular lamina up to the second row of OHCs is pulled toward the basilar membrane, whereas the outer part (third row of OHCs and the Hensen's cells) unexpectedly moves in the opposite direction. A similar differentiated response pattern is observed for sound-evoked vibrations: restoration of the endogenous polarization decreases vibrations of the inner part of the reticular lamina and results in up to a 10-fold increase of vibrations of the outer part. We conclude that the endogenous polarization of scala media affects the function of the hearing organ by altering its geometry, mechanical and electrical properties.

Linking LIMK1 deficiency to hyperacusis and progressive hearing loss in individuals with Williams syndrome

Williams syndrome (a.k.a. Williams-Beuren Syndrome) is a multisystem disorder caused by the hemizygous deletion of a 1.6 Mb region at 7q11.23 encompassing about 26 genes, including that encoding LIM kinase 1 (LIMK1). Individuals with Williams Syndrome manifest hyperacusis and progressive hearing loss, and hyperacusis early onset suggests that it could be associated with one of the deleted genes. Based on our results about the critical role of LIM kinases in the regulation of the motile responses of cochlear outer hair cells (OHC) and cochlear amplification, we propose here that a reduced expression of LIMK1 in OHC would be the major underlying cause of the hyperacusis and progressive hearing loss observed in patients with Williams Syndrome. Moreover, we propose a novel model of gain-control for cochlear amplification based on LIMK-mediated regulation of OHC's slow motility.

Auditory nerve excitation via a non-traveling wave mode of basilar membrane motion

Basilar membrane (BM) motion and auditory nerve fiber (ANF) tuning are generally very similar, but the ANF had appeared to be unresponsive to a plateau mode of BM motion that occurs at frequencies above an ANF's characteristic frequency (CF). We recorded ANF responses from the gerbil, concentrating on this supra-CF region. We observed a supra-CF plateau in ANF responses at high stimulus level, indicating that the plateau mode of BM motion can be excitatory.

Breaking away: violation of distortion emission phase-frequency invariance at low frequencies

The phase versus frequency function of the distortion product otoacoustic emission (DPOAE) at 2f(1) - f(2) is approximately invariant at frequencies above 1.5 kHz in human subjects when recorded with a constant f(2)/f(1). However, a secular break from this invariance has been observed at lower frequencies where the phase-gradient becomes markedly steeper. Apical DPOAEs, such as 2f(1) - f(2), are known to contain contributions from multiple sources. This experiment asked whether the phase behavior of the ear canal DPOAE at low frequencies is driven by the phase of the component from the distortion product (DP) region at 2f(1) - f(2), which exhibits rapid phase accumulation. Placing a suppressor tone close in the frequency to 2f(1) - f(2) reduced the contribution of this component to the ear canal DPOAE in normal-hearing adult human ears. When the contribution of this component was reduced, the phase behavior of the ear canal DPOAE was not altered, suggesting that the breaking from DPOAE phase invariance at low frequencies is an outcome of apical-basal differences in cochlear mechanics. The deviation from DPOAE phase invariance appears to be a manifestation of the breaking from approximate scaling symmetry in the human cochlear apex.

Tectorial membrane material properties in Tecta(Y)(1870C/+) heterozygous mice

The solid component of the tectorial membrane (TM) is a porous matrix made up of the radial collagen fibers and the striated sheet matrix. The striated sheet matrix is believed to contribute to shear impedance in both the radial and longitudinal directions, but the molecular mechanisms involved have not been determined. A missense mutation in Tecta, a gene that encodes for the α-tectorin protein in the striated sheet matrix, causes a 60-dB threshold shift in mice with relatively little reduction in outer hair cell amplification. Here, we show that this threshold shift is coupled to changes in shear impedance, response to osmotic pressure, and concentration of fixed charge of the TM. In Tecta(Y)(1870C/+) mice, the tectorin content of the TM was reduced, as was the content of glycoconjugates reacting with the lectin wheat germ agglutinin. Charge measurements showed a decrease in fixed charge concentration from -6.4±1.4 mmol/L in wild-types to -2.1±0.7 mmol/L in Tecta(Y)(1870C/+) TMs. TMs from Tecta(Y)(1870C/+) mice showed little volume change in response to osmotic pressure compared to those of wild-type mice. The magnitude of both radial and longitudinal TM shear impedance was reduced by 10±1.6 dB in Tecta(Y)(1870C/+) mice. However, the phase of shear impedance was unchanged. These changes are consistent with an increase in the porosity of the TM and a corresponding decrease of the solid fraction. Mechanisms by which these changes can affect the coupling between outer and inner hair cells are discussed.

The remarkable cochlear amplifier

This composite article is intended to give the experts in the field of cochlear mechanics an opportunity to voice their personal opinion on the one mechanism they believe dominates cochlear amplification in mammals. A collection of these ideas are presented here for the auditory community and others interested in the cochlear amplifier. Each expert has given their own personal view on the topic and at the end of their commentary they have suggested several experiments that would be required for the decisive mechanism underlying the cochlear amplifier. These experiments are presently lacking but if successfully performed would have an enormous impact on our understanding of the cochlear amplifier.

