  The hair bundle of sensory cells in the vertebrate ear provides an example of
a noisy oscillator close to a Hopf bifurcation. The analysis of the data from
both spontaneous and forced oscillations shows a strong violation of the
fluctuation-dissipation theorem, revealing the presence of an underlying active
process that keeps the system out of equilibrium. Nevertheless, we show that a
generalized fluctuation-dissipation theorem, valid for non-equilibrium steady
states, is fulfilled within the limits of our experimental accuracy and
computational approximations, when the adequate conjugate degrees of freedom
are chosen.
