ACOUSTICAL AND ELECTRICAL BIASING OF THE COCHLEA PARTITION - EFFECTS ON THE ACOUSTIC 2 TONE DISTORTIONS F(2)-F(1) AND 2F(1)-F(2)

Low frequency acoustical biasing of the cochlear partition with 5 Hz t ones produces phase correlated changes of the acoustic two-tone distor tions 2f(1)-f(2) and f(2)-f(1). Pronounced changes of f(2)-f(1) and on ly small changes of 2f(1)-f(2) for lower bias tone levels indicate tha t 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 (F rank and Kossl, 1996). To further investigate this relationship, the c ochlear partition was additionally biased by current injection into th e 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 fr equency 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 o f 1.3 mu A, the effects are approximately of the same size. A change i n 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 t he setpoint of the cochlear amplifier through the injection of DC curr ent into the scala media during acoustical biasing of the cochlear par tition produces the same changes of f(2)-f(1) biasing patterns as incr easing the primary levels. This indicates that the operating point of the outer hair cells that respond to the primary tones is not only inf luenced by low frequency biasing stimuli but also by shifts with incre asing primary levels.