Responses of the ear to low frequency sounds, infrasound and wind turbines

Infrasonic sounds are generated internally in the body (by respiration, heartbeat, coughing, etc) and by external sources, such as air conditioning systems, inside vehicles, some industrial processes and, now becoming increasingly prevalent, wind turbines. It is widely assumed that infrasound presented at an amplitude below what is audible has no influence on the ear. In this review, we consider possible ways that low frequency sounds, at levels that may or may not be heard, could influence the function of the ear. The inner ear has elaborate mechanisms to attenuate low frequency sound components before they are transmitted to the brain. The auditory portion of the ear, the cochlea, has two types of sensory cells, inner hair cells (IHC) and outer hair cells (OHC), of which the IHC are coupled to the afferent fibers that transmit "hearing" to the brain. The sensory stereocilia ("hairs") on the IHC are "fluid coupled" to mechanical stimuli, so their responses depend on stimulus velocity and their sensitivity decreases as sound frequency is lowered. In contrast, the OHC are directly coupled to mechanical stimuli, so their input remains greater than for IHC at low frequencies. At very low frequencies the OHC are stimulated by sounds at levels below those that are heard. Although the hair cells in other sensory structures such as the saccule may be tuned to infrasonic frequencies, auditory stimulus coupling to these structures is inefficient so that they are unlikely to be influenced by airborne infrasound. Structures that are involved in endolymph volume regulation are also known to be influenced by infrasound, but their sensitivity is also thought to be low. There are, however, abnormal states in which the ear becomes hypersensitive to infrasound. In most cases, the inner ear's responses to infrasound can be considered normal, but they could be associated with unfamiliar sensations or subtle changes in physiology. This raises the possibility that exposure to the infrasound component of wind turbine noise could influence the physiology of the ear.

Otoacoustic estimation of cochlear tuning: validation in the chinchilla

We analyze published auditory-nerve and otoacoustic measurements in chinchilla to test a network of hypothesized relationships between cochlear tuning, cochlear traveling-wave delay, and stimulus-frequency otoacoustic emissions (SFOAEs). We find that the physiological data generally corroborate the network of relationships, including predictions from filter theory and the coherent-reflection model of OAE generation, at locations throughout the cochlea. The results support the use of otoacoustic emissions as noninvasive probes of cochlear tuning. Developing this application, we find that tuning ratios-defined as the ratio of tuning sharpness to SFOAE phase-gradient delay in periods-have a nearly species-invariant form in cat, guinea pig, and chinchilla. Analysis of the tuning ratios identifies a species-dependent parameter that locates a transition between "apical-like" and "basal-like" behavior involving multiple aspects of cochlear physiology. Approximate invariance of the tuning ratio allows determination of cochlear tuning from SFOAE delays. We quantify the procedure and show that otoacoustic estimates of chinchilla cochlear tuning match direct measures obtained from the auditory nerve. By assuming that invariance of the tuning ratio extends to humans, we derive new otoacoustic estimates of human cochlear tuning that remain mutually consistent with independent behavioral measurements obtained using different rationales, methodologies, and analysis procedures. The results confirm that at any given characteristic frequency (CF) human cochlear tuning appears sharper than that in the other animals studied, but varies similarly with CF. We show, however, that the exceptionality of human tuning can be exaggerated by the ways in which species are conventionally compared, which take no account of evident differences between the base and apex of the cochlea. Finally, our estimates of human tuning suggest that the spatial spread of excitation of a pure tone along the human basilar membrane is comparable to that in other common laboratory animals.

Compliance profiles derived from a three-dimensional finite-element model of the basilar membrane

A finite-element analysis is used to explore the impact of elastic material properties, boundary conditions, and geometry, including coiling, on the spatial characteristics of the compliance of the unloaded basilar membrane (BM). It is assumed that the arcuate zone is isotropic and the pectinate zone orthotropic, and that the radial component of the effective Young's modulus in the pectinate zone decreases exponentially with distance from base to apex. The results concur with tonotopic characteristics of compliance and neural data. Moreover, whereas the maximum compliance in a radial profile is located close to the boundary between the two zones in the basal region, it shifts to the midpoint of the pectinate zone for the apical BM; the width of the profile also expands. This shift begins near the 1 kHz characteristic place for guinea pig and the 2.4 kHz place for gerbil. Shift and expansion are not observed for linear rather than exponential decrease of the radial component of Young's modulus. This spatial change of the compliance profile leads to the prediction that mechanical excitation in the apical region of the organ of Corti is different to that in the basal region.

