The role of intermodulation distortion in transient-evoked otoacoustic emissions

Transient-evoked otoacoustic emissions (TEOAEs) are low-intensity sounds recorded in the external ear canal immediately following stimulation by a transient stimulus, typically a click. While the details of their production is unknown, there is evidence to suggest that the amplitude of each component frequency reflects the physiological condition of the corresponding region of the cochlea. Certain observations are at variance with this assumption, however, suggesting that pathology at a basal site within the cochlea might affect the production of emissions at frequencies which are not characteristic for that site. We have recorded click-evoked emissions in guinea pigs using high-pass clicks and found emissions at frequencies which are not present in the stimulus and which could not, therefore, have originated from the characteristic place for those emission frequencies. These new frequencies are, by definition, intermodulation distortion frequencies and must have been generated from combinations of frequencies in the stimulus by non-linear processes within the cochlea. Further processing of the emissions by Kemp's technique of non-linear recovery showed that the magnitude of emissions at frequencies within the stimulus frequency pass-band was approximately the same as that of frequencies not present in the stimulus. We propose that, in guinea pigs at least, most of the click-evoked emission energy is generated as intermodulation distortion, produced by non-linear intermodulation between various frequency components of the stimulus. If this result is confirmed in humans, many of the anomalies in the literature may be resolved.