Scala vestibuli pressure and three-dimensional stapes velocity measured in direct succession in gerbil

Material characterization of human middle ear using machine-learning-based surrogate models

This study aims to introduce a novel non-invasive method for rapid material characterization of middle-ear structures, taking into consideration the invaluable insights provided by the mechanical properties of ear tissues. Valuable insights into various ear pathologies can be gleaned from the mechanical properties of ear tissues, yet conventional techniques for assessing these properties often entail invasive procedures that preclude their use on living patients. In this study, in the first step, we developed machine-learning models of the middle ear to predict its responses with a significantly lower computational cost in comparison to finite-element models. Leveraging findings from prior research, we focused on the most influential model parameters: the Young's modulus and thickness of the tympanic membrane and the Young's modulus of the stapedial annular ligament. The eXtreme Gradient Boosting (XGBoost) method was implemented for creating the machine-learning models. Subsequently, we combined the created machine-learning models with Bayesian optimization (BoTorch) for fast and efficient estimation of the Young's moduli of the tympanic membrane and the stapedial annular ligament. We demonstrate that the resultant surrogate models can fairly represent the vibrational responses of the umbo, stapes footplate, and vibration patterns of the tympanic membrane at most frequencies. Also, our proposed material characterization approach successfully estimated the Young's moduli of the tympanic membrane and stapedial annular ligament (separately and simultaneously) with values of mean absolute percentage error of less than 7%. The remarkable accuracy achieved through the proposed material characterization method underscores its potential for eventual clinical applications of estimating mechanical properties of the middle-ear structures for diagnostic purposes.

The impact of size on middle-ear sound transmission in elephants, the largest terrestrial mammal

Elephants have a unique auditory system that is larger than any other terrestrial mammal. To quantify the impact of larger middle ear (ME) structures, we measured 3D ossicular motion and ME sound transmission in cadaveric temporal bones from both African and Asian elephants in response to air-conducted (AC) tonal pressure stimuli presented in the ear canal (PEC). Results were compared to similar measurements in humans. Velocities of the umbo (VU) and stapes (VST) were measured using a 3D laser Doppler vibrometer in the 7-13,000 Hz frequency range, stapes velocity serving as a measure of energy entering the cochlea-a proxy for hearing sensitivity. Below the elephant ME resonance frequency of about 300 Hz, the magnitude of VU/PEC was an order of magnitude greater than in human, and the magnitude of VST/PEC was 5x greater. Phase of VST/PEC above ME resonance indicated that the group delay in elephant was approximately double that of human, which may be related to the unexpectedly high magnitudes at high frequencies. A boost in sound transmission across the incus long process and stapes near 9 kHz was also observed. We discuss factors that contribute to differences in sound transmission between these two large mammals.

The impact of size on middle-ear sound transmission in elephants, the largest terrestrial mammal

Elephants have a unique auditory system that is larger than any other terrestrial mammal. To quantify the impact of larger middle ear (ME) structures, we measured 3D ossicular motion and ME sound transmission in cadaveric temporal bones from both African and Asian elephants in response to air-conducted (AC) tonal pressure stimuli presented in the ear canal (P EC ). Results were compared to similar measurements in humans. Velocities of the umbo (V U ) and stapes (V ST ) were measured using a 3D laser Doppler vibrometer in the 7-13,000 Hz frequency range, stapes velocity serving as a measure of energy entering the cochlea-a proxy for hearing sensitivity. Below the elephant ME resonance frequency of about 300 Hz, the magnitude of V U /P EC was an order of magnitude greater than in human, and the magnitude of V ST /P EC was 5x greater. Phase of V ST /P EC above ME resonance indicated that the group delay in elephant was approximately double that of human, which may be related to the unexpectedly high magnitudes at high frequencies. A boost in sound transmission across the incus long process and stapes near 9 kHz was also observed. We discuss factors that contribute to differences in sound transmission between these two large mammals.

Relative importance and interactions of parameters of finite-element models of human middle ear

In the last decades, finite-element models of the middle ear have been widely used to predict the middle-ear vibration outputs. Even with the simplest linear assumption for material properties of the structures in the middle ear, these models need tens of parameters. Due to the complexities of measurements of material properties of these structures, accurate estimations of the values of most of these parameters are not possible. In this study, we benefited from the stochastic finite-element model of the middle ear we had developed in the past, to perform global sensitivity analysis. For this aim, we implemented Sobol' sensitivity analysis which ranks the importance of all uncertain parameters and interactions among them at different frequencies. To decrease the computational costs, we found Sobol' indices from surrogate models that we created using stochastic finite-element results and the polynomial chaos expansion method. Based on the results, the Young's modulus and thickness of the tympanic membrane, Young's modulus and damping of the stapedial annular ligaments, and the Young's modulus of ossicles are among the parameters with the greatest impacts on vibrations of the umbo and stapes footplate. Furthermore, the most significant interactions happen between the Young's modulus and thickness of the tympanic membrane.

Mammalian middle ear mechanics: A review

The middle ear is part of the ear in all terrestrial vertebrates. It provides an interface between two media, air and fluid. How does it work? In mammals, the middle ear is traditionally described as increasing gain due to Helmholtz's hydraulic analogy and the lever action of the malleus-incus complex: in effect, an impedance transformer. The conical shape of the eardrum and a frequency-dependent synovial joint function for the ossicles suggest a greater complexity of function than the traditional view. Here we review acoustico-mechanical measurements of middle ear function and the development of middle ear models based on these measurements. We observe that an impedance-matching mechanism (reducing reflection) rather than an impedance transformer (providing gain) best explains experimental findings. We conclude by considering some outstanding questions about middle ear function, recognizing that we are still learning how the middle ear works.

