Detection of low-level sounds by the mammalian cochlea requires electromechanical feedback from outer hair cells (OHCs). This feedback arises due to the electromotile response of OHCs, which is driven by the modulation of their receptor potential caused by the stimulation of mechano-sensitive ion channels. Nonlinearity in these channels distorts impinging sounds, creating distortion-products that are detectable in the ear canal as distortion-product otoacoustic emissions (DPOAEs). Ongoing efforts aim to develop DPOAEs, which reflects the ear’s health, into diagnostic tools for sensory hearing loss. These efforts are hampered by limited knowledge on the cochlear extent contributing to DPOAEs. Here, we report on intracochlear distortion products (IDPs) in OHC electrical responses and intracochlear fluid pressures. Experiments and simulations with a physiologically motivated cochlear model show that widely generated electrical IDPs lead to mechanical vibrations in a frequency-dependent manner. The local cochlear impedance restricts the region from which IDPs contribute to DPOAEs at low to moderate intensity, which suggests that DPOAEs may be used clinically to provide location-specific information about cochlear damage.
The tectorial membrane is an extracellular matrix located in the cochlea. The tectorial membrane is often hypothesized to play an important role in hearing mechanics. Measurements of wave propagation on isolated tectorial membranes have been used in the literature to characterize the intrinsic mechanical properties of tectorial membranes in the auditory frequency range. While most previous studies have made an implicit assumption regarding the width of the tectorial membranes in order to find the properties of the TM using a simple model, we have recently used a more accurate model that takes into account the finite width of the TM and its anisotropy. However, experiments are conducted in an artificial endolymph bath, which we neglected in our previous analysis. In this work, we study the influence of the viscous boundary layer due to the fluid on wave propagation on isolated tectorial membranes. The boundary layer adds damping and mass to the tectorial membrane, which we model using a commercial finite element software. The influence of the boundary layer on the space constant and wave speed of the longitudinally propagating radial motion, and on the spatial patterns of the longitudinal motion, are analyzed.
In response to an external stimulus, the cochlea emits sounds, called stimulus frequency otoacoustic emissions (SFOAEs), at the stimulus frequency. In this article, a three-dimensional computational model of the gerbil cochlea is used to simulate SFOAEs and clarify their generation mechanisms and characteristics. This model includes electromechanical feedback from outer hair cells (OHCs) and cochlear roughness due to spatially random inhomogeneities in the OHC properties. As in the experiments, SFOAE simulations are characterized by a quasiperiodic fine structure and a fast varying phase. Increasing the sound pressure level broadens the peaks and decreases the phase-gradient delay of SFOAEs. A state-space formulation of the model provides a theoretical framework to analyze the link between the fine structure and global modes of the cochlea, which arise as a result of standing wave resonances. The SFOAE fine structure peaks correspond to weakly damped resonant modes because they are observed at the frequencies of nearly unstable modes of the model. Variations of the model parameters that affect the reflection mechanism show that the magnitude and sharpness of the tuning of these peaks are correlated with the modal damping ratio of the nearly unstable modes. The analysis of the model predictions demonstrates that SFOAEs originate from the peak of the traveling wave.
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