The acoustic two-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 mechanical amplification in the cochlea. Generation of the cubic distortion 2f1-f2 depends on asymmetric components of a non-linear transfer function whereas the difference tone f2-f1 relies on symmetric components. Therefore, 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) were used to bias the position of the cochlear partition in the gerbil. Phase-correlated changes of f2-f1 occurred at bias tone levels where there were almost no effects on 2f1-f2. Higher levels of the bias tone induced pronounced changes of both distortions. These results are qualitatively 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) transduction. To influence OHC motility, salicylate was injected. It caused a decrease 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 terms of a shift of the operating point of the cochlear amplifier along a non-linear transfer characteristic. To directly influence the cochlear amplifier, DC current was injected into the scala media. Large negative currents(> -2 mu A) caused a pronounced decrease of 2f1-f2 (> 15 dB) and positive currents had more complex effects with increasing and/or decreasing 2f1-f2 distortion level. The effects were time and primary level dependent. Changes of f2-f1 for DC currents > \mu 2A\ were in most cases larger compared to 2f1-f2 and reversed for certain primary levels. The current effects probably result from a combination of changing the endocochlear potential and shifting the operating point along a non-linear transfer function.