Cochlear outer hair cells undergo an apical circumference remodeling constrained by the hair bundle shape

Epithelial cells acquire diverse shapes relating to their different functions. This is particularly relevant for the cochlear outer hair cells (OHCs), whose apical and basolateral shapes accommodate the functioning of these cells as mechano-electrical and electromechanical transducers, respectively. We uncovered a circumferential shape transition of the apical junctional complex (AJC) of OHCs, which occurs during the early postnatal period in the mouse, prior to hearing onset. Geometric analysis of the OHC apical circumference using immunostaining of the AJC protein ZO1 and Fourier-interpolated contour detection characterizes this transition as a switch from a rounded-hexagon to a non-convex circumference delineating two lateral lobes at the neural side of the cell, with a negative curvature in between. This shape tightly correlates with the 'V'-configuration of the OHC hair bundle, the apical mechanosensitive organelle that converts sound-evoked vibrations into variations in cell membrane potential. The OHC apical circumference remodeling failed or was incomplete in all the mouse mutants affected in hair bundle morphogenesis that we tested. During the normal shape transition, myosin VIIa and myosin II (A and B isoforms) displayed polarized redistributions into and out of the developing lobes, respectively, while Shroom2 and F-actin transiently accumulated in the lobes. Defects in these redistributions were observed in the mutants, paralleling their apical circumference abnormalities. Our results point to a pivotal role for actomyosin cytoskeleton tensions in the reshaping of the OHC apical circumference. We propose that this remodeling contributes to optimize the mechanical coupling between the basal and apical poles of mature OHCs.

In vivo impedance of the gerbil cochlear partition at auditory frequencies

The specific acoustic impedance of the cochlear partition was measured from 4 to 20 kHz in the basal turn of the gerbil cochlea, where the best frequency is approximately 40 kHz. The acoustic impedance was found as the ratio of driving pressure to velocity response. It is the physical attribute that governs cochlear mechanics and has never before been directly measured, to our knowledge. The basilar membrane velocity was measured through the transparent round window membrane. Simultaneously, the intracochlear pressure was measured close to the stapes and quite close to the cochlear partition. The impedance phase was close to -90 degrees and the magnitude decreased with frequency, consistent with stiffness-dominated impedance. The resistive component of the impedance was relatively small. Usually the resistance was negative at frequencies below 8 kHz; this unexpected finding might be due to other vibration modes within the cochlear partition.

Multiple roles for the tectorial membrane in the active cochlea

This review is concerned with experimental results that reveal multiple roles for the tectorial membrane in active signal processing in the mammalian cochlea. We discuss the dynamic mechanical properties of the tectorial membrane as a mechanical system with several degrees of freedom and how its different modes of movement can lead to hair-cell excitation. The role of the tectorial membrane in distributing energy along the cochlear partition and how it channels this energy to the inner hair cells is described.

An adaptive model simulating the somatic motility and the active hair bundle motion of the OHC

The outer hair cells (OHC) of the mammalian inner ear change the sensitivity and frequency selectivity of the filtering system of the cochlea using two kinds of mechanical activity: the somatic motility and the active hair bundle motion. We designed a non-linear adaptive model of the OHC employing both mechanisms of the mechanical activity. The modeling results show that the high sensitivity and frequency selectivity of the filtering system of the cochlea depend on the somatic motility of the OHC. However, both mechanisms of mechanical activity are involved in the adaptation to sound intensity and efferent-synaptic influence. The fast (alternating) component (AC) of the mechanical-electrical transduction signal controls the motor protein prestin and fast changes in axial length of the cell. The slow (direct) component (DC) appearing at high signal intensity affects the axial stiffness, the cell length and the position of the hair bundle. The efferent influence is realized by the same mechanism.

A digital heterodyne laser interferometer for studying cochlear mechanics

Laser interferometry is the technique of choice for studying the smallest displacements of the hearing organ. For low intensity sound stimulation, these displacements may be below 1 nm. This cannot be reliably measured with other presently available techniques in an intact organ of Corti. In a heterodyne interferometer, light is projected against an object of study and motion of the target along the optical axis causes phase and frequency modulations of the back-reflected light. To recover object motion, the reflected light is made to interfere with a reference beam of artificially altered frequency, producing a beating signal. In conventional interferometers, this carrier signal is demodulated with analog electronics. In this paper, we describe a digital implementation of the technique, using direct carrier sampling. In order to obtain the necessary reference signal for demodulation we introduce an additional third light path. Together, this results in lower noise and reduces the cost of the system. Within the hearing organ, different structures may move in different directions. It is therefore necessary to precisely measure the angle of incidence of the laser light, and to precisely localize the anatomical structure where the measurement is performed. Therefore, the interferometer is integrated with a laser scanning confocal microscope that permits us to map crucial morphometric parameters in each experiment. We provide key construction parameters and a detailed performance characterization. We also show that the system accurately measures the diminutive vibrations present in the apical turn of the cochlea during low-level sound stimulation.

The tectorial membrane: one slice of a complex cochlear sandwich

PURPOSE OF REVIEW

The review is both timely and relevant, as recent findings have shown the tectorial membrane plays a more dynamic role in hearing than hitherto suspected, and that many forms of deafness can result from mutations in tectorial membrane proteins.

RECENT FINDINGS

Main themes covered are the molecular composition, the structural organization and properties of the tectorial membrane, the role of the tectorial membrane as a second resonator and a structure within which there is significant longitudinal coupling, and how mutations in tectorial membrane proteins cause deafness in mice and men.

