Cochlear transmission line models and otoacoustic emissions spectral latency as objective measure of cochlear tuning

The spectral latency of otoacoustic emissions (OAEs) is physically due to the cochlear traveling time from the base to the tonotopic site and back. The Greenwood map provides the relation between the position of the tonotopic place along the cochlea and the frequency. As a consequence, the latency is intrinsically dependent on frequency. The relation between OAE latency and frequency is also related to the tuning curve, i.e. the relation between the quality factor of cochlear resonances and frequency. In this work we use a theoretical model for the cochlear transmission, which permits to compute the OAE spectral latency as a function of the tuning spectral curve. By inverting this functional relation, the model was used to estimate the tuning curve in a population of adult and neonate subjects. Spectral latency was measured by wavelet time-frequency analysis of transient evoked OAE data. Independently, we used theoretical models of spontaneous OAEs generation, based on the Bragg condition for coherent reflection by random cochlear inhomogeneities, to get an independent relation between minimum spacing between spontaneous OAEs and spectral tuning. Very good agreement was found between the estimates of spectral tuning obtained on the same neonate subjects with the two independent methods.