Synchronization of two active ears via binaural coupling

Abstract

The two ears of many non-mammalian vertebrates are acoustically coupled through an interaural cavity. Sounds can hit both the external and internal surfaces of the tympanic membranes, providing mechanical directional sensitivity for sound localization. This coupling may also give rise to binaural synchronization. In other words, each inner ear could influence the function of the other. Previous work in lizards demonstrates that binaural coupling affects the spontaneous otoacoustic emissions (SOAEs) measured at each external auditory meatus, with properties indicative of binaural synchronization. However, it is unclear how binaural coupling and, consequently, synchronization contribute to SOAE generation, which is typically modelled as being localized to an individual ear. We simultaneously measured SOAEs at both ears of green anole lizards (Anolis carolinensis) and found robust relationships between them, including evidence that binaural synchronization could not be attributed to one ear uniformly driving the other. Instead, we observed frequency-dependent phase-locking between the two ears, primarily at frequencies where SOAE peaks occurred in both ears. While some pairs of ears were more strongly synchronized than others, we consistently found that binaural coupling could have greater effects on the resulting emissions than the coupling between generators within the same ear. We propose a framework for active hearing that incorporates binaural coupling, accounting for its effects on SOAE generation and sound localization in the green anole.