1. Effects of Low-Frequency Bias Tones on Stimulus-Frequency Otoacoustic Emissions Eric L. Carmichel Mentors Dr. Michael Dorman Dr. Lin Bian

2. Committee Members Dr. Lin Bian (Committee Chair) Dr. Michael Dorman Dr. Andrea Pittman

3. Otoacoustic Emissions (OAEs) Faint acoustical signals originating from the inner ear but can be detected in ear canal They can occur spontaneously (SOAEs) Can occur during and after stimulation of the ear In general, healthy ears produce OAEs, whereas hearing-impaired ears do not

8. Otoacoustic Emissions (OAEs) OAEs are a by-product of the active mechanism of the inner ear (Probst et al., 1991) When OHCs are damaged, the active mechanism is reduced in effectiveness or lost altogether Compressive nonlinearity is associated with the active cochlear process

9. What does “compressive nonlinearity” mean?

11. Previous studies Low-frequency biasing technique used to assess the dynamic nonlinearity of the inner ear The bias tone can shift the cochlear partition. This produced an amplitude modulation of DPOAEs (Bian et al., 2002, 2007; Bian, 2004)

12. Cochlear partition—Consists of the basilar membrane, tectorial membrane, and the Organ of Corti

13. Cochlear partition

14. Biasing the cochlear partition

15. Question: Can the biasing technique be applied to SFOAEs? SFOAEs evoked in response to a single external tone Measuring SFOAEs presents a unique challenge. As the name implies… The emission is at the same frequency as the stimulus (or probe) tone

16. DPOAE visible via spectral analysis SFOAE cannot be “seen”

17. Question: Can the biasing technique be applied to SFOAEs? Cannot “see” the emission Separating a minute signal (i.e.,the emission) requires an indirect method Can we study the effects of amplitude modulating the SFOAE with a bias tone? More theory (oh boy!) and a hardware model

21. “OHC” Probe |Bias| Bias tone

27. Question: Can the biasing technique be applied to SFOAEs? We have a mathematical foundation We have a working hardware model Now all we need… are human ears!

28. Methods Nine participants, ages 21 – 48 yrs (M=25) Ten ears total Eight normal ears with robust DPOAEs Two ears with profound hearing loss Bias tone and probe tone presented simultaneously Ear canal acoustics recorded

29. Participants were comfortably seated

45. Summary Noninvasive nature of biasing technique— shows potential as a research tool and development of new clinical applications Normal and impaired ears can be tested— potentially be applied in the differential diagnosis of cochlear pathologies Just the beginning—present assay can be expanded upon

46. Acknowledgements Thanks to committee members Dr. Bian, Dr. Dorman, and Dr. Pittman Thanks to Tina Stinson for recruiting and screening participants Thanks to the participants Many thanks to all of you!