TWO-TONE DISTORTION ON THE BASILAR MEMBRANE OF THE CHINCHILLA COCHLEA. Luis Robles, Mario A. Ruggero and Nola C. Rich. Departamento de Fisiologa y Biofsica, Facultad de Medicina, Universidad de Chile, Santiago, Chile, Institute for Neuroscience and Hugh Knowles Center (Department of Communication Sciences and Disorders), Northwestern University, Evanston, IL 60208-3550, 1193 Liberty Church Road, Mocksville, NC 27028.
APStracts 4:0024N, 1997.
ABSTRACT
1. Basilar-membrane responses to pairs of tones were measured, using a laser velocimeter, in the basal turn of the cochlea in anaesthetized 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). 2. For equal-level primaries with frequencies such that 2f1-f2 equals CF, the magnitude of the 2f1-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, the 2f1-f2 DP grows at a rate of about 2 dB/dB with f1 level and 1 dB/dB with f2 level . 3. 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. 4. DP phase lags increase with increasing stimulus level for large f2/f1 ratios, but decrease for small ratios. 5. Cochlear exposure to an intense tone that produces large sensitivity losses for the primary frequencies (but little sensitivity loss 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 2f1-f2. 6. 2f1-f2 DPs on the basilar membrane resemble those measured in human psychophysics in most respects, but they do not show the broadly tuned band- pass filter that seemingly shapes DP otoacoustic emissions (Harris, F. P., Lonsbury-Martin, B.L., Stagner, B.B., Coats, A. C. and Martin, G.K., J. Acoust. Soc. Am. 85:220-229, 1989). 

Received 31 May 1996; accepted in final form 30 December 1996.
APS Manuscript Number J432-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 24 January 1997