Dependence of distortion-product otoacoustic emissions on primary levels in normal and impaired ears. II. Asymmetry in L1,L2 space

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Abstract
Previous studies indicate that the amplitude of 2f 1−f 2 distortion‐product otoacoustic emissions (DPOAEs), evoked by two tones of frequencies f 1<f 2, demonstrates a complex dependence on the levels (L 1 and L 2) of the primary tones. In the present study, 2f 1−f 2 DPOAE amplitudes were measured over a wide range of L 1 and L 2 in normal human ears, allowing a systematic, level‐dependent asymmetry of DPOAE amplitude in L 1,L 2 space to be characterized. The L 1,L 2 at which DPOAEs were largest was close to L 1=L 2 at high stimulus levels, but moved monotonically toward L 1≳L 2 as stimulus levels decreased. A related observation was that DPOAE amplitude had a greater dependence on L 1 than on L 2. These asymmetries were quantified in normal human ears, and compared to the corresponding asymmetries apparent in data from animalmodels. Recent studies have demonstrated that the reduction of DPOAE amplitude by cochlear trauma is greater when L 1≳L 2 than when L 1=L 2, suggesting that the reduction of DPOAEs by trauma demonstrates an asymmetry in L 1,L 2 space that is qualitatively similar to that of normative DPOAE amplitude. To investigate this issue, 2f 1−f 2 DPOAE amplitudes were measured over a wide range of L 1 and L 2 in rabbit ears pre‐ and postinjection of the ototoxic loop‐diuretic ethacrynic acid. The results indicate that the asymmetry in L 1,L 2 space of the reduction of DPOAEs by trauma is both qualitatively and quantitatively similar to the asymmetry in L 1,L 2 space of normative DPOAE amplitude. Specifically, the L 1 values that maximized normative DPOAE amplitudes for any specified L 2 (or, equivalently, the L 1 values that allowed L 2 to be minimized for any specified normative DPOAE amplitude) also yielded the greatest reduction of DPOAEs by the diuretic. In humans, the L 1 values that maximize normative DPOAE amplitudes for any specified L 2 are well approximated by a simple equation, with parameters that vary with frequency and f 2/f 1. It is suggested that the L 1,L 2 values defined by this equation may be optimum for use in clinical applications.

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