CONCLUSION

Findings from experimental models imply that the tectorial membrane plays multiple, critical roles in hearing. These include coupling elements along the length of the cochlea, supporting a travelling wave and ensuring the gain and timing of cochlear feedback are optimal. The clinical findings suggest stable, moderate-to-severe forms of hereditary hearing loss may be diagnostic of a mutation in TECTA, a gene encoding one of the major, noncollagenous proteins of the tectorial membrane.

Comment on "Measuring power production in the mammalian cochlea" [Curr. Biol. 17, 1340 (2007)]

Recently, a paper by Lakashkin et al. (2007) ("Power amplification in the mammalian cochlea," Curr. Biol. 17, 1340-1344) was published on how power can be measured in the mammalian cochlea. The general subject is of current widespread interest, so the question of whether the method used by Lakashkin et al. is valid may be of interest to the readers of this journal. Power generation in the cochlea can account for the extraordinary sensitivity of hearing. Lukashkin et al. claimed to provide a direct proof of cochlear power generation. A first-order spring-dashpot system was used to model the organ of Corti. The power flux direction can be derived from the sign of the phase difference between the force and displacement, which can be presented as a "hysteresis plot." Basilar membrane (BM) vibration near the characteristic frequency (CF) was measured while applying a low-frequency modulation tone together with the CF tone. A force was derived from the modulation profile of the BM CF vibration and when plotted versus the displacement at the modulation frequency, the function had a counterclockwise direction of hysteresis, suggesting power generation. In this letter, we present comments on the analysis in the report: (1) that it is not appropriate to analyze at the modulation frequency to derive the power generation at CF; (2) that the derivation of a force from just the displacement profile is not justified, followed by an alternative interpretation of the experimental data.

A MULTISCALE MODEL OF THE ORGAN OF CORTI

The organ of Corti is the sensory epithelium in the cochlea of the inner ear. It is modeled as a shell-of-revolution structure with continuous and discrete components. Our recent work has been on the inclusion of the viscous fluid. Measurements from various laboratories provide the opportunity to refocus on the elastic properties. The current detailed model for the organ of Corti is reasonably consistent with diverse measurements. Most components have little stiffness in the propagation direction. However, the isotropic stiffness of the pillar heads is found to offer an explanation for the difference in point load and pressure measurements. The individual rows of inner hair cell stereocilia with tip links and the Hensen stripe are included, since these details are important for the determination of the neural excitation. The results for low frequency show a phase of tip link tension similar to auditory nerve measurements. The nonlinearity of fluid in the small gaps is considered. A result is that as amplitude increases, because of the near contact with the Hensen stripe, the excitation changes polarity, similar to the peak-splitting neural behavior sometimes observed.

The influence of noise exposure on the parameters of a convolution model of the compound action potential

The influence of noise exposure on the parameters of a convolution model of the compound action potential (CAP) was examined. CAPs were recorded in normal-hearing gerbils and in gerbils exposed to a 117 dB SPL 8 kHz band of noise for various durations. The CAPs were fitted with an analytic CAP to obtain the parameters representing the number of nerve fibers (N), the probability density function [P(t)] from a population of nerve fibers, and the single-unit waveform [U(t)]. The results showed that the analytic CAP fitted the physiologic CAPs well with correlations of approximately 0.90. A subsequent analysis using hierarchical linear modeling quantified the change in the parameters as a function of both signal level and hearing threshold. The results showed that noise exposure caused some of the parameter-level functions to simply shift along the signal level axis in proportion to the amount of hearing loss, whereas others shifted along the signal level axis and steepened. Significant changes occurred in the U(t) parameters, but they were not related to hearing threshold. These results suggest that noise exposure alters the physiology underlying the CAP, some of which can be explained by a simple lack of gain, whereas others may not.

Medial olivocochlear efferent inhibition of basilar-membrane responses to clicks: evidence for two modes of cochlear mechanical excitation

Conceptualizations of mammalian cochlear mechanics are based on basilar-membrane (BM) traveling waves that scale with frequency along the length of the cochlea, are amplified by outer hair cells (OHCs), and excite inner hair cells and auditory-nerve (AN) fibers in a simple way. However, recent experimental work has shown medial-olivocochlear (MOC) inhibition of AN responses to clicks that do not fit with this picture. To test whether this AN-initial-peak (ANIP) inhibition might result from hitherto unrecognized aspects of the traveling-wave or MOC-evoked inhibition, MOC effects on BM responses to clicks in the basal turns of guinea pig and chinchilla cochleae were measured. MOC stimulation inhibited BM click responses in a time and level dependent manner. Inhibition was not seen during the first half-cycle of the responses, but built up gradually, and ultimately increased the responses' decay rates. MOC stimulation also produced small phase leads in the response wave forms, but had little effect on the instantaneous frequency or the waxing and waning of the responses. These data, plus recent AN data, support the hypothesis that the MOC-evoked inhibitions of the traveling wave and of the ANIP response are separate phenomena, and indicate that the OHCs can affect at least two separate modes of excitation in the mammalian cochlea.

