The effect of aspirin on cochlear mechanical tuning.

The level of cubic distortion measured in the human ear canal reaches a peak when the distortion frequency falls approximately half-an-octave below that of the higher of two stimulus tones. The peak is thought to be indicative of a bandpass filter in the cochlea. The properties of the distortion peak have been measured during the administration of aspirin (3.84 g/day for 2 days) in eight human volunteers. In three of the subjects, there was a downward shift of the center frequency on the distortion peak by as much as 200 Hz during aspirin consumption, but no significant changes in Q3 dB of the peak. In two of these subjects, the shift in distortion peak was not associated with a change in psychophysical threshold. The mean group delay of the emitted distortion was reduced by approximately 0.5 ms in four of the eight subjects. The measurement of stimulus frequency emission (SFE) did not reveal any downward shift of features in the spectrum corresponding to those seen for the distortion peak. It is concluded (1) that aspirin affects the resonance frequency of the bandpass filter revealed by distortion measurement, but not its tuning; and (2) that the bandpass filter is dissociated from the mechanism responsible for the "fine structure" in the SFE spectrum.