AIP Conference Proceedings最新文献

Two hydrodynamic effects are introduced in the standard transmission-line formalism, the focusing of the pressure and fluid velocity fields near the basilar membrane and the viscous damping at the fluid-basilar membrane interface, which significantly affect the cochlear response in the short-wave region. In this region, in which the wavelength is shorter than the cochlear duct height, only a layer of fluid of order of the wavelength is effectively involved in the traveling wave. This has been interpreted [8] as a reduced fluid contribution to the system inertia in the peak region, which is a viewpoint common to the 3-D FEM solutions. In this paper we propose an alternative approach, from a slightly different physical viewpoint. Invoking the fluid flux conservation along the traveling wave propagation direction, we can derive a rigorous propagation equation for the pressure integrated along the vertical axis. Consequently, the relation between the average pressure and the local pressure [4] at the fluid-BM interface can be written. The local pressure is amplified by a factor dependent on the local wavenumber with respect to the average pressure, a phenomenon we refer to as "fluid focusing", which plays a relevant role in the BM total amplification gain. This interpretation of the hydrodynamic boost to the pressure provides a physical justification to the strategy [10] of fitting the BM admittance with a polynomial containing both a conjugated pole and a zero. In the short-wave region, the sharp gradients of the velocity field yield a second important effect, a damping force on the BM motion, proportional to the local wavenumber, which stabilizes active models and shifts the peak of the response towards the base, with respect to the resonant place. This way, the peaked BM response is not that of a proper resonance, corresponding to a sharp maximum of the admittance, but rather a focusing-driven growth toward the resonant place, which is "aborted" before reaching it by the sharply increasing viscous losses. The large values of the wavenumber that ensure strong focusing are ultimately fueled, against viscosity, by the nonlinear OHC mechanism, hence the otherwise puzzling observation of a wide nonlinear gain dynamics with almost level-independent admittance.

According to the dominant view, the mammalian cochlea spatially amplifies signals by actively pumping energy into the traveling wave. That is, signals are amplified as they propagate through a region where the medium's resistance is effectively negative. While signal amplification has been extensively studied in active cochlear models, the same cannot be said for amplification of internal noise. According to transmission-line theory, signals are amplified more than internal noise in regions where the net resistance is negative. Here we generalize this finding by showing that a distributed system composed of cascaded "noisy" amplifiers boosts signals more rapidly than the internal noise; the larger the amplifier gain, the larger the signal-to-noise ratio (SNR) of the amplified signal. We further show that this mechanism operates in existing active cochlear models: the cochlear amplifier increases the SNR of cochlear responses, and thus enhances cochlear sensitivity. When considering also that the cochlear amplifier narrows the bandwidth of the "cochlear filters", activation of the cochlear amplifiers dramatically increases the SNR (by about one order of magnitude in our simulations) from the tail to the peak of the traveling wave. We further demonstrate that the tapered ear-horn-like cochlear geometry significantly improves the SNR of basilar-membrane responses.

Distortion Product Otoacoustic Emissions (DPOAE) offer great potential for hearing diagnosis, but are complicated by the interaction of components generated by different mechanisms. Separation of these components from DPOAE measurements may allow conclusions to be drawn about the functionality of these separate mechanisms of a cochlear. However, the signal processing methods for performing this separation are imperfect. Existing methods are based on time windowing of DPOAE generated from frequency sweep stimuli. This paper presents a method in which the entire spectra of both distortion (D) and reflection (R) components are simultaneously estimated. This approach has several advantages. Firstly, the method removes the need for a compromise between frequency precision and signal to noise ratio. Secondly, the method can be made to include models of the stimulus signals, so that the stimulus does not strongly interfere with the estimation process. Thirdly, the method can be arranged to make efficient use of data that has been corrupted by measurement artefacts. Fourthly, the method can be easily adapted to track DPOAEs that are changing in response to chemical or acoustic treatments. The basic modelling assumptions made are that the sum of R and D measurements can be represented as the sum of convolutions with the stimulus signal, the frequency representation of the D component is more smooth than the R component, and that a reasonable estimate of the noise level in the signal is available. These assumptions are combined into a linear convex problem. In this paper we compare the proposed approach with three other methods. While it is not superior to the earlier methods at every frequency, it does offer some improvement, particularly with regards reducing the contamination of D by R.

The cochlear tonotopic map determines where along the basilar membrane traveling waves of different frequencies peak. Endolymphatic hydrops has been hypothesized to shift the tonotopic map by altering the stiffness of the cochlear partition, especially in the apex. In this exploratory study performed in a handful of normal and hydropic ears, we report preliminary measurements of interaural differences assayed using behavioral pitch-matching supplemented by measurements of reflection otoacoustic-emission phase-gradient delays.

When elicited by two stimulus tones (at frequencies f₁ and f₂, f₂ > f₁), the amplitudes of specific distortion-product otoacoustic emission (DPOAE) components exhibit a characteristic bandpass shape as the ratio between f₂ and f₁ is varied. This bandpass shape has been attributed to various mechanisms including intracochlear resonance, suppression, and wave interference, and has been proposed to be related to cochlear frequency tuning. While human studies suggest modest correlations between psychophysical tuning and the tuning of DPOAE amplitude vs. f₂/f₁ ratio functions, a relationship between the latter and the tuning of cochlear mechanical responses has yet to be established. This was addressed here through direct comparisons of DPOAEs and cochlear vibrations in wild-type CBA/CaJ mice. DPOAEs were elicited using a fixed-f₂, swept-f₁ paradigm, and optical coherence tomography was used to measure displacements from cochlear locations with characteristic frequencies near f₂. The tuning sharpness of 2f₁-f₂ DPOAE ratio functions was found to be remarkably similar to that of basilar membrane and/or tectorial membrane responses to single tones, with the tuning sharpness of all responses increasing similarly with decreasing stimulus level. This relationship was observed for f₂ frequencies ranging from ~8 to 22 kHz. Intracochlear distortion products did not exhibit a bandpass shape as the f₂/f₁ ratio was varied, indicating that interference between distortion products traveling to the stapes may be responsible for the tuning of the DPOAE ratio function. While these findings suggest that DPOAE ratio functions could be used to noninvasively infer cochlear tuning, it remains to be determined whether this relationship holds in other species and for lower frequency regions.

Mammalian auditory epithelium (the organ of Corti) stands out among different inner-ear epithelia in that it has large extracellular fluid spaces such as the tunnel of Corti, Nuel's space, outer tunnel, and spacing between outer hair cells. We tested the hypothesis that advective flow facilitates mass transport in the cochlear fluids, using computational simulations of cochlear fluid dynamics and ex vivo experiments to investigate mass transport in extracellular fluid spaces of the cochlea. Three model simulations were performed in series-cochlear mechanics, nonlinear fluid dynamics, and mass transport. In nonlinear fluid dynamics, we incorporated convection terms for more accurate computation of drift flow. For mass transport, both diffusion and advection were considered. For experiments, we measured vibrations of excised cochlear turns using optical coherence tomography. The excised OoC was subjected to acoustic and electrical stimulations.