Cochlear mechanics: no shout but a twist in the absence of prestin

Mammalian hearing is boosted by mechanically active auditory receptor cells, the outer hair cells which amplify the actions of incoming sounds. Recent evidence indicates that the molecular motor that drives this amplification, prestin, may do more than boogie.

Testing coherent reflection in chinchilla: Auditory-nerve responses predict stimulus-frequency emissions

Coherent-reflection theory explains the generation of stimulus-frequency and transient-evoked otoacoustic emissions by showing how they emerge from the coherent "backscattering" of forward-traveling waves by mechanical irregularities in the cochlear partition. Recent published measurements of stimulus-frequency otoacoustic emissions (SFOAEs) and estimates of near-threshold basilar-membrane (BM) responses derived from Wiener-kernel analysis of auditory-nerve responses allow for comprehensive tests of the theory in chinchilla. Model predictions are based on (1) an approximate analytic expression for the SFOAE signal in terms of the BM traveling wave and its complex wave number, (2) an inversion procedure that derives the wave number from BM traveling waves, and (3) estimates of BM traveling waves obtained from the Wiener-kernel data and local scaling assumptions. At frequencies above 4 kHz, predicted median SFOAE phase-gradient delays and the general shapes of SFOAE magnitude-versus-frequency curves are in excellent agreement with the measurements. At frequencies below 4 kHz, both the magnitude and the phase of chinchilla SFOAEs show strong evidence of interference between short- and long-latency components. Approximate unmixing of these components, and association of the long-latency component with the predicted SFOAE, yields close agreement throughout the cochlea. Possible candidates for the short-latency SFOAE component, including wave-fixed distortion, are considered. Both empirical and predicted delay ratios (long-latency SFOAE delay/BM delay) are significantly less than 2 but greater than 1. Although these delay ratios contradict models in which SFOAE generators couple primarily into cochlear compression waves, they are consistent with the notion that forward and reverse energy propagation in the cochlea occurs predominantly by means of traveling pressure-difference waves. The compelling overall agreement between measured and predicted delays suggests that the coherent-reflection model captures the dominant mechanisms responsible for the generation of reflection-source otoacoustic emissions.

Outer hair cell somatic, not hair bundle, motility is the basis of the cochlear amplifier

Sensitivity, dynamic range and frequency tuning of the cochlea are attributed to amplification involving outer hair cell stereocilia and/or somatic motility. We measured acoustically and electrically elicited basilar membrane displacements from the cochleae of wild-type and Tecta(DeltaENT/DeltaENT) mice, in which stereocilia are unable to contribute to amplification near threshold. Electrically elicited responses from Tecta(DeltaENT/DeltaENT) mice were markedly similar to acoustically and electrically elicited responses from wild-type mice. We conclude that somatic, and not stereocilia, motility is the basis of cochlear amplification.

Mechanics of the exceptional anuran ear

The anuran ear is frequently used for studying fundamental properties of vertebrate auditory systems. This is due to its unique anatomical features, most prominently the lack of a basilar membrane and the presence of two dedicated acoustic end organs, the basilar papilla and the amphibian papilla. Our current anatomical and functional knowledge implies that three distinct regions can be identified within these two organs. The basilar papilla functions as a single auditory filter. The low-frequency portion of the amphibian papilla is an electrically tuned, tonotopically organized auditory end organ. The high-frequency portion of the amphibian papilla is mechanically tuned and tonotopically organized, and it emits spontaneous otoacoustic emissions. This high-frequency portion of the amphibian papilla shows a remarkable, functional resemblance to the mammalian cochlea.

Cochlear outer hair cell motility

Normal hearing depends on sound amplification within the mammalian cochlea. The amplification, without which the auditory system is effectively deaf, can be traced to the correct functioning of a group of motile sensory hair cells, the outer hair cells of the cochlea. Acting like motor cells, outer hair cells produce forces that are driven by graded changes in membrane potential. The forces depend on the presence of a motor protein in the lateral membrane of the cells. This protein, known as prestin, is a member of a transporter superfamily SLC26. The functional and structural properties of prestin are described in this review. Whether outer hair cell motility might account for sound amplification at all frequencies is also a critical question and is reviewed here.

Prestin's role in cochlear frequency tuning and transmission of mechanical responses to neural excitation