Forward and Reverse Middle Ear Transmission in Gerbil with a Normal or Spontaneously Healed Tympanic Membrane

Tympanic membranes (TM) that have healed spontaneously after perforation present abnormalities in their structural and mechanical properties; i.e., they are thickened and abnormally dense. These changes result in a deterioration of middle ear (ME) sound transmission, which is clinically presented as a conductive hearing loss (CHL). To fully understand the ME sound transmission under TM pathological conditions, we created a gerbil model with a controlled 50% pars tensa perforation, which was left to heal spontaneously for up to 4 weeks (TM perforations had fully sealed after 2 weeks). After the recovery period, the ME sound transmission, both in the forward and reverse directions, was directly measured with two-tone stimulation. Measurements were performed at the input, the ossicular chain, and output of the ME system, i.e., at the TM, umbo, and scala vestibuli (SV) next to the stapes. We found that variations in ME transmission in forward and reverse directions were not symmetric. In the forward direction, the ME pressure gain decreased in a frequency-dependent manner, with smaller loss (within 10 dB) at low frequencies and more dramatic loss at high frequency regions. The loss pattern was mainly from the less efficient acoustical to mechanical coupling between the TM and umbo, with little changes along the ossicular chain. In the reverse direction, the variations in these ears are relatively smaller. Our results provide detailed functional observations that explain CHL seen in clinical patients with abnormal TM, e.g., caused by otitis media, that have healed spontaneously after perforation or post-tympanoplasty, especially at high frequencies. In addition, our data demonstrate that changes in distortion product otoacoustic emissions (DPOAEs) result from altered ME transmission in both the forward and reverse direction by a reduction of the effective stimulus levels and less efficient transfer of DPs from the ME into the ear canal. This confirms that DPOAEs can be used to assess both the health of the cochlea and the middle ear.

Comparison of sheep and human middle-ear ossicles: anatomy and inertial properties

The sheep middle ear has been used in training to prepare physicians to perform surgeries and to test new ways of surgical access. This study aimed to (1) collect anatomical data and inertial properties of the sheep middle-ear ossicles and (2) explore effects of these features on sound transmission, in comparison to those of the human. Characteristic dimensions and inertial properties of the middle-ear ossicles of White-Alpine sheep (n = 11) were measured from high-resolution micro-CT data, and were assessed in comparison with the corresponding values of the human middle ear. The sheep middle-ear ossicles differed from those of human in several ways: anteroinferior orientation of the malleus handle, relatively small size of the incus with a relatively short distance to the lenticular process, a large area of the articular surfaces at the incudostapedial joint, and a relatively small moment of inertia along the anterior-posterior axis. Analysis in this study suggests that structure and orientation of the middle-ear ossicles in the sheep are conducive to an increase in the hinge-like ossicular-lever-action around the anterior-posterior axis. Considering the substantial anatomical differences, outcomes of middle-ear surgeries would presumably be difficult to assess from experiments using the sheep middle ear.

Limitations of present models of blast-induced sound power conduction through the external and middle ear

The use of models to predict the effect of blast-like impulses on hearing function is an ongoing topic of investigation relevant to hearing protection and hearing-loss prevention in the modern military. The first steps in the hearing process are the collection of sound power from the environment and its conduction through the external and middle ear into the inner ear. Present efforts to quantify the conduction of high-intensity sound power through the auditory periphery depend heavily on modeling. This paper reviews and elaborates on several existing models of the conduction of high-level sound from the environment into the inner ear and discusses the shortcomings of these models. A case is made that any attempt to more accurately define the workings of the middle ear during high-level sound stimulation needs to be based on additional data, some of which has been recently gathered.

Effects of tympanic membrane perforation on middle ear transmission in gerbil

Perforations of the tympanic membrane (TM) alter its structural and mechanical properties, thus resulting in a deterioration of sound transmission through the middle ear (ME), which presents itself clinically as a conductive hearing loss (CHL). The resulting CHL is proposed to be due to the loss of the pressure difference across the TM between the outer ear canal space and the ME cavity, a hypothesis which has been tested with both theoretical and experimental approaches. In the past, direct experimental observations had been either from the ME input (umbo) or the output of the stapes, and were focused mainly on the low frequency region. However, there was little documentation providing a thorough picture of the influence of systematically increasing sizes of TM perforations on ME sound transmission from the input (i.e., pressure at the TM or motion of the umbo) to the output (pressure produced by the motion of the stapes). Our study explored ME transmission in gerbil under conditions of a normal, intact TM followed by the placement of mechanically-induced TM perforations ranging from miniscule to complete removal of the pars tensa, leaving the other parts of ME intact. Testing up to 50 kHz, variations of ME transmission were characterized in simultaneously measured tone induced pressure responses at the TM (P_TM), pressure responses in the scala vestibuli next to the stapes (P_SV), and velocity measurements of the umbo (V_umbo), as well as by detailed descriptions of sound transmission from the TM to the stapes, i.e., the umbo transfer function (TF), the transfer of the sound stimulus along the ossicular chain as found from the ratio of cochlear pressure to umbo motion, and ME pressure gain (MEPG). Our results suggested that increasing the size of TM perforations led to a reduction in MEPG, which appeared to be primarily due to the reduction in the effective/initial mechanical drive to the umbo, with a relatively smaller decrease of sound transfer along the ossicular chain. Expansion of the perforation more than 25% appeared to drastically reduce sound transmission through the ME, especially for the higher frequencies.

