Imaging the ratio dependence of distortion products at apical regions of the gerbil cochlea

Distortion product otoacoustic emissions (DPOAEs) exhibit features at low frequencies that are distinct from high-frequency ones in that they are less ratio-dependent: their characteristics (amplitude and phase) vary little with the frequency separation of the primary tones. The underlying mechanism may arise from the unique apical cochlear mechanics that are poorly understood. Intracochlear observations provide evidence for the generation mechanism of DPOAEs. In the current study, two-tone induced distortion products (DPs) were recorded from the apically located organ of Corti complex [OCC includes the organ of Corti, the basilar membrane, and the tectorial membrane] of alive gerbil cochleae using optical coherence tomography (OCT) together with simultaneously measured ear canal DPOAEs. Within the OCC, DPs were robust and recorded from all key structures. The ratio-dependency of DPs appeared to link to the characteristics of the primary tones that were broadly tuned and exhibited nonlinearity in a broad region. The upper sideband, 2f₂− f₁ DP was found ratio independent due to the invariant overlapping region of the primary tones. The lower sideband, 2f₁ − f₂ DP differed, and varied with the f₂/f₁ ratio. It included both local and nonlocal (i.e. traveling) DP components. Thus, their correlation to related DPOAEs was not straightforward. Our observation provides insights into the generation of low-frequency DPOAEs and related cochlear mechanics, which helps to improve the precision of DPOAEs in detecting auditory sensory damage in the clinic.