The effects of body temperature on the characteristics of spontaneous otoacoustic emissions from tokay gecko

Abstract

Spontaneous Otoacoustic emissions (SOAEs) are low-level sounds emitted from the ear in the absence of acoustic stimulations. The generation mechanisms of SOAEs are currently undetermined. However, several evidences indicate that SOAEs may be produced from hair cells, the acoustic receptors of the inner ear. Each hair cell has been mathematically described by a nonlinear oscillator that operates on a verge of an instability. We hypothesize that changing of the body temperature alters the level of random fluctuations in the inner ear, which in turn can strongly influence the dynamics of hair cells. This project aims to provide a further evidence that hair cells are responsible for the production of SOAEs by investigating the effects of changes in the body temperature on the spectrum of SOAE. We showed that the frequency of SOAEs increased with the body temperature at a rate of 30 to 130 Hz/˚C. The effect was stronger for emissions observed at higher frequencies. The amplitudes of SOAEs reached a maximum at the temperature levels between 28 and 30˚C. Experimental results were compared with results from numerical simulations of a nonlinear oscillator near a supercritical Hopf bifurcation in the presence of noise. The oscillation frequency of a non-isochronous oscillator increased with the noise intensity. However, the model cannot capture the frequency-dependent temperature sensitivity, nor the alteration of SOAE amplitude observed in the experiment. Our results suggest that the generation of SOAEs is partly consistent with a non-isochronous nonlinear oscillator. A more complete development of the model is required to capture the experimental observations.