The remarkable power amplifier [1] of the cochlea boosts low-level and compresses high-level vibrations of the basilar membrane (BM) [2]. By contributing maximally at the characteristic frequency (CF) of each point along its length, the amplifier ensures the exquisite sensitivity, narrow frequency tuning, and enormous dynamic range of the mammalian cochlea. The motor protein prestin in the outer hair cell (OHC) lateral membrane is a prime candidate for the cochlear power amplifier [3]. The other contender for this role is the ubiquitous calcium-mediated motility of the hair cell stereocilia, which has been demonstrated in vitro and is based on fast adaptation of the mechanoelectrical transduction channels [4, 5]. Absence of prestin [6] from OHCs results in a 40-60 dB reduction in cochlear neural sensitivity [7]. Here we show that sound-evoked BM vibrations in the high-frequency region of prestin(-/-) mice cochleae are, surprisingly, as sensitive as those of their prestin(+/+) siblings. The BM vibrations of prestin(-/-) mice are, however, broadly tuned to a frequency approximately a half octave below the CF of prestin(+/+) mice at similar BM locations. The peak sensitivity of prestin(+/+) BM tuning curves matches the neural thresholds. In contrast, prestin(-/-) BM tuning curves at their best frequency are >50 dB more sensitive than the neural responses. We propose that the absence of prestin from OHCs, and consequent reduction in stiffness of the cochlea partition, changes the passive impedance of the BM at high frequencies, including the CF. We conclude that prestin influences the cochlear partition's dynamic properties that permit transmission of its vibrations into neural excitation. Prestin is crucial for defining sharp and sensitive cochlear frequency tuning by reducing the sensitivity of the low-frequency tail of the tuning curve, although this necessitates a cochlear amplifier to determine the narrowly tuned tip.

Mouse models for human hereditary deafness

Hearing impairment is a frequent condition in humans. Identification of the causative genes for the early onset forms of isolated deafness began 15 years ago and has been very fruitful. To date, approximately 50 causative genes have been identified. Yet, limited information regarding the underlying pathogenic mechanisms can be derived from hearing tests in deaf patients. This chapter describes the success of mouse models in the elucidation of some pathophysiological processes in the auditory sensory organ, the cochlea. These models have revealed a variety of defective structures and functions at the origin of deafness genetic forms. This is illustrated by three different examples: (1) the DFNB9 deafness form, a synaptopathy of the cochlear sensory cells where otoferlin is defective; (2) the Usher syndrome, in which deafness is related to abnormal development of the hair bundle, the mechanoreceptive structure of the sensory cells to sound; (3) the DFNB1 deafness form, which is the most common form of inherited deafness in Caucasian populations, mainly caused by connexin-26 defects that alter gap junction communication between nonsensory cochlear cells.

Laser amplification with a twist: traveling-wave propagation and gain functions from throughout the cochlea

Except at the handful of sites explored by the inverse method, the characteristics-indeed, the very existence-of traveling-wave amplification in the mammalian cochlea remain largely unknown. Uncertainties are especially pronounced in the apex, where mechanical and electrical measurements lack the independent controls necessary for assessing damage to the preparation. At a functional level, the form and amplification of cochlear traveling waves are described by quantities known as propagation and gain functions. A method for deriving propagation and gain functions from basilar-membrane mechanical transfer functions is presented and validated by response reconstruction. Empirical propagation and gain functions from locations throughout the cochlea are obtained in mechanically undamaged preparations by applying the method to published estimates of near-threshold basilar membrane responses derived from Wiener-kernel (chinchilla) and zwuis analysis (cat) of auditory-nerve responses to broadband stimuli. The properties of these functions, and their variation along the length of the cochlea, are described. In both species, and at all locations examined, the gain functions reveal a region of positive power gain basal to the wave peak. The results establish the existence of traveling-wave amplification throughout the cochlea, including the apex. The derived propagation and gain functions resemble those characteristic of an active optical medium but rotated by 90 degrees in the complex plane. Rotation of the propagation and gain functions enables the mammalian cochlea to operate as a wideband, hydromechanical laser analyzer.

Longitudinally propagating traveling waves of the mammalian tectorial membrane

Sound-evoked vibrations transmitted into the mammalian cochlea produce traveling waves that provide the mechanical tuning necessary for spectral decomposition of sound. These traveling waves of motion that have been observed to propagate longitudinally along the basilar membrane (BM) ultimately stimulate the mechano-sensory receptors. The tectorial membrane (TM) plays a key role in this process, but its mechanical function remains unclear. Here we show that the TM supports traveling waves that are an intrinsic feature of its visco-elastic structure. Radial forces applied at audio frequencies (2-20 kHz) to isolated TM segments generate longitudinally propagating waves on the TM with velocities similar to those of the BM traveling wave near its best frequency place. We compute the dynamic shear storage modulus and shear viscosity of the TM from the propagation velocity of the waves and show that segments of the TM from the basal turn are stiffer than apical segments are. Analysis of loading effects of hair bundle stiffness, the limbal attachment of the TM, and viscous damping in the subtectorial space suggests that TM traveling waves can occur in vivo. Our results show the presence of a traveling wave mechanism through the TM that can functionally couple a significant longitudinal extent of the cochlea and may interact with the BM wave to greatly enhance cochlear sensitivity and tuning.