Middle-Ear Sound Transmission Under Normal, Damaged, Repaired, and Reconstructed Conditions

HYPOTHESIS

We hypothesize that current clinical treatment strategies for the disarticulated or eroded incus have the effect of combining the incus and stapes of the human middle ear (ME) into one rigid structure, which, while capable of adequately transmitting lower-frequency sounds, fails for higher frequencies.

BACKGROUND

ME damage causes conductive hearing loss (CHL) and while great progress has been made in repairing or reconstructing damaged MEs, the outcomes are often far from ideal.

METHODS

Temporal bones (TBs) from human cadavers, a laser Doppler vibrometer (LDV), and a fiber-optic based micro-pressure sensor were used to characterize ME transmission under various ME conditions: normal; with a disarticulated incus; repaired using medical glue; or reconstructed using a partial ossicular replacement prosthesis (PORP).

RESULTS

Repairing the disarticulated incus using medical glue, or replacing the incus using a commercial PORP, provided similar restoration of ME function including almost perfect function at frequencies below 4 kHz, but with more than a 20-dB loss at higher frequencies. Associated phase responses under these conditions sometimes varied and seemed dependent on the degree of coupling of the PORP to the remaining ME structure. A new ME-prosthesis design may be required to allow the stapes to move in three-dimensional (3-D) space to correct this deficiency at higher frequencies.

CONCLUSIONS

Fixation of the incus to the stapes or ossicular reconstruction using a PORP limited the efficiency of sound transmission at high frequencies.

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.

Mechanical model of an arched basilar membrane in the gerbil cochlea

The frequency selectivity of a gerbil cochlea, unlike other mammals, does not depend on varying thickness and width of its basilar membrane from the basal to the apical end. We model the gerbil arched basilar membrane focusing on the radial tension, embedded fiber thickness, and the membrane arch, which replace the functionality of the variation in thickness and width. The model is verified with the previous gerbil cochlea model which estimated the equivalent basilar membrane thickness and is shown to be more accurate than the flat sandwiched basilar membrane model. The simple sinusoidal-shaped bending mode assumption in previous models is found to be valid in the present model with <12% error. Parametric study on the present model shows that fiber thickness contribution to the membrane stiffness is close to the 3rd order, higher than the 1st order estimation of previous models. We found that the effective Young's modulus of the fiber bundle is at least 6 orders higher than the shear modulus of the soft-cells and the membrane radial bending stiffness is more sensitive to the membrane arch and the shear modulus of the soft-cells near the apical end.

Finite-Element Modelling of the Response of the Gerbil Middle Ear to Sound

We present a finite-element model of the gerbil middle ear that, using a set of baseline parameters based primarily on a priori estimates from the literature, generates responses that are comparable with responses we measured in vivo using multi-point vibrometry and with those measured by other groups. We investigated the similarity of numerous features (umbo, pars-flaccida and pars-tensa displacement magnitudes, the resonance frequency and break-up frequency, etc.) in the experimental responses with corresponding ones in the model responses, as opposed to simply computing frequency-by-frequency differences between experimental and model responses. The umbo response of the model is within the range of variability seen in the experimental data in terms of the low-frequency (i.e., well below the middle-ear resonance) magnitude and phase, the main resonance frequency and magnitude, and the roll-off slope and irregularities in the response above the resonance frequency, but is somewhat high for frequencies above the resonance frequency. At low frequencies, the ossicular axis of rotation of the model appears to correspond to the anatomical axis but the behaviour is more complex at high frequencies (i.e., above the pars-tensa break-up). The behaviour of the pars tensa in the model is similar to what is observed experimentally in terms of magnitudes, phases, the break-up frequency of the spatial vibration pattern, and the bandwidths of the high-frequency response features. A sensitivity analysis showed that the parameters that have the strongest effects on the model results are the Young's modulus, thickness and density of the pars tensa; the Young's modulus of the stapedial annular ligament; and the Young's modulus and density of the malleus. Displacements of the tympanic membrane and manubrium and the low-frequency displacement of the stapes did not show large changes when the material properties of the incus, stapes, incudomallear joint, incudostapedial joint, and posterior incudal ligament were changed by ±10 % from their values in the baseline parameter set.

