Dynamic nonlinear cochlear model predictions of click-evoked otoacoustic emission suppression

A comprehensive set of results from 2-click suppression experiments on otoacoustic emissions (OAEs) have been presented by Kapadia and Lutman [Kapadia, S., Lutman, M.E., 2000a. Nonlinear temporal interactions in click-evoked otoacoustic emissions. I. Assumed model and polarity-symmetry. Hear. Res. 146, 89-100]. They found that the degree of suppression of an OAE evoked by a test click varied systematically with the timing and the level of a suppressor click, being greatest for suppressor clicks occurring some time before the test click, particularly at lower levels of suppression. Kapadia and Lutman also showed that although the general shape of the graph of suppression against suppressor click timing could be predicted by a static power law model, this did not predict the asymmetry with respect to the timing of the suppressor click. A generalised automatic gain control (AGC) is presented as a simple example of a dynamic nonlinear system. Its steady state nonlinear behaviour, as quantified by its level curve, and its dynamic behaviour, as quantified by its transient response, can be independently set by the feedback gain law and detector time constant, respectively. The previously reported suppression results, with the asymmetry in the timing, are found to be predicted better by such an AGC having a level curve with a slope of about 0.5 dB/dB, and a detector time constant of about twice the period at the characteristic frequency. Although this gives adequate predictions for high suppression levels, it under predicts the suppression and the asymmetry for lower levels. Further research is required to establish whether simple peripheral feedback models can explain OAE suppression of this type.