Acoustical and electrical biasing of the cochlea partition. Effects on the acoustic two tone distortions f(2)-f(1) and 2f(1)-f(2)

Low frequency acoustical biasing of the cochlear partition with 5 Hz tones produces phase correlated changes of the acoustic two-tone distortions 2f(1)-f(2) and f(2)-f(1). Pronounced changes of f(2)-f(1) and only small changes of 2f(1)-f(2) for lower bias tone levels indicate that there is a close relation between changes in the difference tone f(2)-f(1) and changes in the operating point of the cochlear amplifier (Frank and Kossl, 1996). To further investigate this relationship, the cochlear partition was additionally biased by current injection into the scala media of the gerbil. The injection of low frequency (5 Hz) AC currents (max. 1.3 mu A) has a similar effect to that caused by low frequency tones in that both produce phase correlated changes of the two distortions (so-called biasing patterns), with stronger effects on f(2)-f(1). For bias tone levels of about 105 dB SPL and current values of 1.3 mu A, the effects are approximately of the same size. A change in the f(2)-f(1) biasing pattern that can be found for increasing bias tone levels can also be seen for increasing primary levels. Changing the setpoint of the cochlear amplifier through the injection of DC current into the scala media during acoustical biasing of the cochlear partition produces the same changes of f(2)-f(1) biasing patterns as increasing the primary levels. This indicates that the operating point of the outer hair cells that respond to the primary tones is not only influenced by low frequency biasing stimuli but also by shifts with increasing primary levels.