Distortion-product source unmixing: A test of the two-mechanism model for DPOAE generation

Abstract

This paper tests key predictions of the "two-mechanism model" for the generation of distortion-product otoacoustic emissions (DPOAEs). The two-mechanism model asserts that lower-sideband DPOAEs constitute a mixture of emissions arising not simply from two distinct cochlear locations (as is now well established) but, more importantly, by two fundamentally different mechanisms: nonlinear distortion induced by the traveling wave and linear coherent reflection off pre-existing micromechanical impedance perturbations. The model predicts that (1) DPOAEs evoked by frequency-scaled stimuli (e.g., at fixed f2/f1) can be unmixed into putative distortion- and reflection-source components with the frequency dependence of their phases consistent with the presumed mechanisms of generation; (2) The putative reflection-source component of the total DPOAE closely matches the reflection-source emission (e.g., low level stimulus-frequency emission) measured at the same frequency under similar conditions. These predictions were tested by unmixing DPOAEs into components using two completely different methods: (a) selective suppression of the putative reflection source using a third tone near the distortion-product frequency and (b) spectral smoothing (or, equivalently, time-domain windowing). Although the two methods unmix in very different ways, they yield similar DPOAE components. The properties of the two DPOAE components are consistent with the predictions of the two-mechanism model.

Simultaneous Intracochlear Pressure Measurements from Two Cochlear Locations: Propagation of Distortion Products in Gerbil

“…Numerous studies support OHC electromechanics as the fundamental source of the emissions, because OAEs are reduced when OHCs are damaged or when the endocochlear potential, which is necessary for OHC electromotility is reduced (Henley et al 1996;Avan et al 2003;Mills 2003). For low primary levelevoked DPs, strong evidence has been shown that the generation of DPs can be summarized into a twosource model in which the generation sites are expected to be well localized to the f 2 and f dp places (Mauermann et al 1999a, b;Kalluri and Shera 2001). At moderate to high primary-tone levels, the cochlear excitation pattern broadens and regions responsible for the generation of OAEs appear to extend further basal (Martin et al 1999;Knight and Kemp 2001;Martin et al 2010).…”

Distortion product otoacoustic emissions measured as vibration on the eardrum of human subjects

“…The two backward waves sum to yield the DPOAE. The quasiperiodic, so-called fine structure of the DPOAE appears to derive from constructive and destructive interference between these backward waves (15). Recently, Boege and Janssen (19) showed that, under certain stimulus conditions, the cubic DPOAE can be used objectively to predict auditory threshold at f 2 .…”

“…Although the underlying mechanisms remain largely unknown (e.g., refs. 11-13), there is strong evidence that DPOAEs, at least at low sound intensities, derive from two different regions along the cochlea (14), with a different mechanism being involved at each region (15)(16)(17)(18): the distortion-source emission and the reflection-source emission. The distortion-source emission appears to result from nonlinear interaction of the two primaries near and apical to the f 2 place, yielding the cubic distortion component of frequency 2f 1 Ϫ f 2 , which is directed both backward to the cochlear input and forward apically along the cochlea.…”

Distortion-product otoacoustic emission reflection-component delays and cochlear tuning: Estimates from across the human lifespan

“…In addition, the presence of the primary tones in the DPOAE paradigm suppresses amplification and/or reflection near the 2f 1 Àf 2 place, potentially modifying the results. Indeed, obtaining a close correspondence between the DPOAE R component and SFOAEs at the same frequency entails the use of an f 1 primary "mimicker" during the measurement of SFOAEs (Kalluri and Shera, 2001). Finally, possible biases introduced by the swept-tone analysis, including parameters such as the durations of the LSF and IFFT windows, remain to be systematically investigated.…”