How hair bundles respond
(a) When a frog hair bundle is shaken using a microneedle, its response (red) is characteristic of a noisy Hopf oscillator. The applied force, which is related to the amplitude of the displacement of the needle (blue), progressively increases down the figure. When the bundle is shaken gently, the Hopf oscillator's gain - its response divided by the input stimulus - is large.
(b) The Fourier transform of the bundle displacement has a peak at the stimulus frequency. The height of this peak grows as the cube root of the applied force, indicating that the gain increases as the force decreases (data courtesy of Pascal Martin and Jim Hudspeth).