Effects of Medial Olivocochlear Auditory Reflex Activation on Cochlear Vibration

In the cochlea, the basilar membrane (BM) vibrates in response to sound. The BM separates the different frequency components of sound along its length. BM vibration is a combination of passive mechanics and an active “cochlear amplifier.” The cochlear amplifier arises from the motion of cochlear outer hair cells, which change their length in response to specific frequencies. This boosts the amplitude of BM vibration, which improves hearing of soft sounds and frequency resolution. The Medial Olivocochlear Reflex (MOCR) is a feedback pathway in the brainstem which, in the presence of background noise, lessens the boost of the cochlear amplifier. MOCR can be indirectly measured using Otoacoustic Emissions (OAEs), which are soft sounds originating in the cochlea as a byproduct of cochlear amplifier activity. Past research suggests that the MOCR alters the magnitude and phase of OAEs and BM vibration. However, the apparent phase change may be a change in frequency. This would suggest that activation of the MOCR changes the resonant frequency of the BM. The present study sought to explore MOCR effects on OAE frequency, magnitude, and phase. The results indicated high variability between subjects, but many subjects showed changes in OAE frequency without changes to OAE phase. This paradigm shift may have implications for reanalysis of past research on this topic.