Some effects of spatial randomness along the length of the cochlear on its performance

Abstract

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 very significant active enhancements of its response, of the order of 90 dB. In a real biological system, however, such uniform distributions will always be disrupted by random spatial variations, and a state space model of the cochlea has been developed that can be used to investigate the effects of such random variations. One effect is found to be that the cochlear amplifier gains have to be reduced to maintain stability, so that maximum active enhancements of the cochlear response is more typically 40 dB. The cochlear amplifier, however, is nonlinear as well as active, and unstable poles in the linear model generate instabilities that settle into limit cycle oscillations at discrete frequencies in the nonlinear model. These oscillations are thought to be the origin of spontaneous otoacoustic emissions, and have a significant effect on sound detection at low level. 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.