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Long-delay components showing a symmetrical pattern with positive and negative delays can be observed in the time-frequency representation (or in the inverse Fourier transform) of distortion product otoacoustic emissions. Positive-only phase-gradient delays are predicted by place-fixed backscattering mechanisms, such as coherent reflection due to roughness, whereas the nonlinear distortion wave-fixed mechanism should generate an almost null-delay component. The symmetrical delay pattern arises whenever spectral amplitude fluctuations are not fully correlated to phase fluctuations. An interpretation of this phenomenon is proposed, involving place-fixed modulation of the spectral strength of the wave–fixed nonlinear generator. Experimental data are shown in which these negative-delay sources are particularly strong, and further amplified by contralateral stimulation, suggesting that this effect could be dynamically enhanced. Analytical solutions of a linear 1-d transmission line model, in which cubic nonlinearity and roughness were added as small perturbations, have been used to test this hypothesis. [ABSTRACT FROM AUTHOR]