NONINVASIVE MEASUREMENT OF THE COCHLEAR TRAVELING-WAVE RATIO

The microstructure of threshold hearing curves and the frequency spect ra of evoked otoacoustic emissions both often evince a roughly periodi c series of maxima and minima. Current models for the generation of ot oacoustic emissions explain the observed spectral regularity by suppos ing that since the cochlea maps frequency into position the spectral p eriodicity mirrors a spatial oscillation in the mechanics of the organ of Corti. In this view emissions are generated when forward-traveling waves reflect from periodic corrugations in the mechanics, suggesting that the amplitude of the cochlear traveling-wave ratio-defined to be the ratio of the backward- and forward-traveling cochlear waves at th e stapes-should manifest pronounced maxima and minima with a correspon ding periodicity. This paper describes measurements of stimulus-freque ncy emissions, establishes their analyticity properties, and uses them to explore the spatial distribution of mechanical inhomogeneities (em ission ''generators'') in the human cochlea. The approximate form and frequency dependence of the cochlear traveling-wave ratio are determin ed noninvasively. The amplitude of the empirical traveling-wave ratio is a slowly varying, nonperiodic function of frequency, suggesting tha t the distribution of inhomogeneities is uncorrelated with the periodi city found in the threshold microstructure. The observed periodicities arise predominantly from the cyclic variation in relative phase betwe en the forward-and backward-traveling waves at the stapes.