Middle ear forward and reverse transmission in gerbil.

The middle ear transmits environmental sound to the inner ear. It also transmits acoustic energy sourced within the inner ear out to the ear canal, where it can be detected with a sensitive microphone as an otoacoustic emission. Otoacoustic emissions are an important noninvasive measure of the condition of sensory hair cells and to use them most effectively one must know how they are shaped by the middle ear. In this contribution, forward and reverse transmissions through the middle ear were studied by simultaneously measuring intracochlear pressure in scala vestibuli near the stapes and ear canal pressure. Measurements were made in gerbil, in vivo, with acoustic two-tone stimuli. The forward transmission pressure gain was about 20-25 dB, with a phase-frequency relationship that could be fit by a straight line, and was thus characteristic of a delay, over a wide frequency range. The forward delay was about 32 micros. The reverse transmission pressure loss was on average about 35 dB, and the phase-frequency relationship was again delaylike with a delay of about 38 mus. Therefore to a first approximation the middle ear operates similarly in the forward and reverse directions. The observation that the amount of pressure reduction in reverse transmission was greater than the amount of pressure gain in forward transmission suggests that complex motions of the tympanic membrane and ossicles affect reverse more than forward transmission.