The behavior of evoked otoacoustic emissions during and after postural changes

Click-evoked and stimulus frequency otoacoustic emissions (CEOAEs and SFOAE s, respectively) were studied in humans during and after postural changes. The subjects were tilted from upright to a recumbent position (head down 30 deg) and upright again. Due to the downward posture change, CEOAEs showed a phase increase (80 deg at 1 kHz) and a level decrease (0.5 at I kHz), esp ecially for frequency components below 2 kHz. For SFOAEs, the typical rippl e pattern showed a positive shift along the frequency axis, which can be in terpreted as a phase shift of the inner-ear component of the microphone sig nal (90 deg at I kHz). This also occur-red mainly for frequencies below 2 k Hz. The altered posture is thought to cause an increase of the intracranial pressure, and consequently of the intracochlear fluid pressure, which resu lts in an increased stiffness of the stapes system. The observed emission c hanges are in agreement with predictions from a model in which the stiffnes s of the cochlear windows was altered. For CEOAEs, the time to regain stabi lity after a downward turn was of the order of 30 s, while this took about 20 s after an upward turn. For SFOAEs, this asymmetry was not found to be p resent (about 11 s, both for up- and downward turns). (C) 2001 Acoustical S ociety of America.