The phase shift of the envelope-following response as an indicator of the traveling wave propagation in the beluga cochlea

Abstract

Auditory system of the toothed whales features a number of modifications that allow to adapt to aquatic life and echolocation. One of unanswered questions is how the characteristics of the traveling wave changed due to extension of frequency range. We hypothesize that the phase shift of the envelope-following response (EFR) reflects the traveling wave propagation along the basilar membrane in the cochlea. EFRs were non-invasively recorded in the beluga whale. The sound stimuli were tone burst of carrier frequencies of 16 to 128 kHz and envelope frequency of 1 kHz. Thus, all EFR contained the fluctuations with 1 kHz frequency. The EFR phase shift depended on the carrier frequency and the sound intensity: the higher intensity and frequency, the greater the phase shift. If it is accepted that at the EFR threshold the phase depends only on the carrier frequency, the phase shift allows to calculate the time of the traveling wave propagation along the basilar membrane. The travelling wave time between the regions corresponding to 16 and 128 kHz was approximately 0.8 ms.