The Journal of Neurophysiology Vol. 77 No. 5 May 1997, pp. 2385-2399
Copyright ©1997 The American Physiological Society

Two-Tone Distortion on the Basilar Membrane of the Chinchilla Cochlea

Luis Robles¹, Mario A. Ruggero², and Nola C. Rich³

¹ Departamento de Fisiología y Biofísica, Facultad de Medicina, Universidad de Chile, Santiago, Chile; ² Institute for Neuroscience and Hugh Knowles Center Department of Communication Sciences and Disorders, Northwestern University, Evanston, Illinois 60208-3550; and ³ Department of Otolaryngology, University of Michigan, Minneapolis, Minnesota 55455 

Robles, Luis, Mario A. Ruggero, and Nola C. Rich. Two-tone distortion on the basilar membrane of the chinchilla cochlea. J. Neurophysiol. 77: 2385-2399, 1997. 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 (f₁, f₂) 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 2f₁  f₂, 3f₁  2f₂, 2f₂  f₁ and 3f₂  2f₁). For equal-level primaries with frequencies such that 2f₁  f₂ equals CF, the magnitude of the2f₁  f₂ 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 2f₁  f₂ DP is a nonmonotonic function of the level of the other primary tone. For low intensities of the variable tone, the2f₁  f₂ DP grows at a rate of ~2 dB/dB with f₁ level and 1 dB/dB with f₂ level. DP magnitudes decrease rapidly with increasing primary frequency ratio (f₂/f₁) 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 f₂/f₁ ratios, but exhibit leads for small f₂/f₁ 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 2f₁  f₂) causes a substantial decrease in magnitude of the 2f₁  f₂ DP. This result demonstrates that the 2f₁  f₂ 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. 2f₁  f₂ 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 f₂/f₁ ratio evident in DP otoacoustic emissions.

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