Generation place of the long- and short-latency components of transient-evoked otoacoustic emissions in a nonlinear cochlear model.

Time-domain numerical solutions of a nonlinear active cochlear model forced by click stimuli are analyzed with a time-frequency wavelet technique to identify the components of the otoacoustic response associated with different generation mechanisms/places. Previous experimental studies have shown evidence for the presence of at least two components in the transient otoacoustic response: A long-latency response, growing compressively with increasing stimulus level, and a shorter-latency response, characterized by faster growth. The possible mechanisms for the generation of the two components are discussed using the results of the numerical simulations. The model is a one-dimensional (1-D) transmission line model with nonlinear and nonlocal active terms representing the anti-damping action of the "cochlear amplifier." The dependence on the stimulus level of latency and level was measured for the different components of the response. The generation mechanisms/places of the different components were identified by varying the stimulus level and by turning off the cochlear roughness in well-defined cochlear regions. The results suggest that reflections from roughness coming from basal regions of the cochlea may give a relevant contribution to the early otoacoustic response, whereas nonlinear mechanisms seem to produce a much smaller additional contribution.

Wavelet and matching pursuit estimates of the transient-evoked otoacoustic emission latency.

Different time-frequency techniques may be used to investigate the relation between latency and frequency of transient-evoked otoacoustic emissions. In this work, the optimization of these techniques and the interpretation of the experimental result are discussed. Time-frequency analysis of click-evoked otoacoustic emissions of 42 normal-hearing young subjects has been performed, using both wavelet and matching pursuit algorithms. Wavelet techniques are very effective to provide fast and reliable evaluation of the average latency of large samples of subjects. A major advantage of the matching pursuit technique, as observed by Jedrzejczak et al. [J. Acoust. Soc. Am. 115, 2148-2158 (2004)], is to provide detailed information about the time evolution of the response of single ears at selected frequencies. A hybrid matching pursuit algorithm that includes Fourier spectral information was developed, capable of speeding-up computation times and of identifying "spurious" atoms, whose latency-frequency relation is apparently anomalous. These atoms could be associated with several known phenomena, either intrinsic, such as intermodulation distortion, spontaneous emissions and multiple internal reflections, or extrinsic, such as instrumental noise, linear ringing and the acquisition window onset. A correct interpretation of these phenomena is important to get accurate estimates of the otoacoustic emission latency.