The compression curves of different models of cochlear nonlinearities

Abstract

A widely used method for representing the nonlinear response of the basilar membrane (BM) is a graph of log amplitude of the BM motion against the log amplitude of the sinusoidal driving pressure, known as the input–output level curve. At low sound pressure levels (less than approximately 30 dB) the level of the BM response rises with sound pressure level at a slope of about 1 dB/dB, indicating a linear response. Above this region the slope of the level curve decreases, typically to about 1/2 to 1/3 dB/dB, indicating a compressive nonlinearity. Various models for cochlear nonlinearity will be presented, particularly contrasting the characteristics and behavior of an instantaneously acting nonlinear function and those of level-dependent systems. Both of these models have been used historically to model the nonlinearities present in the cochlea. Comparisons will be made between the properties of these models, highlighting potential methods of distinguishing between them. Similar nonlinear responses to those of the BM are observed in otoacoustic emissions. Experimental results will be reported which attempt to distinguish between the instantaneously acting and level-dependent models of this nonlinearity, which suggest that the latter is more likely.