Probing Apical-Basal Differences in the Human Cochlea Using Distortion-Product Otoacoustic Emission Phase.

Distortion-product otoacoustic emission (DPOAE) phase is shaped by interaction between the evoking stimulus waves. Near-invariant at high frequencies, DPOAE phase-vs-frequency functions measured at fixed ratios bend into sloping functions at low frequencies. The different phase behaviors observed suggest that the mechanics underlying the generation of OAEs differ in the halves of the cochlea. To map out the phenomenological extent of low-to-mid frequency phase bends, this study recorded DPOAE responses from 20 normal-hearing human adult ears for a wide range of stimulus frequencies, f1 and f2, where f2 frequency sweeps from 0.25 to 8 kHz, and the f2/ f1 ratio varies from 1.05 to 1.49. Our preliminary results show two transitions in the phase slopes. One near 2.6 kHz in agreement with the literature, and another of opposite polarity near 0.75 kHz which has not been reported before. We find that the f2 frequencies marking these defining phase features are invariant with stimulus ratio. Even as the underlying mechanics remain unknown, the invariance opens the door for DPOAE phase to reliably characterize apical-basal differences across age groups and species.