Distortion products are generated by the cochlea in response to a two-tone stimulus, of frequency f1 and f2. Once generated distortion product propagate toward the middle ear; distortion product otoacoustic emissions (DPOAEs) can be measured in the ear canal. Distortion products are consequence of the nonlinearity of outer hair cells. Hence, measuring distortion product can be used to assess the functional status of outer hair cells. Measuring DPOAEs in the ear canal is commonly used to diagnose sensorineurinal hearing loss.
However, how distortion product propagate in the cochlea toward the middle ear has been debated in the scientific literature. To clarify how distortion product propagate inside the cochlea, we used a physiologically-motivated computational model of the mammalian cochlea. For the first time, we analyzed how the distortion product in the intracochlear fluid pressure varies spatially. As seen in the movie below, the total pressure (shown in the 1st panel) is maximum close to the basilar membrane (BM) around the DP (the location of maximum BM response at the DP frequency). The pressure is highly three-dimensional close to the DP BP and becomes more one-dimensional (1D) close to the base. The total pressure can be decomposed into a antisymmetric component (which interacts directly with the BM) and a symmetric component (which is uniform and is generated due to impedance mismatch between the stapes and round window). The antisymmetric component is quite complex, because it is the superposition of a reverse wave component and a forward wave component. The reverse component indicates that the DP propagate as a slowly propagating reverse wave toward the middle ear.
More details about this work can be found in your paper: Bowling, T., Meaud, J., 2018, Forward and reverse waves: modeling distortion products in the intracochlear fluid pressure, Biophysical Journal, 114(3):747:757