Similar tuning of distortion-product otoacoustic emission ratio functions and cochlear vibrations in mice

When elicited by two stimulus tones (at frequencies f₁ and f₂, f₂ > f₁), the amplitudes of specific distortion-product otoacoustic emission (DPOAE) components exhibit a characteristic bandpass shape as the ratio between f₂ and f₁ is varied. This bandpass shape has been attributed to various mechanisms including intracochlear resonance, suppression, and wave interference, and has been proposed to be related to cochlear frequency tuning. While human studies suggest modest correlations between psychophysical tuning and the tuning of DPOAE amplitude vs. f₂/f₁ ratio functions, a relationship between the latter and the tuning of cochlear mechanical responses has yet to be established. This was addressed here through direct comparisons of DPOAEs and cochlear vibrations in wild-type CBA/CaJ mice. DPOAEs were elicited using a fixed-f₂, swept-f₁ paradigm, and optical coherence tomography was used to measure displacements from cochlear locations with characteristic frequencies near f₂. The tuning sharpness of 2f₁−f₂ DPOAE ratio functions was found to be remarkably similar to that of basilar membrane and/or tectorial membrane responses to single tones, with the tuning sharpness of all responses increasing similarly with decreasing stimulus level. This relationship was observed for f₂ frequencies ranging from ∼8 to 22 kHz. Intracochlear distortion products did not exhibit a bandpass shape as the f₂/f₁ ratio was varied, indicating that interference between distortion products traveling to the stapes may be responsible for the tuning of the DPOAE ratio function. While these findings suggest that DPOAE ratio functions could be used to noninvasively infer cochlear tuning, it remains to be determined whether this relationship holds in other species and for lower frequency regions.