Two-tone distortion on the basilar membrane of the chinchilla cochlea

Basilar membrane responses to pairs of tones were measured, with the use of a laser velocimeter, in the basal turn of the cochlea in anesthetized chinchillas. Frequency spectra of basilar membrane responses to primary tones with frequencies (f(1), f(2)) close to the characteristic frequency (CF) contain prominent odd-order two-tone distortion products (DPs) at frequencies both higher and lower than CF (such as 2f(1) - f(2), 3f(1) - 2f(2), 2f(2) - f(1) and 3f(2) - 2f(1)). For equal-level primaries with frequencies such that 2f(1) - f(2) equals CF, the magnitude of the 2f(1) - f(2) DP grows with primary level at linear or faster rates at low stimulus levels, but it saturates or decreases slightly at higher levels. For a fixed level of one of the primary tones, the magnitude of the 2f(1) - f(2) DP is a nonmonotonic function of the level of the other primary tone. For low intensities of the variable tone, the 2f(1) - f(2) DP grows at a rate of similar to 2 dB/dB with f(1) level and 1 dB/dB with f(2) level. DP magnitudes decrease rapidly with increasing primary frequency ratio (f(2)/f(1)) at low stimulus levels. For more intense stimuli, DP magnitudes remain constant or decrease slowly over a wide range of frequency ratios until a critical value is reached, at which DP magnitudes fall with slopes as steep as -300 dB/octave. As stimulus level grows, DP phases increasingly lag for large f(2)/(1) ratios, but exhibit leads for small f(2)/f(1) ratios. Cochlear exposure to an intense tone that produces large sensitivity losses for the primary frequencies (but only small losses for tones with frequency equal to 2f(1) - f(2)) causes a substantial decrease in magnitude of the 2f(1) - f(2) DP. This result demonstrates that the 2f(1) - f(2) DP originates at the basilar membrane region with CFs corresponding to the primary frequencies and propagates to the location with CF equal to the DP frequency. 2f(1) - f(2) DPs on the basilar membrane resemble those measured in human psychophysics in most respects. However, the magnitude of basilar membrane DPs does not show the nonmonotonic dependence on f(2)/f(1) ratio evident in DP otoacoustic emissions.