Comparison of a hair bundle's spontaneous oscillations with its response to mechanical stimulation reveals the underlying active process

Hearing relies on active filtering to achieve exquisite sensitivity and sha rp frequency selectivity. In a quiet environment, the ears of many vertebra tes become unstable and emit one to several tones. These spontaneous otoaco ustic emissions, the most striking manifestation of the inner ear's active process, must result from self-sustained mechanical oscillations of aural c onstituents. The mechanoreceptive hair bundles of hair cells in the bullfro g's sacculus have the ability to amplify mechanical stimuli and oscillate s pontaneously. By comparing a hair bundle's spontaneous oscillations with it s response to small mechanical stimuli, we demonstrate a breakdown in a gen eral principle of equilibrium thermodynamics, the fluctuation-dissipation t heorem. We thus confirm that a hair bundle's spontaneous movements are prod uced by energy-consuming elements within the hair cell. To characterize the dynamical behavior of the active process, we introduce an effective temper ature that, for each frequency component, quantifies a hair bundle's deviat ion from thermal equilibrium. The effective temperature diverges near the b undle's frequency of spontaneous oscillation. This behavior, which is not g eneric for active oscillators, can be accommodated by a simple model that c haracterizes quantitatively the fluctuations of the spontaneous movements a s well as the hair bundle's linear response function.