Altered traveling wave propagation and reduced endocochlear potential associated with cochlear dysplasia in the BETA2/NeuroD1 null mouse

The BETA2/NeuroD1 null mouse has cochlear dysplasia. Its cochlear duct is shorter than normal, there is a lack of spiral ganglion neurons, and there is hair cell disorganization. We measured vertical movements of the tectorial membrane at acoustic frequencies in excised cochleae in response to mechanical stimulation of the stapes using laser doppler vibrometry. While tuning curve sharpness was similar between wild-type, heterozygotes, and null mice in the base, null mutants had broader tuning in the apex. At both the base and the apex, null mice had less phase lag accumulation with increasing stimulus frequency than wild-type or heterozygote mice. In vivo studies demonstrated that the null mouse lacked distortion product otoacoustic emissions, and the cochlear microphonic and endocochlear potential were found to be severely reduced. Electrically evoked otoacoustic emissions could be elicited, although the amplitudes were lower than those of wild-type mice. Cochlear cross-sections revealed an incomplete partition malformation, with fenestrations within the modiolus that connected the cochlear turns. Outer hair cells from null mice demonstrated the normal pattern of prestin expression within their lateral walls and normal FM 1-43 dye entry. Overall, these data demonstrate that while tonotopicity can exist with cochlear dysplasia, traveling wave propagation is abnormally fast. Additionally, the presence of electrically evoked otoacoustic emissions suggests that outer hair cell reverse transduction is present, although the acoustic response is shaped by the alterations in cochlear mechanics.

Shroom2, a myosin-VIIa- and actin-binding protein, directly interacts with ZO-1 at tight junctions

Defects in myosin VIIa lead to developmental anomalies of the auditory and visual sensory cells. We sought proteins interacting with the myosin VIIa tail by using the yeast two-hybrid system. Here, we report on shroom2, a submembranous PDZ domain-containing protein that is associated with the tight junctions in multiple embryonic and adult epithelia. Shroom2 directly interacts with the C-terminal MyTH4-FERM domain of myosin VIIa and with F-actin. In addition, a shroom2 fragment containing the region of interaction with F-actin was able to protect actin filaments from cytochalasin-D-induced disruption in MDCK cells. Transfection experiments in MDCK and LE (L fibroblasts that express E-cadherin) cells led us to conclude that shroom2 is targeted to the cell-cell junctions in the presence of tight junctions only. In Ca(2+)-switch experiments on MDCK cells, ZO-1 (also known as TJP1) preceded GFP-tagged shroom2 at the differentiating tight junctions. ZO-1 directly interacts with the serine- and proline-rich region of shroom2 in vitro. Moreover, the two proteins colocalize in vivo at mature tight junctions, and could be coimmunoprecipitated from brain and cochlear extracts. We suggest that shroom2 and ZO-1 form a tight-junction-associated scaffolding complex, possibly linked to myosin VIIa, that bridges the junctional membrane to the underlying cytoskeleton, thereby contributing to the stabilization of these junctions.

Sound-evoked radial strain in the hearing organ

The hearing organ contains sensory hair cells, which convert sound-evoked vibration into action potentials in the auditory nerve. This process is greatly enhanced by molecular motors that reside within the outer hair cells, but the performance also depends on passive mechanical properties, such as the stiffness, mass, and friction of the structures within the organ of Corti. We used resampled confocal imaging to study the mechanical properties of the low-frequency regions of the cochlea. The data allowed us to estimate an important mechanical parameter, the radial strain, which was found to be 0.1% near the inner hair cells and 0.3% near the third row of outer hair cells during moderate-level sound stimulation. The strain was caused by differences in the motion trajectories of inner and outer hair cells. Motion perpendicular to the reticular lamina was greater at the outer hair cells, but inner hair cells showed greater radial vibration. These differences led to deformation of the reticular lamina, which connects the apex of the outer and inner hair cells. These results are important for understanding how the molecular motors of the outer hair cells can so profoundly affect auditory sensitivity.

Basilar membrane mechanics in the 6-9 kHz region of sensitive chinchilla cochleae

The vibration of the basilar membrane in the 6-9 kHz region in the chinchilla cochlea has been studied using a displacement sensitive interferometer. Displacements of 0.7-1.4 nm at 0 dB sound pressure level have been obtained. At the characteristic frequency (CF), rate-of-growth (ROG) functions computed as the slope of input-output (IO) functions can be as low as 0.1 dB/dB. IO functions for frequencies > CF have ROGs near 0 dB/dB and can have notches characterized by both negative slopes and expansive ROGs, i.e., > 1 dB/dB. For frequencies < 0.6*CF, ROGs > 1.2 dB/dB were found. Cochlear gain is shown to be greater than 60 dB in sensitive preparations with a single cochlea having nearly 80 dB gain. The compressive nature of the cochlea remains at all levels though it is masked at frequencies > CF when the amplitude of a compression wave exceeds that of the traveling wave. The compression wave produces the plateau region of the mechanical response at high intensities and has a nearly constant phase versus frequency function implying a high velocity. The summation of the traveling and compression waves explains the occurrence of the notches in both the IO and iso-intensity functions. Vibration of the osseous spiral limbus may alter the drive to inner hair cells.

