Suppression of Otoacoustic Emission using Schroeder Harmonic Complexes as the Suppressing Stimuli

Introduction: The medial olivocochlear reflex (MOCR) causes inhibition of outer hair cell activity upon noise stimulation, and acts as a protective mechanism of the ear against noise. The MOCR can be recorded through the suppression of otoacoustic emissions (OAEs). The mechanism of MOCR function is still unclear, and whether the function is affected by the phase of incoming noise requires further investigation. This research aimed to identify the effect of MOCR activity: i) at different frequencies; and ii) when using different phases of Schroeder harmonic complexes as the suppressor noise. Method: 26 normal hearing subjects were recruited. The suppression was analysed by looking at the amplitude difference of distortion-product otoacoustic emissions (DPOAEs) in quiet and in the presence of noise in contralateral ear, at different tested frequencies (i.e.; 1, 1.5, 2, 3, 4, and 6 kHz). Result: There was significantly higher suppression amplitude for the 1-2 kHz frequency compared to higher frequencies (p < 0.05). Significantly higher suppression was observed when negative phase Schroeder harmonic complexes (c: -1) were used as supressing stimuli compared to positive phase complexes (c: +1) (p < 0.05). Conclusion: These findings showed that the MOCR in humans: i) is enhanced at the mid-frequency region; and ii) has the ability to inhibit the outer hair cell active mechanism differently upon stimulation with different phases of noise.