Chinchilla middle-ear admittance and sound power: high-frequency estimates and effects of inner-ear modifications

Dynamic X-ray Microtomography vs. Laser-Doppler Vibrometry: A Comparative Study

Purpose

There are challenges in understanding the biomechanics of the human middle ear, and established methods for studying this system show significant limitations. In this study, we evaluate a novel dynamic imaging technique based on synchrotron X-ray microtomography designed to assess the biomechanical properties of the human middle ear by comparing it to laser-Doppler vibrometry (LDV).

Methods

We examined three fresh-frozen temporal bones (TB) using dynamic synchrotron-based X-ray microtomography for 256 Hz and 512 Hz, stimulated at 110 dB and 120 dB SPL. In addition, we performed measurements on these TBs using 1D LDV, a well-established method.

Results

The normalized displacement values (μm/Pa) at the umbo and the posterior crus of the stapes are consistent or within 5-10 dB differences between all LDV and dynamic microtomography measurements and previously reported literature references. In general, the overall behavior is similar between the two measurement techniques.

Conclusion

In conclusion, our results demonstrate the suitability of dynamic synchrotron-based X-ray microtomography in studying the middle ear's biomechanics. However, this study shows that better standardization regarding acoustic stimulation and measurement points is needed to better compare the two measurement techniques.

The onset of nonlinear growth of middle-ear responses to high intensity sounds

The human tympanic membrane (TM) and ossicles are generally considered to act as a linear system as they conduct low and moderate level environmental sounds to the cochlea. At intense stimulus levels (> 120 dB SPL) there is evidence that the TM and ossicles no longer act linearly. The anatomical structures that contribute to the nonlinear responses and their level and frequency dependences are not well defined. We used cadaveric human ears to characterize middle-ear responses to continuous tones between 200 and 20,000 Hz with levels between 60 and 150 dB SPL. The responses of the TM and ossicles are essentially sinusoidal, even at the highest stimulus level, but grow nonlinearly with increased stimulus level. The umbo and the stapes show different nonlinear behaviors: The umbo displacement grows faster than the stimulus level (expansive growth) at frequencies below 2000 Hz, while the stapes exhibits mostly compressive growth (grows slower than the stimulus level) over a wide frequency range. The sound pressure level where the nonlinearity first becomes obvious and the displacement at that level are lower at the stapes than at the umbo. These observations suggest the presence of multiple nonlinear processes within the middle ear. The existence of an expansive growth of umbo displacement that has limited effect on the stapes compressive growth suggests that the ossicular joints reduce the coupling between multiple nonlinear mechanisms within the middle ear. This study provides new data to test and refine middle-ear nonlinear models.

Mouse middle-ear forward and reverse acoustics

The mouse is an important animal model for hearing science. However, our knowledge of the relationship between mouse middle-ear (ME) anatomy and function is limited. The ME not only transmits sound to the cochlea in the forward direction, it also transmits otoacoustic emissions generated in the cochlea to the ear canal (EC) in the reverse direction. Due to experimental limitations, a complete characterization of the mouse ME has not been possible. A fully coupled finite-element model of the mouse EC, ME, and cochlea was developed and calibrated against experimental measurements. Impedances of the EC, ME, and cochlea were calculated, alongside pressure transfer functions for the forward, reverse, and round-trip directions. The effects on sound transmission of anatomical changes such as removing the ME cavity, pars flaccida, and mallear orbicular apophysis were also calculated. Surprisingly, below 10 kHz, the ME cavity, eardrum, and stapes annular ligament were found to significantly affect the cochlear input impedance, which is a result of acoustic coupling through the round window. The orbicular apophysis increases the delay of the transmission line formed by the flexible malleus, incus, and stapes, and improves the forward sound-transmission characteristics in the frequency region of 7-30 kHz.

Mass spectrometry imaging of blast overpressure induced modulation of GABA/glutamate levels in the central auditory neuraxis of Chinchilla

Acoustic trauma damages inner ear neural structures including cochlear hair cells which result in hearing loss and neurotransmitter imbalances within the synapses of the central auditory pathway. Disruption of GABA/glutamate levels underlies, tinnitus, a phantom perception of sound that persists post-exposure to blast noise which may manifest in tandem with acute/chronic loss of hearing. Many putative theories explain tinnitus physiology based on indirect and direct assays in animal models and humans, although there is no comprehensive evidence to explain the phenomenon. Here, GABA/glutamate levels were imaged and quantified in a blast overpressure model of chinchillas using Fourier transform ion cyclotron resonance mass spectrometry imaging. The direct measurement from whole-brain sections identified the relative levels of GABA/glutamate in the central auditory neuraxis centers including the cochlear nucleus, inferior colliculus, and auditory cortex. These preliminary results provide insight on the homeostasis of GABA/glutamate within whole-brain sections of chinchilla for investigation of the pathomechanism of blast-induced tinnitus.

