Synchronization of two active ears via binaural coupling

The two ears of many non-mammalian vertebrates are acoustically coupled through an interaural cavity, providing mechanical directional sensitivity for sound localization. Previous studies using lizards demonstrate that this coupling affects the spontaneous otoacoustic emissions (SOAEs) measured at each ear, with properties indicative of binaural synchronization. In other words, each active inner ear could influence the function of the other. However, it is unclear how binaural coupling and, consequently, synchronization contribute to SOAE generation, which is typically modeled 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. Notably, these relationships could also occur in the regions between SOAE peaks. While binaural synchronization was stronger in some individuals 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 heuristic model for active hearing that incorporates binaural coupling, accounting for its effects on SOAE generation and sound localization in the green anole. Our data strengthen the evidence that, in lizards, emissions can be generated binaurally, a principle that can inspire new strategies for designing hearing assistive technologies.