Effects of middle ear pressure on otoacoustic emission measures

An animal surgical model was established to manipulate pressure in the middle ear. The 2 f1-f2 cochlear microphonic distortion product (CMDP) and distortion product otoacoustic emission (DPOAE) were simultaneously measured in order to analyze the effects of middle ear pressure on sound transmission. The frequency ratio of the two stimulus tones f2/f1 was held at 1.2 and the level ratio L1/L2 was 10 dB. The effect of middle ear pressure on forward transmission was determined form the CMDP data. The effect on backward transmission was determined from the DPOAE data after the effect on forward transmission was subtracted out. The results showed a similar effect on forward and backward transmission. In general, negative pressure had a greater effect than positive pressure. Positive pressures of 10 and 20-cm H2O affected transmission for low-frequency stimuli (f2 = 1620 and 2680 Hz) but had little effect for high-frequency stimuli (f2 = 6980 and 10,250 Hz). Negative pressures of -2.5 to -10-cm H2O affected transmission across all frequencies tested. The effect at low frequencies is hypothesized to be related to tympanic membrane stiffness. The effect of negative pressure at high frequencies may be related to changes in the incudostapedial joint.