Sound localization in the lizard using internally coupled ears: A finite-element approach.

A number of interesting differences become apparent when comparing the hearing systems of terrestrial vertebrates, especially between mammals and non-mammals. Almost all non-mammals possess only a single ossicle, enabling impedance matching and hearing below 10 kHz. The middle ear (ME) evolved as a chain of three ossicles in mammals, enabling sound transmission up to higher frequencies than in similar-sized non-mammals. The relatively low-frequency hearing in non-mammals is associated with audible wavelengths that are significantly larger than the head. Therefore, it is unlikely that localization of the sound source can be obtained by using external cues between the ears (intensity and time difference between both sides), especially when compared to similarly sized mammals. The heads of many non-mammals contain large air-filled cavities, which acoustically couple both MEs. This article studies acoustic responses and sound-source localization capabilities of the coupled MEs of the brown anole (Anolis sagrei), using finite-element modeling. Based on high-resolution μCT data, 3D finite-element models of the ME and interaural cavity were constructed. The parameter values in the ME model were determined such that the response of the isolated ME matches experimental data of literature and the velocity ratio between the tympanic membrane (apex) and footplate reflects the anatomical arrangement of the columellar lever in the anole. It was found from simulation of the coupled ME model that the interaural connection amplifies intensity differences between both sides and thus enhances the capability of sound-source localization. In addition, the interaural canal doubles the phase differences of the incident external sound waves between the eardrums. In isolated ears, generating such phase differences would require head sizes twice as large. Effects of the inner-ear loading on the sound-source localization of the coupled MEs were investigated as well. The inner-ear load lowered the peak velocity ratios between the ears, but created broader plateaus of useful directionality, indicating that inner-ear loading not only influences sound perception but also sound localization in internally connected ears.