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Re: two-tone suppression
I suspect that both saturation and automatic gain control will enter a complete description.
As I've commented in my various publications on basilar membrane dc-shifts, the outer hair cells provide huge gain at very low frequencies, e.g. they may easily change length by microns while the operating point of the inner hair cell stereocilia may vary only by fractions of a micron to take it from fully off to fully on. I see outer hair cell length change as being the primary mechanism in support of Lyon's ideas about automatic gain control, but I see the low-frequency control signal as more than capable of saturating the IHCs, not to mention being responsible for the dc-shifts in IHC membrane potentials under high frequency tonal stimulation as originally documented by Russell and Sellick.. My 1989 Hearing Research article data may not have all been reproducible since, but the ideas developed in the discussion section are all valid and timely. These ideas were so far ahead of their time, the journey has been the reward.
I have continued to follow not just the ac-ramifications of dc-shifts/AGC, but have also recently demonstrated that these very low frequency baseline changes may be recorded in the human ear canal directly, i.e. not reflected in distortion product behaviour. I presented the initial report of these measurements at the Portland Cochlear Mechanics conference. The Portland poster on 2TS shifts is downloadable from my web page (below) where many references may be accessed. While there, please take a few minutes to fill out the ping tinnitus questionnaire - open for all readers.