Onset of basilar membrane non-linearity reflected in cubic distortion tone input-output functions

The basilar membrane (BM) input output (I/O) function is a non-linear compressive function over much of its operating range. A low level non-compressive region with a break-point or compression threshold between 20 and 40 dB SPL has been identified. To date, no similar compression threshold in cubic distortion tone otoacoustic emission (CDT) data, which would illustrate the dependence of the CDT on BM growth, has been demonstrated. A Taylor series expansion of the outer hair cell gating function yields an amplitude term for 2f1-f2 of p.A1(2).A2, where A1 and A2 are the displacement amplitudes of the BM for two pure tone input stimuli of levels L1 and L2, p a constant. By selectively varying either L1 or L2 with f2/f1 appropriately chosen to reduce suppression effects, the CDT I/O function can be examined for deviation from the power law. In particular, if the amplitude of the CDT were dependent on BM displacement amplitude, then it should be possible by an appropriate choice of parameters to measure compression threshold. We have examined CDT I/O functions for an f2 of 8 kHz in the guinea pig and found them to be consistent with the expected power law. With L1 held constant, L2 varied and f2/f1 = 1.6, a low level region with a slope of one and a compressive region with a slope of 0.14-0.27 corresponding to the analogous regions of the BM I/O function was identified, with a break-point or compression threshold of 22-33 dB SPL.