Stapes Vibration in the Chinchilla Middle Ear: Relation to Behavioral and Auditory-Nerve Thresholds

The vibratory responses to tones of the stapes and incus were measured in the middle ears of deeply anesthetized chinchillas using a wide-band acoustic-stimulus system and a laser velocimeter coupled to a microscope. With the laser beam at an angle of about 40 ° relative to the axis of stapes piston-like motion, the sensitivity-vs.-frequency curves of vibrations at the head of the stapes and the incus lenticular process were very similar to each other but larger, in the range 15-30 kHz, than the vibrations of the incus just peripheral to the pedicle. With the laser beam aligned with the axis of piston-like stapes motion, vibrations of the incus just peripheral to its pedicle were very similar to the vibrations of the lenticular process or the stapes head measured at the 40 ° angle. Thus, the pedicle prevents transmission to the stapes of components of incus vibration not aligned with the axis of stapes piston-like motion. The mean magnitude curve of stapes velocities is fairly flat over a wide frequency range, with a mean value of about 0.19 mm(.)(s Pa(-1)), has a high-frequency cutoff of 25 kHz (measured at -3 dB re the mean value), and decreases with a slope of about -60 dB/octave at higher frequencies. According to our measurements, the chinchilla middle ear transmits acoustic signals into the cochlea at frequencies exceeding both the bandwidth of responses of auditory-nerve fibers and the upper cutoff of hearing. The phase lags of stapes velocity relative to ear-canal pressure increase approximately linearly, with slopes equivalent to pure delays of about 57-76 μs.

Three-dimensional vibration of the malleus and incus in the living gerbil

In previous studies, 3D motion of the middle-ear ossicles in cat and human was explored, but models for hearing research have shifted in the last few decades to smaller mammals, and gerbil, in particular, has become a popular hearing model. In the present study, we have measured with an optical interferometer the 3D motion of the malleus and incus in anesthetized gerbil for sound of moderate intensity (90-dB sound pressure level) over a broad frequency range. To access the ossicles, the pars flaccida was removed exposing the neck and head of the malleus and the incus from the malleus-incus joint to the plate of the lenticular process. Vibration measurements were done at six to eight points per ossicle while the angle of observation was varied over approximately 30 ° to enable calculation of the 3D rigid-body velocity components. These components were expressed in an intrinsic reference frame, with one axis along the anatomical suspension axis of the malleus-incus block and a second axis along the stapes piston direction. Another way of describing the motion that does not assume an a priori rotation axis is to calculate the instantaneous rotation axis (screw axis) of the malleus/incus motion. Only at frequencies below a few kilohertz did the screw axis have a maximum rotation in a direction close to that of the ligament axis. A slight slippage in the malleus-incus joint developed with increasing frequency. Our findings are useful in determining the sound transfer characteristics through the middle ear and serve as a reference for validation of mathematical middle-ear models. Last but not least, comparing our present results in gerbil with those of previously measured species (human and cat) exposes similarities and dissimilarities among them.

The effect of rocking stapes motions on the cochlear fluid flow and on the basilar membrane motion

The basilar membrane (BM) and perilymph motion in the cochlea due to rocking stapes motion (RSM) and piston-like stapes motion (PSM) is modeled by numerical simulations. The full Navier-Stokes equations are solved in a two-dimensional box geometry. The BM motion is modeled by independent oscillators using an immersed boundary technique. The traveling waves generated by both stimulation modes are studied. A comparison of the peak amplitudes of the BM motion is presented and their dependence on the frequency and on the model geometry (stapes position and cochlear channel height) is investigated. It is found that the peak amplitudes for the RSM are lower and decrease as frequency decreases whereas those for the PSM increase as frequency decreases. This scaling behavior can be explained by the different mechanisms that excite the membrane oscillation. Stimulation with both modes at the same time leads to either a slight increase or a slight decrease of the peak amplitudes compared to the pure PSM, depending on the phase shift between the two modes. While the BM motion is dominated by the PSM mode under normal conditions, the RSM may lead to hearing if no PSM is present or possible, e.g., due to round window atresia.

Alternative fixation of an active middle ear implant at the short incus process

INTRODUCTION

Since 1996, the preferred approach for positioning the active middle-ear implant Vibrant Soundbridge© is a mastoidectomy and a posterior tympanotomy. With this device, placement of the floating mass transducer (FMT) on the long incus process is the standard method for treatment of mild-to-severe sensorineural hearing loss in the case of normal middle-ear anatomy. The aim of this study was to determine the vibrational effectiveness of FMT placement at the short incus process.

MATERIALS AND METHODS

An extended antrotomy and a posterior tympanotomy were performed in 5 fresh human temporal bones. As a control for normal middle-ear function, the tympanic membrane was stimulated acoustically and the vibration of the stapes footplate and the round-window (RW) membrane were (sequentially) measured by laser Doppler vibrometry. Vibration responses for coupling of an FMT to the long incus process (standard coupling) were compared to those for coupling to the short incus process.

RESULTS

Apart from narrow frequency bands near 3 and 9 kHz for the stapes footplate and RW membrane, respectively, the velocity responses presented no significant differences between standard coupling of the FMT and coupling to the short incus process.

CONCLUSION

Coupling the FMT to the short incus process may be a viable alternative in cases where the surgical approach is limited to an extended antrotomy. A reliable technique for attachment to the short incus process has yet to be developed.

Middle-ear velocity transfer function, cochlear input immittance, and middle-ear efficiency in chinchilla

The transfer function H(V) between stapes velocity V(S) and sound pressure near the tympanic membrane P(TM) is a descriptor of sound transmission through the middle ear (ME). The ME power transmission efficiency (MEE), the ratio of sound power entering the cochlea to power entering the middle ear, was computed from H(V) measured in seven chinchilla ears and previously reported measurements of ME input admittance Y(TM) and ME pressure gain G(MEP) [Ravicz and Rosowski, J. Acoust. Soc. Am. 132, 2437-2454 (2012); J. Acoust. Soc. Am. 133, 2208-2223 (2013)] in the same ears. The ME was open, and a pressure sensor was inserted into the cochlear vestibule for most measurements. The cochlear input admittance Y(C) computed from H(V) and G(MEP) is controlled by a combination of mass and resistance and is consistent with a minimum-phase system up to 27 kHz. The real part Re{Y(C)}, which relates cochlear sound power to inner-ear sound pressure, decreased gradually with frequency up to 25 kHz and more rapidly above that. MEE was about 0.5 between 0.1 and 8 kHz, higher than previous estimates in this species, and decreased sharply at higher frequencies.