A lumped-element model of the chinchilla middle ear

An air-conduction circuit model was developed for the chinchilla middle ear and cochlea. The lumped-element model is based on the classic Zwislocki model of the same structures in human. Model parameters were fit to various measurements of chinchilla middle-ear transfer functions and impedances, using a combination of error-minimization-driven computer-automated and manual fitting methods. The measurements used to fit the model comprise a newer, more-extensive data set than previously used, and include measurements of stapes velocity and inner-ear sound pressure within the vestibule and the scala tympani near the round window. The model is in agreement with studies of the effects of middle-ear cavity holes in experiments that require access to the middle-ear air space. The structure of the model allows easy addition of other sources of auditory stimulation, e.g., the multiple sources of bone-conducted sound-the long-term goal for the model's development-and mechanical stimulation of the ossicles and round window.

Chinchilla middle ear transmission matrix model and middle-ear flexibility

The function of the middle ear (ME) in transforming ME acoustic inputs and outputs (sound pressures and volume velocities) can be described with an acoustic two-port transmission matrix. This description is independent of the load on the ME (cochlea or ear canal) and holds in either direction: forward (from ear canal to cochlea) or reverse (from cochlea to ear canal). A transmission matrix describing ME function in chinchilla, an animal commonly used in auditory research, is presented, computed from measurements of forward ME function: input admittance YTM, ME pressure gain GMEP, ME velocity transfer function HV, and cochlear input admittance YC, in the same set of ears [Ravicz and Rosowski (2012b). J. Acoust. Soc. Am. 132, 2437-2454; (2013a). J. Acoust. Soc. Am. 133, 2208-2223; (2013b). J. Acoust. Soc. Am. 134, 2852-2865]. Unlike previous estimates, these computations require no assumptions about the state of the inner ear, effectiveness of ME manipulations, or measurements of sound transmission in the reverse direction. These element values are generally consistent with physical constraints and the anatomical ME "transformer ratio." Differences from a previous estimate in chinchilla [Songer and Rosowski (2007). J. Acoust. Soc. Am. 122, 932-942] may be due to a difference in ME flexibility between the two subject groups.

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.

Sound pressure distribution within natural and artificial human ear canals: forward stimulation

This work is part of a study of the interaction of sound pressure in the ear canal (EC) with tympanic membrane (TM) surface displacement. Sound pressures were measured with 0.5-2 mm spacing at three locations within the shortened natural EC or an artificial EC in human temporal bones: near the TM surface, within the tympanic ring plane, and in a plane transverse to the long axis of the EC. Sound pressure was also measured at 2-mm intervals along the long EC axis. The sound field is described well by the size and direction of planar sound pressure gradients, the location and orientation of standing-wave nodal lines, and the location of longitudinal standing waves along the EC axis. Standing-wave nodal lines perpendicular to the long EC axis are present on the TM surface >11-16 kHz in the natural or artificial EC. The range of sound pressures was larger in the tympanic ring plane than at the TM surface or in the transverse EC plane. Longitudinal standing-wave patterns were stretched. The tympanic-ring sound field is a useful approximation of the TM sound field, and the artificial EC approximates the natural EC.

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.

Inner-ear sound pressures near the base of the cochlea in chinchilla: further investigation

The middle-ear pressure gain GMEP, the ratio of sound pressure in the cochlear vestibule PV to sound pressure at the tympanic membrane PTM, is a descriptor of middle-ear sound transfer and the cochlear input for a given stimulus in the ear canal. GMEP and the cochlear partition differential pressure near the cochlear base ΔPCP, which determines the stimulus for cochlear partition motion and has been linked to hearing ability, were computed from simultaneous measurements of PV, PTM, and the sound pressure in scala tympani near the round window PST in chinchilla. GMEP magnitude was approximately 30 dB between 0.1 and 10 kHz and decreased sharply above 20 kHz, which is not consistent with an ideal transformer or a lossless transmission line. The GMEP phase was consistent with a roughly 50-μs delay between PV and PTM. GMEP was little affected by the inner-ear modifications necessary to measure PST. GMEP is a good predictor of ΔPCP at low and moderate frequencies where PV >> PST but overestimates ΔPCP above a few kilohertz where PV ≈ PST. The ratio of PST to PV provides insight into the distribution of sound pressure within the cochlear scalae.