State space model of a nonlinear inhomogeneous cochlea
At Applied Nonlinear Mathematics: Making it Real.
10 - 14 Sep 2007.
Full text not available from this repository.
A state space model of the coupled mechanics of the cochlea has been developed. It can be used to investigate the low level, linear, response of the cochlear with different spatial inhomogeneities along its length and also to simulate the nonlinear time domain response at higher levels. The model combines a macromechanical formulation for fluid coupling with a discrete distribution of active micromechanical elements, each with a nonlinear feedback model of the cochlear amplifier.
If the active cochlear is modelled as having a uniform distribution of parameters, the system is stable even for high gains in the cochlear amplifiers, resulting in extremely large active enhancements of its response. In a real biological system, however, such uniform distributions will always be disrupted by random spatial variations, and the state space model of the cochlea has been used to investigate the effects of such random variations. It is found that the cochlear amplifier gains have to be reduced to maintain stability in this case, so that the maximum active enhancement of the cochlear response is more typically 40 dB, which is similar to that observed in measurements on the living cochlear.
The cochlear amplifier, however, is nonlinear as well as active, and any unstable poles in the linear model generate instabilities, which settle into limit cycle oscillations at discrete frequencies in the nonlinear model. These oscillations are thought to be the origin of spontaneous otoacoustic emissions. They also have a significant effect on sound detection at very low sound pressure levels. At higher sound pressure levels the nonlinearity in the cochlear amplifier suppresses such oscillations and the sound detection has a more uniform frequency response, although the amplification is reduced.
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