Characterization of stapes anatomy: investigation of human and guinea pig

The accuracy of any stapes model relies on the accuracy of the anatomical information upon which it is based. In many previous models and measurements of the stapes, the shape of the stapes has been considered as symmetric with respect to the long and short axes of the footplate. Therefore, the reference frame has been built based upon this assumption. This study aimed to provide detailed anatomical information on the dimensions of the stapes, including its asymmetries. High-resolution microcomputed tomography data from 53 human stapes and 11 guinea pig stapes were collected, and their anatomical features were analyzed. Global dimensions of the stapes, such as the size of the footplate, height, and volume, were compared between human and guinea pig specimens, and asymmetric features of the stapes were quantitatively examined. Further, dependence of the stapes dimensions on demographic characteristics of the subjects was explored. The height of the stapes relative to the footplate size in the human stapes was found to be larger than the corresponding value in guinea pig. The stapes showed asymmetry of the footplate with respect to the long axis and offset of the stapes head from the centroid of the medial surface of the footplate for both humans and guinea pigs. The medial surface of the footplate was curved, and the longitudinal arches of the medial surface along the long axis of the footplate were shaped differently between humans and guinea pigs. The dimension of the footplate was gender-dependent, with the size greater in men than in women.

Sound transmission along the ossicular chain in common wild-type laboratory mice

The use of genetically modified mice can accelerate progress in auditory research. However, the fundamental profile of mouse hearing has not been thoroughly documented. In the current study, we explored mouse middle ear transmission by measuring sound-evoked vibrations at several key points along the ossicular chain using a laser-Doppler vibrometer. Observations were made through an opening in pars flaccida. Simultaneously, the pressure at the tympanic membrane close to the umbo was monitored using a micro-pressure-sensor. Measurements were performed in C57BL mice, which are widely used in hearing research. Our results show that the ossicular local transfer function, defined as the ratio of velocity to the pressure at the tympanic membrane, was like a high-pass filter, almost flat at frequencies above ∼15 kHz, decreasing rapidly at lower frequencies. There was little phase accumulation along the ossicles. Our results suggested that the mouse ossicles moved almost as a rigid body. Based on these 1-dimensional measurements, the malleus-incus-complex primarily rotated around the anatomical axis passing through the gonial termination of the anterior malleus and the short process of the incus, but secondary motions were also present. This article is part of a special issue entitled "MEMRO 2012".

In-plane motions of the stapes in human ears

The piston-like (translation normal to the footplate) and rocking-like (rotation along the long and short axes of the footplate) are generally accepted as motion components of the human stapes. It has been of issue whether in-plane motions, i.e., transversal movements of the footplate in the oval window, are comparable to these motion components. In order to quantify the in-plane motions the motion at nine points on the medial footplate was measured in five temporal bones with the cochlea drained using a three-dimensional (3D) laser Doppler vibrometer. It was found that the stapes shows in-plane movements up to 19.1 ± 8.7% of the piston-like motion. By considering possible methodological errors, i.e., the effects of the applied reflective glass beads and of alignment of the 3D laser Doppler system, such value was reduced to be about 7.4 ± 3.1%. Further, the in-plane motions became minimal (≈ 4.2 ± 1.4% of the piston-like motion) in another plane, which was anatomically within the footplate. That plane was shifted to the lateral direction by 118 μm, which was near the middle of the footplate, and rotated by 4.7° with respect to the medial footplate plane.

Reverse transmission along the ossicular chain in gerbil

In a healthy cochlea stimulated with two tones f (1) and f (2), combination tones are generated by the cochlea's active process and its associated nonlinearity. These distortion tones travel "in reverse" through the middle ear. They can be detected with a sensitive microphone in the ear canal (EC) and are known as distortion product otoacoustic emissions. Comparisons of ossicular velocity and EC pressure responses at distortion product frequencies allowed us to evaluate the middle ear transmission in the reverse direction along the ossicular chain. In the current study, the gerbil ear was stimulated with two equal-intensity tones with fixed f (2)/f (1) ratio of 1.05 or 1.25. The middle ear ossicles were accessed through an opening of the pars flaccida, and their motion was measured in the direction in line with the stapes piston-like motion using a laser interferometer. When referencing the ossicular motion to EC pressure, an additional amplitude loss was found in reverse transmission compared to the gain in forward transmission, similar to previous findings relating intracochlear and EC pressure. In contrast, sound transmission along the ossicular chain was quite similar in forward and reverse directions. The difference in middle ear transmission in forward and reverse directions is most likely due to the different load impedances-the cochlea in forward transmission and the EC in reverse transmission.

