Demonstrating True DPOAE Modulation by the Efferent Pathway

Abstract

Objective: 1) To isolate the distortion product otoacoustic emission (DPOAE) components using an ipsilateral and individualized suppressor tone. 2) To demonstrate the modulation of DPOAEs by the medial olivocochlear efferent pathway once component interference has been eliminated.

Method: DPOAE (2f1-f2 ) recordings using a frequency-sweep paradigm with ipsilateral suppressor tones and CAS were made on 8 normal-hearing subjects in a sound-treated audiology booth from January to February 2012. Estimates of DPOAE level and phase were obtained using fast Fourier transform and least-squares-fit techniques.

Results: Fine structure depth was significantly reduced with the use of an ideal ipsilateral suppressor tone, demonstrating the effectiveness of the suppressor tone at separating DPOAE components. In addition, once the DPOAE components were separated to remove phase interference, MOC activation resulted in consistent suppression of DPOAE level as compared to nonseparated DPOAE level which showed enhancement as well as suppression. These results help to support the theory of DPOAE component interference leading to enhancements often seen with efferent activation and to demonstrate a more consistent way to view DPOAE modulation by the efferent pathway.

Conclusion: This study demonstrates that DPOAE fine structure enhancement seen with activation of the MOC pathway is likely due to DPOAE component interference as opposed to true DPOAE modulation. In addition, the results show that DPOAE component isolation can be achieved by an ipsilateral suppressor tone, however it must be individualized.