  Experimental records of active bundle motility are used to demonstrate the
presence of a low-dimensional chaotic attractor in hair cell dynamics.
Dimensionality tests from dynamic systems theory are applied to estimate the
number of independent variables sufficient for modeling the hair cell response.
Poincare maps are constructed to observe a quasiperiodic transition from chaos
to order with increasing amplitudes of mechanical forcing. The onset of this
transition is accompanied by a reduction of Kolmogorov entropy in the system
and an increase in mutual information between the stimulus and the hair bundle,
indicative of signal detection. A simple theoretical model is used 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.