Békésy's contributions to our present understanding of sound conduction to the inner ear

In our daily lives we hear airborne sounds that travel primarily through the external and middle ear to the cochlear sensory epithelium. We also hear sounds that travel to the cochlea via a second sound-conduction route, bone conduction. This second pathway is excited by vibrations of the head and body that result from substrate vibrations, direct application of vibrational stimuli to the head or body, or vibrations induced by airborne sound. The sensation of bone-conducted sound is affected by the presence of the external and middle ear, but is not completely dependent upon their function. Measurements of the differential sensitivity of patients to airborne sound and direct vibration of the head are part of the routine battery of clinical tests used to separate conductive and sensorineural hearing losses. Georg von Békésy designed a careful set of experiments and pioneered many measurement techniques on human cadaver temporal bones, in physical models, and in human subjects to elucidate the basic mechanisms of air- and bone-conducted sound. Looking back one marvels at the sheer number of experiments he performed on sound conduction, mostly by himself without the aid of students or research associates. Békésy's work had a profound impact on the field of middle-ear mechanics and bone conduction fifty years ago when he received his Nobel Prize. Today many of Békésy's ideas continue to be investigated and extended, some have been supported by new evidence, some have been refuted, while others remain to be tested.

Forward and reverse transfer functions of the middle ear based on pressure and velocity DPOAEs with implications for differential hearing diagnosis

Recently it was shown that distortion product otoacoustic emissions (DPOAEs) can be measured as vibration of the human tympanic membrane in vivo, and proposed to use these vibration DPOAEs to support a differential diagnosis of middle-ear and cochlear pathologies. Here, we investigate how the reverse transfer function (r-TF), defined as the ratio of DPOAE-velocity of the umbo to DPOAE-pressure in the ear canal, can be used to diagnose the state of the middle ear. Anaesthetized guinea pigs served as the experimental animal. Sound was delivered free-field and the vibration of the umbo measured with a laser Doppler vibrometer (LDV). Sound pressure was measured 2-3 mm from the tympanic membrane with a probe-tube microphone. The forward transfer function (f-TF) of umbo velocity relative to ear-canal pressure was obtained by stimulating with multi-tone pressure. The r-TF was assembled from DPOAE components generated in response to acoustic stimulation with two stimulus tones of frequencies f(1) and f(2); f(2)/f(1) was constant at 1.2. The r-TF was plotted as function of DPOAE frequencies; they ranged from 1.7 kHz to 23 kHz. The r-TF showed a characteristic shape with an anti-resonance around 8 kHz as its most salient feature. The data were interpreted with the aid of a middle-ear transmission-line model taken from the literature for the cat and adapted to the guinea pig. Parameters were estimated with a three-step fitting algorithm. Importantly, the r-TF is governed by only half of the 15 independent, free parameters of the model. The parameters estimated from the r-TF were used to estimate the other half of the parameters from the f-TF. The use of r-TF data - in addition to f-TF data - allowed robust estimates of the middle-ear parameters to be obtained. The results highlight the potential of using vibration DPOAEs for ascertaining the functionality of the middle ear and, therefore, for supporting a differential diagnosis of middle-ear and cochlear pathologies.

Anatomical boundary of the tympanic membrane pars flaccida of the Meriones unguiculatus (Mongolian gerbil)

The pars flaccida of the Meriones unguiculatus (Mongolian gerbil) was in previous studies shown to bulge almost spherically when pressurized, a behavior suggesting that it is suspended by a fixed circular boundary. The question arises whether this "functional" boundary is based on an underlying circular anatomical boundary, an important issue for modeling the middle-ear mechanics. In this article, the boundaries of the Mongolian gerbil pars flaccida were visualized in situ with otomicroscopy and in slides with light microscopy and by micro-CT radiology. For the major part of its circumference, the pars flaccida was found to be suspended by rigid bone, that is, the tympanic legs. The remaining boundary is made up of the terminal portion of the handle of the malleus and the soft tissue of the terminal arches. The attachment to these structures is simple and uncomplicated, and the geometry is regular and symmetric: deviating by only 3.5% from a perfect circular shape. The findings justify the use of a fixed circular boundary as a good approximation for the modeling of pars flaccida deformation in the Mongolian gerbil.

Feed-forward and feed-backward amplification model from cochlear cytoarchitecture: an interspecies comparison

The high sensitivity and wide bandwidth of mammalian hearing are thought to derive from an active process involving the somatic and hair-bundle motility of the thousands of outer hair cells uniquely found in mammalian cochleae. To better understand this, a biophysical three-dimensional cochlear fluid model was developed for gerbil, chinchilla, cat, and human, featuring an active "push-pull" cochlear amplifier mechanism based on the cytoarchitecture of the organ of Corti and using the time-averaged Lagrangian method. Cochlear responses are simulated and compared with in vivo physiological measurements for the basilar membrane (BM) velocity, V(BM), frequency tuning of the BM vibration, and Q₁₀ values representing the sharpness of the cochlear tuning curves. The V(BM) simulation results for gerbil and chinchilla are consistent with in vivo cochlea measurements. Simulated mechanical tuning curves based on maintaining a constant V(BM) value agree with neural-tuning threshold measurements better than those based on a constant displacement value, which implies that the inner hair cells are more sensitive to V(BM) than to BM displacement. The Q₁₀ values of the V(BM) tuning curve agree well with those of cochlear neurons across species, and appear to be related in part to the width of the basilar membrane.