Imaging electrically evoked micromechanical motion within the organ of corti of the excised gerbil cochlea

The outer hair cell (OHC) of the mammalian inner ear exhibits an unusual form of somatic motility that can follow membrane-potential changes at acoustic frequencies. The cellular forces that produce this motility are believed to amplify the motion of the cochlear partition, thereby playing a key role in increasing hearing sensitivity. To better understand the role of OHC somatic motility in cochlear micromechanics, we developed an excised cochlea preparation to visualize simultaneously the electrically-evoked motion of hundreds of cells within the organ of Corti (OC). The motion was captured using stroboscopic video microscopy and quantified using cross-correlation techniques. The OC motion at approximately 2-6 octaves below the characteristic frequency of the region was complex: OHC, Deiter's cell, and Hensen's cell motion were hundreds of times larger than the tectorial membrane, reticular lamina (RL), and pillar cell motion; the inner rows of OHCs moved antiphasic to the outer row; OHCs pivoted about the RL; and Hensen's cells followed the motion of the outer row of OHCs. Our results suggest that the effective stimulus to the inner hair cell hair bundles results not from a simple OC lever action, as assumed by classical models, but by a complex internal motion coupled to the RL.

Sharpened cochlear tuning in a mouse with a genetically modified tectorial membrane

Frequency tuning in the cochlea is determined by the passive mechanical properties of the basilar membrane and active feedback from the outer hair cells, sensory-effector cells that detect and amplify sound-induced basilar membrane motions. The sensory hair bundles of the outer hair cells are imbedded in the tectorial membrane, a sheet of extracellular matrix that overlies the cochlea's sensory epithelium. The tectorial membrane contains radially organized collagen fibrils that are imbedded in an unusual striated-sheet matrix formed by two glycoproteins, alpha-tectorin (Tecta) and beta-tectorin (Tectb). In Tectb(-/-) mice the structure of the striated-sheet matrix is disrupted. Although these mice have a low-frequency hearing loss, basilar-membrane and neural tuning are both significantly enhanced in the high-frequency regions of the cochlea, with little loss in sensitivity. These findings can be attributed to a reduction in the acting mass of the tectorial membrane and reveal a new function for this structure in controlling interactions along the cochlea.

Properties of distortion product otoacoustic emissions and neural suppression tuning curves attributable to the tectorial membrane resonance

Mechanically coupled cochlear structures are likely to form a resonator with several degrees of freedom. Consequently one can expect complex, frequency-dependent relative movements between these structures, particularly between the tectorial membrane and reticular lamina. Shearing movement between these two structures excites the cochlear receptors. This excitation should be minimal at the frequency of the hypothesized tectorial membrane resonance. In each preparation, simultaneous masking neural tuning curves and distortion product otoacoustic emissions were recorded. The position of the low-frequency minima in the tuning curves, frequency dependence of the emission bandpass structure, and level-dependent phase reversal were compared to determine if they were generated by a common phenomenon, for example the tectorial membrane resonance. The notch in the masking curves and the phase inversion of the emission growth functions at the auditory thresholds are both situated half an octave below the probe frequency and the high-frequency primary, respectively, and show similar frequency dependence. The emission bandpass structure is, however, likely to be generated by a combination of mechanisms with different ones dominating at different stimulus parameters.

[Electromechanical transduction: influence of the outer hair cells on the motion of the organ of Corti]

BACKGROUND

The somatic electromotility of the outer hair cells can be induced by an extracellular electrical field. This enables us to investigate the electromechanically induced motion of the organ of Corti.

METHODS

The electrically induced motion of the guinea-pig organ of Corti was measured with a laser Doppler vibrometer in three cochlear turns at ten radial positions on the reticular lamina (RL) and six on each of the upper and lower surfaces of the tectorial membrane (TM).

RESULTS AND CONCLUSIONS

We found a complex vibration pattern of the RL and TM, leading to a stimulus synchronous modulation of the depth of the subtectorial space in the region of the inner hair cells (IHCs). This modulation causes radial fluid motion inside the space up to at least 3 kHz. This motion is capable of deflecting the IHC stereocilia and provides an amplification mechanism additional to that associated with basilar-membrane motion.

Autocorrelation Analysis of Hair Bundle Structure in the Utricle

The ability of hair bundles to signal head movements and sounds depends significantly on their structure, but a quantitative picture of bundle structure has proved elusive. The problem is acute for vestibular organs because their hair bundles exhibit complex morphologies that vary with endorgan, hair cell type, and epithelial locus. Here we use autocorrelation analysis to quantify stereociliary arrays (the number, spacing, and distribution of stereocilia) on hair cells of the turtle utricle. Our first goal was to characterize zonal variation across the macula, from medial extrastriola, through striola, to lateral extrastriola. This is important because it may help explain zonal variation in response dynamics of utricular hair cells and afferents. We also use known differences in type I and II bundles to estimate array characteristics of these two hair cell types. Our second goal was to quantify variation in array orientation at single macular loci and use this to estimate directional tuning in utricular afferents. Our major findings are that, of the features measured, array width is the most distinctive feature of striolar bundles, and within the striola there are significant, negatively correlated gradients in stereocilia number and spacing that parallel gradients in bundle heights. Together with previous results on stereocilia number and bundle heights, our results support the hypothesis that striolar hair cells are specialized to signal high-frequency/acceleration head movements. Finally, there is substantial variation in bundle orientation at single macular loci that may help explain why utricular afferents respond to stimuli orthogonal to their preferred directions.