The origin of periodicity in the spectrum of evoked otoacoustic emissions.

Current models of evoked otoacoustic emissions explain the striking periodicity in their frequency spectra by suggesting that it originates through the reflection of forward-traveling waves by a corresponding spatial corrugation in the mechanics of the cochlea. Although measurements of primate cochlear anatomy find no such corrugation, they do indicate a considerable irregularity in the arrangement of outer hair cells. It is suggested that evoked emissions originate through a novel reflection mechanism, representing an analogue of Bragg scattering in nonuniform, disordered media. Forward-traveling waves reflect off random irregularities in the micromechanics of the organ of Corti. The tall, broad peak of the traveling wave defines a localized region of coherent reflection that sweeps along the organ of Corti as the frequency is varied monotonically. Coherent scattering occurs off irregularities within the peak with spatial period equal to half the wavelength of the traveling wave. The phase of the net reflected wave rotates uniformly with frequency at a rate determined by the wavelength of the traveling wave in the region of its peak. Interference between the backward-traveling wave and the stimulus tone creates the observed spectral periodicity. Ear-canal measurements are related to cochlear mechanics by assuming that the transfer characteristics of the middle ear vary slowly with frequency compared to oscillations in the emission spectrum. The relationship between cochlear mechanics at low sound levels and the frequency dependence of evoked emissions is made precise for one-dimensional models of cochlear mechanics. Measurements of basilar-membrane motion in the squirrel monkey are used to predict the spectral characteristics of their emissions. And conversely, noninvasive measurements of evoked otoacoustic emissions are used to predict the width and wavelength of the peak of the traveling wave in humans.