2-TONE DISTORTION ON THE BASILAR-MEMBRANE OF THE CHINCHILLA COCHLEA

Basilar membrane responses to pairs of tones were measured, with the u se of a laser velocimeter, in the basal turn of the cochlea in anesthe tized chinchillas. Frequency spectra of basilar membrane responses to primary tones with frequencies (f(1), f(2)) close to the characteristi c frequency (CF) contain prominent odd-order two-tone distortion produ cts (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-leve l primaries with frequencies such that 2f(1) - f(2) equals CF, the mag nitude of the 2f(1) - f(2) DP grows with primary level at linear or fa ster rates at low stimulus levels, but it saturates or decreases sligh tly at higher levels. For a fixed level of one of the primary tones, t he 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 t one, 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 w ith increasing primary frequency ratio (f(2)/f(1)) at low stimulus lev els. For more intense stimuli, DP magnitudes remain constant or decrea se 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 lar ge f(2)/(1) ratios, but exhibit leads for small f(2)/f(1) ratios. Coch lear exposure to an intense tone that produces large sensitivity losse s for the primary frequencies (but only small losses for tones with fr equency equal to 2f(1) - f(2)) causes a substantial decrease in magnit ude 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 correspondi ng to the primary frequencies and propagates to the location with CF e qual to the DP frequency. 2f(1) - f(2) DPs on the basilar membrane res emble 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.