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Comparing time domain simulations of different nonlinear models of cochlear micromechanics
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Does the human cochlea work like a laser?
No full textThe sharply tuned sense of hearing in humans is believed to be due to active amplification in the cochlea. One seemingly natural consequence of this ‘cochlear amplifier’ is the existence of spontaneous otoacoustic emissions (SOAEs), narrow-band tones that are detected in the ear canals of 33% to 70% of all normal-hearing individuals (1). The mechanisms underlying the features and generation of SOAEs have long been a subject of debate. Zweig and Shera (2) argue that SOAEs are created by multiple travelling-wave reflections between the middle ear boundary and a dense array of inhomogeneities scattered throughout the cochlea; Shera (3) likens this process to activity in a laser cavity. This theory is contrary to previous ideas which assume independently unstable oscillators in the cochlea.
This work uses a state space formulation of the cochlea to test the predictions made by Zweig and Shera (2). The Elliott et al. model of the cat cochlea (4) has been revised to describe characteristics of the human cochlea. Linear instabilities arise across a wide bandwidth of frequencies when the smooth spatial variation of basilar membrane impedance is disturbed. The salient features of Zweig and Shera’s theory (2) are observed in this active model given perturbations in the distribution of feedback gain along the cochlea. A step change in gain is used to demonstrate system instability
Modelling random changes in the parameters along the length of the cochlea and the effect on hearing sensitivity
No full textA state space model for cochlear mechanics
No full textThe stability of a linear model of the active cochlea is difficult to determine from its calculated frequency response alone. A state space model of the cochlea is presented, which includes a discretized set of general micromechanical elements coupled via the cochlear fluid. The stability of this time domain model can be easily determined in the linear case, and the same framework used to simulate the time domain response of nonlinear models. Examples of stable and unstable behavior are illustrated using the active micromechanical model of Neely and Kim. The stability of this active cochlea is extremely sensitive to abrupt spatial inhomogeneities, while smoother inhomogeneities are less likely to cause instability. The model is a convenient tool for investigating the presence of instabilities due to random spatial inhomogeneities. The number of unstable poles is found to rise sharply with the relative amplitude of the inhomogeneities up to a few percent, but to be significantly reduced if the spatial variation is smoothed. In a saturating nonlinear model, such instabilities generate limit cycles that are thought to produce spontaneous otoacoustic emissions. An illustrative time domain simulation is presented, which shows how an unstable model evolves into a limit cycle, distributed along the cochlea
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