[Forward masking of transiently evoked otoacoustic emissions]

To explore the mechanisms of cochlear regulation systems, two tone phenomena of transiently evoked otoacoustic emissions (EOAEs) were measured using a forward masking paradigm in normal human ears. EOAEs were elicited by probe tone (1 kHz, 25 dBnHL, 3 ms duration), with or without a preceding masker tone (841 Hz, 35-85 dBnHL, 10-300 ms duration, 5-160 ms interval). In the intensity study, EOAEs showed decrements in amplitude according to the masker intensity. To evaluate the effects of the stapedial reflex (SR), the forward masking in facial palsy cases without SR was also measured. For the higher intensity masker, SR effects were observed in normal cases. The duration study showed a little but almost constant decrement with lower intensities, below the SR threshold, and a 20 ms or more duration masker. The interval study showed the decrement even with low and middle (35/55 dBnHL) intensity masker when the intervals were short. As the intervals were shortened, the decrement in EOAEs amplitude became greater. These results suggest that not only the olivocochlear bundle but also other mechanisms, e.g. short-term adaptation and/or lateral inhibition, may affect the forward masking of EOAEs.