The fine structure of distortion product otoacoustic emissions: The primary origin

The 2f₁-f₂ distortion product otoacoustic emission (DPOAE) fine structure may be a more sensitive indicator of cochlear damage than DPOAE level alone. The evaluation of fine structure could considerably improve the clinical use of DPOAEs for early identification of hearing loss. However, the clinical interpretation of the DPOAE fine structure needs the clarification of the origin of the DPOAE fine structure in terms of frequency selectivity and site of lesion. Previous studies hypothesized that the DPOAE fine structure was due to the interference between the two generation sources. However, some of the data from previous studies do support an alternative hypothesis that the DPOAE fine structure is dominated by the local impedance property of the cochlear partition at the distortion product (DP) place. This study investigated this hypothesis by measuring DPOAEs and separating the 2f₁-f₂ DPOAE into its early and late components under different test protocols. Three findings support the hypothesis that the DPOAE fine structure is dominated by the local impedance property of the cochlear partition at the characteristic DP place. First, the 2f₁-f ₂ DPOAE and its late component fine structure have similar fine structure pattern in terms of peak-to-peak frequency space and peak-to-valley level difference for the same test protocol and the same test frequency range for each individual subject. Second, both early and late components of 2f ₁-f₂ DPOAE showed smooth patterns when the fdp was held constant. This indicates that when the characteristic DP place along the cochlear partition was held constant across the test frequency range, the DPOAE late components are generated at the same place and minima and maxima were not observed. Third, the 2f₁-f₂ DPOAE late components obtained from the same characteristic DP frequency range under different test protocols showed a similar pattern of DPOAE fine structure despite the interference between the two DPOAE generation sources. The data obtained from this study support the hypothesis that the impedance property of the cochlear partition at the particular characteristic f_dp place dominates the DPOAE fine structure rather than the interference between the two DPOAE generation sources.