REVERSE MIDDLE-EAR TRANSFER-FUNCTION IN THE GUINEA-PIG MEASURED WITH CUBIC DIFFERENCE TONES

Otoacoustic emissions are increasingly useful for determining cochlear function noninvasively. It is widely agreed that these acoustic signa ls reflect micromechanical processes in the cochlea. However, their qu antitative interpretation requires knowledge of the ways in which vibr ations travelling back to the ear canal from the cochlea are shaped by the middle ear. An intracochlear source is needed to derive the rever se middle-ear transfer function (rMETF) by comparing pressure in the e xternal ear canal to the corresponding pressure in scala vestibuli. In the present study, the rMETF was obtained in vivo in the guinea pig u sing as intracochlear sound source the cubic difference tones (CDTs) g enerated by a pair of external pure tones. With a closed ear canal and open bulla, the rMETF was found to be flat (-35 dB) over a broad freq uency range (1.5-8 kHz). The differences between forward and reverse M ETF could be explained by different loads acting on the middle ear net work, which depends on the direction of signal transmission. With know ledge of the rMETF, it becomes possible to quantify CDTs within the co chlea by measuring them noninvasively in the ear canal.