Middle ear function and cochlear input impedance in chinchilla

Simultaneous measurements of middle ear-conducted sound pressure in the cochlear vestibule P(V) and stapes velocity V(S) have been performed in only a few individuals from a few mammalian species. In this paper, simultaneous measurements of P(V) and V(S) in six chinchillas are reported, enabling computation of the middle ear pressure gain G(ME) (ratio of P(V) to the sound pressure in the ear canal P(TM)), the stapes velocity transfer function SVTF (ratio of the product of V(S) and area of the stapes footplate A(FP) to P(TM)), and, for the first time, the cochlear input impedance Z(C) (ratio of P(V) to the product of V(S) and A(FP)) in individuals. mid R:G(ME)mid R: ranged from 25 to 35 dB over 125 Hz-8 kHz; the average group delay between 200 Hz and 10 kHz was about 52 mus. SVTF was comparable to that of previous studies. Z(C) was resistive from the lowest frequencies up to at least 10 kHz, with a magnitude on the order of 10(11) acoustic ohms. P(V), V(S), and the acoustic power entering the cochlea were good predictors of the shape of the audiogram at frequencies between 125 Hz and 2 kHz.

Complex stapes motions in human ears

It has been reported that the physiological motion of the stapes in human and several animals in response to acoustic stimulation is mainly piston-like at low frequencies. At higher frequencies, the pattern includes rocking motions around the long and short axes of the footplate in human and animal ears. Measurements of such extended stapes motions are highly sensitive to the exact angulation of the stapes in relation to the measurement devices and to measurement errors. In this study, velocity in a specific direction was measured at multiple points on the footplates of human temporal bones using a Scanning Laser Doppler Vibrometer (SLDV) system, and the elementary components of the stapes motions, which were the piston-like motion and the rocking motions about the short and long axes of the footplate, were calculated from the measurements. The angular position of a laser beam with respect to the stapes and coordinates of the measurement points on the footplate plane were calculated by correlation between the SLDV measurement frame and the footplate-fixed frame, which was obtained from micro-CT images. The ratios of the rocking motions relative to the piston-like motion increased with frequency and reached a maximum around 7 kHz.A novel method for quantitatively assessing measurements of complex stapes motions and error boundaries of the motion components is presented. In the frequency range of 0.5 to 8 kHz, the magnitudes of the piston-like and two rocking motions were larger than estimated values of the corresponding upper error bounds.

Anatomy and physics of the exceptional sensitivity of dolphin hearing (Odontoceti: Cetacea)

During the past 50 years, the high acoustic sensitivity and the echolocation behavior of dolphins and other small odontocetes have been studied thoroughly. However, understanding has been scarce as to how the dolphin cochlea is stimulated by high frequency echoes, and likewise regarding the ear mechanics affecting dolphin audiograms. The characteristic impedance of mammalian soft tissues is similar to that of water, and thus no radical refractions of sound, nor reflections of sound, can be expected at the water/soft tissue interfaces. Consequently, a sound-collecting terrestrial pinna and an outer ear canal serve little purpose in underwater hearing. Additionally, compared to terrestrial mammals whose middle ear performs an impedance match from air to the cochlea, the impedance match performed by the odontocete middle ear needs to be reversed to perform an opposite match from water to the cochlea. In this paper, we discuss anatomical adaptations of dolphins: a lower jaw collecting sound, thus replacing the terrestrial outer ear pinna, and a thin and large tympanic bone plate replacing the tympanic membrane of terrestrial mammals. The paper describes the lower jaw anatomy and hypothetical middle ear mechanisms explaining both the high sensitivity and the converted acoustic impedance match.

Middle-ear pressure gain and cochlear partition differential pressure in chinchilla

An important step to describe the effects of inner-ear impedance and pathologies on middle- and inner-ear mechanics is to quantify middle- and inner-ear function in the normal ear. We present middle-ear pressure gain G(MEP) and trans-cochlear-partition differential sound pressure DeltaP(CP) in chinchilla from 100 Hz to 30 kHz derived from measurements of intracochlear sound pressures in scala vestibuli P(SV) and scala tympani P(ST) and ear-canal sound pressure near the tympanic membrane P(TM). These measurements span the chinchilla's auditory range. G(MEP) had constant magnitude of about 20 dB between 300 Hz and 20 kHz and phase that implies a 40-micros delay, values with some similarities to previous measurements in chinchilla and other species. DeltaP(CP) was similar to G(MEP) below about 10 kHz and lower in magnitude at higher frequencies, decreasing to 0 dB at 20 kHz. The high-frequency rolloff correlates with the audiogram and supports the idea that middle-ear transmission limits high-frequency hearing, providing a stronger link between inner-ear macromechanics and hearing. We estimate the cochlear partition impedance Z(CP) from these and previous data. The chinchilla may be a useful animal model for exploring the effects of non-acoustic inner-ear stimulation such as "bone conduction" on cochlear mechanics.

