TWO-TONE DISTORTION ON THE BASILAR MEMBRANE OF THE CHINCHILLA COCHLEA.
Luis Robles*, Mario A. Ruggero+ and Nola C. Rich#.
* Departamento de Fisiología y Biofísica, 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 4:0028N, 1997.
ABSTRACT
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). 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 of the variable tone, the 2f1-f2 DP grows at a rate of
about 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.
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