  Frequency locking to an external forcing frequency is a {well} known
phenomenon. In the auditory system, it results in a localized traveling wave,
the shape of which is essential for efficient discrimination between incoming
frequencies. An amplitude equation approach is used to show that the shape of
the localized traveling wave depends crucially on the relative strength of
additive vs. parametric forcing components; the stronger the parametric forcing
the more asymmetric the response profile and the sharper the traveling-wave
front. The analysis captures the empirically observed regions of linear and
nonlinear responses and highlights the significance of parametric forcing
mechanisms in shaping the resonant response in the inner ear.
