Chaotic Dynamics of Inner Ear Hair Cells

Hair cells of the auditory and vestibular systems are capable of detecting sounds that induce sub-nanometer vibrations of the hair bundle, far below the stochastic noise levels of the surrounding fluid. Hair cells of certain species are also known to oscillate without external stimulation. The role of these spontaneous oscillations is not yet understood, but they are believed to be a manifestation of an underlying active mechanism. A deeper understanding of spontaneous motility could impact our understanding of the extreme sensitivity of hearing. Our experiments suggest that chaos exists in the dynamical system of the active hair cell. We observe a transition from chaos to order as increasingly stronger stimulus is applied to the hair bundle. This transition is accompanied by an increase in information transmission from the stimulus to the hair bundle, indicative of signal detection. Further, we use a simple theoretical model to describe the observed chaotic dynamics. The model exhibits an enhancement of sensitivity to weak stimuli when the system is poised in the chaotic regime. We propose that chaos may play a role in the hair cell's ability to detect low-amplitude sounds.