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Description
It has been proposed that a "single strategy" can account for numerous characteristics of the active ear [e.g., spontaneous otoacoustic emission (SOAE), frequency-specific amplification/compression]. The basic heuristic employed essentially amounts to an individual nonlinear oscillator, describable by a single ordinary differential equation, that has the propensity to exhibit a limit cycle oscillation (i.e., self-oscillation). A common benchmark to assess the validity of such a supercritical "Hopf Oscillator" (HopfO) is to compare its level growth response with respect to a sinusoidal drive to experiment, seeking out a specific degree of compressive behavior. However, while empirical studies generally report nonlinear responses at moderate to high sound levels, motions appear linear closer to threshold. The current study examines these foundations, taking a two-fold approach combining both computational and empirical observations. First, we simulated various scenarios of a noise/sinusoidally-driven HopfO (e.g., vicinity of a bifurcation, degree of noise and detuning). Our results are broadly consistent with, but expand upon, those of O Maoileidigh & Hudspeth (2018). Further, we observed a narrow region over which entrainment occurs that creates significant intermodulation distortion. Second, we report novel vibrometry measurements from the Anolis lizard tympanic membrane that show spontaneous oscillations with displacements on the order of 10 pm and are commensurate with SOAE activity. Crucially, sound-evoked level growth near (as well as away from) those frequencies tended to be highly linear. Linking both paths raises critical questions for the validity of a "single strategy' that relies upon an individual HopfO.