Deviations from Scaling Symmetry in the Apical Half of the Human Cochlea.

Invariant distortion product otoacoustic emission (DPOAE) phase elucidates scaling symmetry in the cochlea. Below some low-frequency boundary, DPOAE phase slope steepens. The origin of this break in phase invariance is not clear. Stimulus frequency (SF)OAE delays computed from the slope of phase also manifest discontinuities at low frequencies, though the relationship between the breaking of cochlear scaling as defined by SFOAE and DPOAE metrics has not been examined. In this study, OAEs were recorded in normal-hearing human adults to probe cochlear scaling and its breaking and to examine the correspondence between two OAE metrics of scaling. Results indicate: (1) the apical break in DPOAE phase invariance cannot be explained by contributions from the reflection-source component; (2) DPOAE phase signals a break from scaling near 1.5 kHz and (3) DPOAE and SFOAE metrics of cochlear scaling produce phase discontinuities within approximately one-quarter octave of each other and show comparable rates of breaking, suggesting a common underlying origin.