Bilateral spontaneous otoacoustic emissions through interaural coupling
Spontaneous otoacoustic emissions (SOAEs) are weak sounds that emanate from the ears of tetrapods in the absence of acoustic stimulation. These emissions are an epiphenomenon of the inner ear's active process, which enhances the auditory system’s sensitivity to weak sounds, but their mechanism of production remains a matter of debate. Recording SOAEs simultaneously from both ears of the tokay gecko, we find that some emissions occur at the same frequency in both ears and are highly synchronized. Suppression of the emissions in one ear often changes the amplitude or shifts the frequency of emissions in the other. Decreasing the frequency of emissions from one ear by lowering its temperature usually reduces the frequency of the contralateral emissions. To understand the relationship between binaural SOAEs, we use a mathematical model of the eardrums as noisy nonlinear oscillators coupled by the air within an animal’s mouth. By according with the model, the results indicate that some SOAEs are generated bilaterally through acoustic coupling across the oral cavity.
The spectra of spontaneous otoacoustic emissions (SOAEs) recorded simultaneously from the left ear (blue) and right ear (red) display seven identical-frequency peaks (arrowheads). These signals reflect acoustic coupling between active oscillators in the two ears.
