THE ORIGIN OF PERIODICITY IN THE SPECTRUM OF EVOKED OTOACOUSTIC EMISSIONS

Current models of evoked otoacoustic emissions explain the striking pe riodicity in their frequency spectra by suggesting that it originates through the reflection of forward-traveling waves by a corresponding s patial corrugation in the mechanics of the cochlea. Although measureme nts of primate cochlear anatomy fmd no such corrugation, they do indic ate a considerable irregularity in the arrangement of outer hair cells . It is suggested that evoked emissions originate through a novel refl ection mechanism, representing an analogue of Bragg scattering in nonu niform, disordered media. Forward-traveling waves reflect off random i rregularities in the micromechanics of the organ of Corti. The tall, b road peak of the traveling wave defines a localized region of coherent reflection that sweeps along the organ of Corti as the frequency is v aried monotonically. Coherent scattering occurs off irregularities wit hin the peak with spatial period equal to half the wavelength of the t raveling wave. The phase of the net reflected wave rotates uniformly w ith frequency at a rate determined by the wavelength of the traveling wave in the legion of its peak. Interference between the backward-trav eling wave and the stimulus tone creates the observed spectral periodi city. Ear-canal measurements are related to cochlear mechanics by assu ming that the transfer characteristics of the middle ear vary slowly w ith frequency compared to oscillations in the emission spectrum The re lationship between cochlear mechanics at low sound levels and the freq uency dependence of evoked emissions is made precise for one-dimension al models of cochlear mechanics. Measurements of basilar-membrane moti on in the squirrel monkey are used to predict the spectral characteris tics of their emissions. And conversely, noninvasive measurements of e voked otoacoustic emissions are used to predict the width and waveleng th of the peak of the traveling wave in humans. (C) 1995 Acoustical So ciety of America.