Bilateral spontaneous otoacoustic emissions show coupling between active oscillators in the two ears

Abstract

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 systems sensitivity to weak sounds, but their mechanism of production remains a matter of debate. To understand the relationship between SOAEs that we recorded simultaneously from the two ears of the tokay gecko, we developed a mathematical model of the eardrums as noisy nonlinear oscillators coupled by the air within a lizards mouth. We found that binaural emissions could be strongly correlated: some emissions occurred at the same frequency in both ears and were highly synchronized. Suppression of the emissions in one ear often changed the amplitude or shifted the frequency of emissions in the other. Decreasing the frequency of emissions from one ear by lowering its temperature usually reduced the frequency of the contralateral emissions. By according with the model, the results indicate that some SOAEs are generated bilaterally through acoustic coupling across the oral cavity. The model predicts that sound localization through the acoustic coupling between ears is influenced by the active processes of both ears.