THE ACOUSTIC 2-TONE DISTORTIONS 2F1-F2 AND F2-F1 AND THEIR POSSIBLE RELATION TO CHANGES IN THE OPERATING POINT OF THE COCHLEAR AMPLIFIER

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.