The mechanical waveform of the basilar membrane. III. Intensity effects

Mechanical responses in the basal turn of the guinea-pig cochlea were measu red with broad-band noise stimuli and expressed as input-output cross-corre lation functions. The experiments were performed over the full range of sti mulus intensities in order to try to understand the influence of cochlear n onlinearity on frequency selectivity, tuning, signal compression and the im pulse response. The results are interpreted within the framework of a nonli near, locally active, three-dimensional model of the cochlea. The data have been subjected to inverse analysis in order to recover the basilar-membran e (BM) impedance, a parameter function that, when inserted into the (linear ized version of that) model, produces a model response that is similar to t he measured response. This paper reports details about intensity effects fo r noise stimulation, in particular, the way the BM impedance varies with st imulus intensity. In terms of the underlying cochlear model, the decrease o f the "activity component" in the BM impedance with increasing stimulus lev el is attributed to saturation of transduction in the outer hair cells. In the present paper this property is brought into a quantitative form. Accord ing to the theory [the EQ-NL theorem, de Beer, Audit. Neurosci. 3, 377-388 (1997)], the BM impedance is composed of two components, both intrinsically independent of stimulus level. One is the passive impedance Z(pass) and th e other one is the ''extra'' impedance Z(extra). The latter impedance is to be multiplied by a real factor gamma (0 less than or equal to gamma less t han or equal to 1) that depends on stimulus level. This concept about the c omposition of the BM impedance is termed the "two-component theory of the B M impedance." In this work both impedances are entirely derived from experi mental data. The dependence of the factor gamma on stimulus level can be de rived by using a unified form of the outer-hair-cell transducer function. F rom an individual experiment, the two functions Z(pass) and Z(extra) are de termined, and an approximation (Z(pass) + gamma Z(extra)) to the BM impedan ce constructed. Next, the model response (the ''resynthesized'' response) c orresponding to this "artificial" impedance is computed. The same procedure is executed for several stimulus-level values. For all levels, the results show a close correspondence with the original experimental data; this incl udes correct prediction of the compression of response amplitudes, the redu ction of frequency selectivity, the shift in peak frequency and, most impor tantly, the preservation of timing in the impulse response. All these findi ngs illustrate the predictive power of the underlying model. (C) 2000 Acous tical Society of America. [S0001-4966(00)02803-4].