G. Frank et M. Kossl, THE ACOUSTIC 2-TONE DISTORTIONS 2F1-F2 AND F2-F1 AND THEIR POSSIBLE RELATION TO CHANGES IN THE OPERATING POINT OF THE COCHLEAR AMPLIFIER, Hearing research, 98(1-2), 1996, pp. 104-115
Acoustic two-tone distortions are generated during non-linear mechanic
al amplification in the cochlea. Generation of the cubic distortion 2f
1-f2 depends on asymmetric components of a non-linear transfer functio
n whereas the difference tone f2-f1 relies on symmetric components. Th
erefore, a change of the operating point and hence the symmetry of the
cochlear amplifier could be strongly reflected in the level of the f2
-f1 distortion. To test this hypothesis, low-frequency tones (5 Hz) we
re used to bias the position of the cochlear partition in the gerbil.
Phase-correlated changes of f2-f1 occurred at bias tone levels where t
here were almost no effects on 2f1-f2. Higher levels of the bias tone
induced pronounced changes of both distortions. These results are qual
itatively in good agreement with the results of a simulation in which
the operating point of a Boltzman Function was shifted. This function
is similar to those used to describe outer hair cell (OHC) transductio
n. To influence OHC motility, salicylate was injected. It caused a dec
rease of the 2f1-f2 level and an increase in the level of f2-f1. Such
reciprocal changes of both distortions, again, can be interpreted in t
erms of a shift of the operating point of the cochlear amplifier along
a non-linear transfer characteristic. To directly influence the cochl
ear amplifier, DC current was injected into the scala media. Large neg
ative currents(> -2 mu A) caused a pronounced decrease of 2f1-f2 (> 15
dB) and positive currents had more complex effects with increasing an
d/or decreasing 2f1-f2 distortion level. The effects were time and pri
mary level dependent. Changes of f2-f1 for DC currents > \mu 2A\ were
in most cases larger compared to 2f1-f2 and reversed for certain prima
ry levels. The current effects probably result from a combination of c
hanging the endocochlear potential and shifting the operating point al
ong a non-linear transfer function.