Relations among compression, suppression, and combination tones in mechanical responses of the basilar membrane: data...

Recent observations of combination tones and low-frequency suppression in the mechanical response of the basilar membrane have eliminated uncertainty of the existence of these non-linear phenomena at this stage of the cochlea (Robles et al., 1991; Ruggero et al., 1992b; Rhode and Cooper, 1993), and provide the first opportunity to test and extend theoretical understanding of these non-linear responses. This paper reports a systematic study of the new data for the chinchilla with the MBPNL (multiple band-pass non-linearity) non-linear model for rapid compression in cochlear filtering (Goldstein, 1990). The two key findings are that: (1) the large differences in suppression growth rates for low- and high-frequency suppressors are closely predicted by the model, although a statistically significant small bias in the prediction was found; and (2) combination tones of frequency 2f2-f1 were well predicted by the model, while reported levels of 2f1-f2 were underestimated by the model. The most likely explanation of the result for 2f1-f2 is that the model correctly predicts the distributed generation of combination tones, but does not include a propagating medium that allows combination tones to accumulate at their tuned site. Alternative explanations requiring experimental clarification are suggested. The general finding of this study is that the complex level-dependent properties of suppression and combination tones are closely related to the three regions characterizing the tuned response of the basilar membrane, in which linear-like responses at low and high sound levels are joined by a compressive range. The MBPNL model describes this general relationship quantitatively and is a reliable basis for further modeling research.