Indications of different distortion product otoacoustic emission mechanisms from a detailed f1,f2 area study

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Abstract
The primary site of generation on the basilar membrane for the 2 f1–f2 distortion product (DP) is generally considered to be near where the higher-frequency stimulus tone peaks. This site has also been shown to be a source of DP otoacoustic emission (DPOAE) in the ear canal, but a second source of emission is known to exist in the region of the DP frequency place. The DPOAE phase versus frequency gradient provides a means of investigating the emission mechanisms. “Wave-fixed” and “place-fixed” mechanisms have been proposed to account for the very different phase gradients found depending on whether the 2 f1–f2 DPOAE is evoked by a small or large stimulus-frequency ratio. DPOAE phase versus frequency gradients can be investigated either by sweeping f1,f2 or by sweeping both frequencies maintaining a constant frequency ratio. Each manipulation gives only a partial description of DP behavior. In this study, the place-fixed/wave-fixed dichotomy is analyzed using extensive 2 f1–f2 and 2 f2–f1 DP stimulus-frequency sweep data presented on matrices of f1 vs f2 and f2/f1 ratio versus DP frequency. These show how the DPs are related and provide a more complete picture of 2 f1–f2 and 2 f2–f1 DPOAE phase and amplitude versus frequency behavior. The phase data contain evidence for a systematic variation in the proportions of wave- and place-fixed emission. The results suggest that 2 f1–f2 DPOAEs with a wide stimulus frequency ratio are wave fixed, while all other DPOAEs are place fixed. A transition occurs within the 2 f1–f2 DP data region at a frequency ratio of about f2/f1=1.1. The 2 f1–f2 DP and 2 f2–f1 DP phase behavior is continuous across the f2/f1=1 boundary. As the 2 f2–f1 DP generation region must be strongly influenced by the DP frequency place, the results imply that the place-fixed component of the 2 f1–f2 DP is also linked to its frequency place. A similar pattern was obtained with the 3f1–2 f2 and 3f2–2 f1 DPs. The results support the following model: For the limited set of stimulus conditions that gives rise to 2 f1–f2 wave-fixed emissions, DP energy is largely generated in the f2 region and is emitted directly. All other DPOAEs are place-fixed emissions, and while nonlinearity within the f2 stimulus envelope remains the generator, the DP is not directly emitted but travels apically until it is re-emitted basally via a separate reflection mechanism in the region of the DP place.

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