Contralateral suppression of distortion product otoacoustic emissions and the middle-ear muscle reflex in human ears.

Distortion product otoacoustic emissions (DPOAEs) were measured in the absence and presence of contralateral noise at five levels--below, equal to, and above the middle-ear muscle (MEM) reflex threshold. The resultant changes in DPOAE level and phase were dependent on stimulus frequency and noise level. Both low-level noise, believed to elicit the medial olivocochlear (MOC) reflex, and high-level noise, thought to activate both MOC and MEM reflexes, significantly decreased the DPOAE level. However, the shift from sole MOC effect to mixed MOC and MEM effects was not as dramatic as we thought. While low-level noise resulted in a minimum DPOAE phase change, high-level noise caused a substantial phase lead for 1 and 2kHz. With increasing frequency, phase lag became more notable. The present study suggests the following: (1) DPOAE contralateral suppression by low-level sound most likely does not involve the effect of the MEM reflex and signal crossover; and (2) combined analysis of DPOAE level and phase changes warrants further investigations to overcome the difficulty in separating the effects of MOC efferents and MEM contraction. The results also imply that OAE measurement has the potential for being used to investigate the effect of the MEM reflex on sound transmission.