Salient features of otoacoustic emissions are common across tetrapod groups and suggest shared properties of generation mechanisms

Otoacoustic emissions (OAEs) are faint sounds generated by healthy inner ears that provide a window into the study of auditory mechanics. All vertebrate classes exhibit OAEs to varying degrees, yet the biophysical origins are still not well understood. Here, we analyzed both spontaneous (SOAE) and stimulus-frequency (SFOAE) otoacoustic emissions from a bird (barn owl, Tyto alba ) and a lizard (green anole, Anolis carolinensis ). These species possess highly disparate macromorphologies of the inner ear relative to each other and to mammals, thereby allowing for novel insights into the biomechanical mechanisms underlying OAE generation. All ears exhibited robust OAE activity, and our chief observation was that SFOAE phase accumulation between adjacent SOAE peak frequencies clustered about an integral number of cycles. Being highly similar to published results from human ears, we argue that these data indicate a common underlying generator mechanism of OAEs across all vertebrates, despite the absence of morphological features thought essential to mammalian cochlear mechanics. We suggest that otoacoustic emissions originate from phase coherence in a system of coupled oscillators, which is consistent with the notion of “coherent reflection” but does not explicitly require a mammalian-type traveling wave. Furthermore, comparison between SFOAE delays and auditory nerve fiber responses for the barn owl strengthens the notion that most OAE delay can be attributed to tuning. Significance Healthy ears not only detect sound but can emit it as well. These sounds can occur either spontaneously and continuously or in response to acoustic stimulation. Evoked emissions are increasingly used as clinical diagnostic tools. The mechanisms underlying their generation, however, are incompletely understood. In particular, otoacoustic emissions occur in a wide variety of species, from frogs to humans, suggesting basic, shared principles. Here, we analyzed both spontaneous and evoked otoacoustic emissions from a bird and a lizard. We relate these data to those from humans and suggest that otoacoustic emissions originate from similar biomechanical principles despite gross differences in the underlying anatomy of the ear.