Speaker
Description
The ear is a complex system that acts as a mechanoelectrical transducer, allowing organisms to sense acoustic signals emanating from the world around them. To improve its ability to detect sound, the ear is active in that it uses metabolic energy to generate force that counteracts both frictional forces and noise (e.g., viscous and thermal properties of the inner ear fluid). The healthy vertebrate ear can even generate and emit coherent sound (spontaneous otoacoustic emission, or SOAE) that can be measured in the ear canal using a sensitive microphone. Significant debate however exists about the collective biophysical processes of the hair cells underlying this power amplification, as well as the related SOAE generation mechanisms. One proposed framework considers the ear as collection of oscillatory elements poised on the verge of an instability: a negative damping is paired with a stabilizing nonlinearity that could allow for self-sustained oscillation (e.g., a Hopf bifurcation). A key prediction of such a model is that the response to external driving force is highly compressive: sound-evoked motions would grow as the cube root of the stimulus level. Here, to test this prediction, we use scanning laser Doppler vibrometry (sLDV) to noninvasively measure the spontaneous and sound-evoked motions of the anole lizard tympanic membrane (TyM, or "eardrum"). We observed spontaneous oscillations (SO) of the TyM as a series of stable spectral peaks that appear consistent with acoustic SOAE measures. These oscillations are readily observable, even with amplitudes as low as 10-20 pm. For reference, the diameter of a hydrogen atom is ~100 pm, and thus the low noise floor of our measurements allow us to observe the sub-atomic nature of TyM displacements down near threshold. While sound-evoked motions appear slightly compressive near SO peak frequencies, it was generally observed that level growth was close to linear. As a heuristic benchmark for a simplified "ear", we also computationally explored several related scenarios (e.g., a coupled pair of Hopf and damped harmonic oscillators). Ultimately our observations contrast predictions of the Hopf-based framework, indicating that the vertebrate ear acts in a linearistic fashion near threshold.
| Keyword-1 | Auditory Biomechanics |
|---|---|
| Keyword-2 | Scanning Laser Vibrometer |
| Keyword-3 | Nonlinear dynamics |