Bilateral Spontaneous Otoacoustic Emissions Show Coupling between Active Oscillators in the Two Ears

Abstract

Spontaneous otoacoustic emissions (SOAEs) are weak sounds that emanate from the ears of tetrapods in the absence of acoustic stimulation. These emissions are an epiphenomenon of the inner ear's active process, which enhances the auditory system’s sensitivity to weak sounds, but their mechanism of production remains a matter of debate. We recorded SOAEs simultaneously from the two ears of the tokay gecko and found that binaural emissions could be strongly correlated: some emissions occurred at the same frequency in both ears and were highly synchronized. Suppression of the emissions in one ear often changed the amplitude or shifted the frequency of emissions in the other. Decreasing the frequency of emissions from one ear by lowering its temperature usually reduced the frequency of the contralateral emissions. To understand the relationship between binaural SOAEs, we developed a mathematical model of the eardrums as noisy nonlinear oscillators coupled by the air within an animal’s mouth. By according with the model, the results indicate that some SOAEs are generated bilaterally through acoustic coupling across the oral cavity. The model predicts that sound localization through the acoustic coupling between ears is influenced by the active processes of both ears.

Complex dynamics of hair bundle of auditory nervous system (I): spontaneous oscillations and two cases of steady states

“…Different from the most of previous studies wherein Hopf bifurcation is mainly the bifurcation related to spontaneous oscillations Deng et al (2019), Maoile ´idigh and D, Hudspeth AJ, (2013), Reichenbach and Hudspeth (2014), in the present paper, 5 codimension-1 bifurcations (H sub , H sup , Flc, HC, and Fold) and 4 codimension-2 bifurcations (GH, BT, Cusp, and P1) are acquired in multiple physiological parameter-planes including (S, D) plane, which presents more complex nonlinear dynamics related to the spontaneous oscillations. Such a result shows that multiple physiological parameters can be used as modulation measures to adjust the bifurcations related to the spontaneous oscillations, which is the basis of otoacoustic emission via enhancing the sensitivity of inner ear to weak sounds Roongthumskul et al (2019). Furthermore, the vector fields, nullcline, and fast/slow dynamics of variables which have been widely used in the study of nervous system are attempted to analyze the dynamics of spontaneous oscillations of hair bundles in the present paper.…”

“…The spontaneous oscillations play an important role in enhancing the sensitivity of the auditory system to sound Roongthumskul et al (2019) and the response of steady state to external stimuli shows the active regulation of hair cells Tinevez et al (2007). Identification of dynamic of spontaneous oscillations, steady states, and relationships between them is helpful to understand the intrinsic mechanism, which is important for the modulations to the physiological functions.…”

Synchronization and chaos in systems of coupled inner-ear hair cells

“…Although SOAEs serve as a diagnostic for hearingrelated disorders in humans, there is currently no consensus on the mechanism responsible for generating them. [16][17][18] One theory suggests that they arise from frequency clustering of actively oscillating coupled hair cells. 19,20 In vivo, hair bundles are attached to overlying structures, which provide coupling between the individual active oscillators.…”

“…However, the inner ear does spontaneously emit faint tones in the absence of stimulus [15]. These spontaneous otoacoustic emissions (SOAEs) are ubiquitious across vertebrate species and occur only in live animals with intact inner ears, suggesting that they arise from an active process [16]. The mechanism responsible for their production has not yet been established, but several theoretical studies suggest they may arise from the spontaneous motion of actively oscillating coupled hair bundles, through a phenomenon known as frequency clustering [17,18].…”

Chaotic Dynamics Enhance the Sensitivity of Inner Ear Hair Cells

“…Spontaneous oscillations of the hair bundle hence provide a useful experimental probe for studying the underlying active mechanism in vitro 8 . While their role in vivo remains to be determined, the presence of spontaneous otoacoustic emissions 11,12 suggests that active innate oscillators may be present in the inner ears of intact animals. Several numerical studies have applied dynamical systems models to demonstrate that spontaneous motility could produce otoacoustic emissions 13,14 .…”