Gerbil middle-ear sound transmission from 100 Hz to 60 kHz

Middle-ear sound transmission was evaluated as the middle-ear transfer admittance H(MY) (the ratio of stapes velocity to ear-canal sound pressure near the umbo) in gerbils during closed-field sound stimulation at frequencies from 0.1 to 60 kHz, a range that spans the gerbil's audiometric range. Similar measurements were performed in two laboratories. The H(MY) magnitude (a) increased with frequency below 1 kHz, (b) remained approximately constant with frequency from 5 to 35 kHz, and (c) decreased substantially from 35 to 50 kHz. The H(MY) phase increased linearly with frequency from 5 to 35 kHz, consistent with a 20-29 micros delay, and flattened at higher frequencies. Measurements from different directions showed that stapes motion is predominantly pistonlike except in a narrow frequency band around 10 kHz. Cochlear input impedance was estimated from H(MY) and previously-measured cochlear sound pressure. Results do not support the idea that the middle ear is a lossless matched transmission line. Results support the ideas that (1) middle-ear transmission is consistent with a mechanical transmission line or multiresonant network between 5 and 35 kHz and decreases at higher frequencies, (2) stapes motion is pistonlike over most of the gerbil auditory range, and (3) middle-ear transmission properties are a determinant of the audiogram.

Simultaneous measurements of ossicular velocity and intracochlear pressure leading to the cochlear input impedance in gerbil

Recent measurements of three-dimensional stapes motion in gerbil indicated that the piston component of stapes motion was the primary contributor to intracochlear pressure. In order to make a detailed correlation between stapes piston motion and intracochlear pressure behind the stapes, simultaneous pressure and motion measurements were undertaken. We found that the scala vestibuli pressure followed the piston component of the stapes velocity with high fidelity, reinforcing our previous finding that the piston motion of the stapes was the main stimulus to the cochlea. The present data allowed us to calculate cochlear input impedance and power flow into the cochlea. Both the amplitude and phase of the impedance were quite flat with frequency from 3 kHz to at least 30 kHz, with a phase that was primarily resistive. With constant stimulus pressure in the ear canal the intracochlear pressure at the stapes has been previously shown to be approximately flat with frequency through a wide range, and coupling that result with the present findings indicates that the power that flows into the cochlea is quite flat from about 3 to 30 kHz. The observed wide-band intracochlear pressure and power flow are consistent with the wide-band audiogram of the gerbil.

Supporting evidence for reverse cochlear traveling waves

As a result of the cochlea's nonlinear mechanics, stimulation by two tones results in the generation of distortion products (DPs) at frequencies flanking the primary tones. DPs are measurable in the ear canal as oto-acoustic emissions, and are used to noninvasively explore cochlear mechanics and diagnose hearing loss. Theories of DP emissions generally include both forward and reverse cochlear traveling waves. However, a recent experiment failed to detect the reverse-traveling wave and concluded that the dominant emission path was directly through the fluid as a compression pressure [Ren, 2004, Nat. Neurosc.7, 333-334]. To explore this further, we measured intracochlear DPs simultaneously with emissions over a wide frequency range, both close to and remote from the basilar membrane. Our results support the existence of the reverse-traveling wave: (1) They show spatial variation in DPs that is at odds with a compression pressure. (2) Although they confirm a forward-traveling character of intraocochlear DPs in a broad frequency region of the best frequency, this behavior does not refute the existence of reverse-traveling waves. (3) Finally, the results show that, in cases in which it can be expected, the DP emission is delayed relative to the DP in a way that supports reverse-traveling-wave theory.

Sound pressure distribution and power flow within the gerbil ear canal from 100 Hz to 80 kHz

Sound pressure was mapped in the bony ear canal of gerbils during closed-field sound stimulation at frequencies from 0.1 to 80 kHz. A 1.27-mm-diam probe-tube microphone or a 0.17-mm-diam fiber-optic miniature microphone was positioned along approximately longitudinal trajectories within the 2.3-mm-diam ear canal. Substantial spatial variations in sound pressure, sharp minima in magnitude, and half-cycle phase changes occurred at frequencies >30 kHz. The sound frequencies of these transitions increased with decreasing distance from the tympanic membrane (TM). Sound pressure measured orthogonally across the surface of the TM showed only small variations at frequencies below 60 kHz. Hence, the ear canal sound field can be described fairly well as a one-dimensional standing wave pattern. Ear-canal power reflectance estimated from longitudinal spatial variations was roughly constant at 0.2-0.5 at frequencies between 30 and 45 kHz. In contrast, reflectance increased at higher frequencies to at least 0.8 above 60 kHz. Sound pressure was also mapped in a microphone-terminated uniform tube-an "artificial ear." Comparison with ear canal sound fields suggests that an artificial ear or "artificial cavity calibration" technique may underestimate the in situ sound pressure by 5-15 dB between 40 and 60 kHz.

The role of organ of Corti mass in passive cochlear tuning

The mechanism for passive cochlear tuning remains unsettled. Early models considered the organ of Corti complex (OCC) as a succession of spring-mass resonators. Later, traveling wave models showed that passive tuning could arise through the interaction of cochlear fluid mass and OCC stiffness without local resonators. However, including enough OCC mass to produce local resonance enhanced the tuning by slowing and thereby growing the traveling wave as it approached its resonant segment. To decide whether the OCC mass plays a role in tuning, the frequency variation of the wavenumber of the cochlear traveling wave was measured (in vivo, passive cochleae) and compared to theoretical predictions. The experimental wavenumber was found by taking the phase difference of basilar membrane motion between two longitudinally spaced locations and dividing by the distance between them. The theoretical wavenumber was a solution of the dispersion relation of a three-dimensional cochlear model with OCC mass and stiffness as the free parameters. The experimental data were only well fit by a model that included OCC mass. However, as the measurement position moved from a best-frequency place of 40 to 12 kHz, the role of mass was diminished. The notion of local resonance seems to only apply in the very high-frequency region of the cochlea.