Fig. 4.

Sensitivity and spontaneous movements as a function of the hair bundle's operating point. (A) Maximal (black symbols) and minimal (purple symbols) sensitivity |χ| as a function of the maximal motor force f_max for P₀ = 0.1 (♦), 0.3 (▴), 0.4 (▪), 0.5 (•), 0.6 (□), 0.7 (▵), and 0.9 (⋄). The sensitivity displays a global maximum for f_max ≃ 439 pN and P_o = 0.5. Responses of maximal sensitivity were obtained in response to small sinusoidal stimuli at the characteristic frequency of spontaneous bundle oscillation, whereas the minimal sensitivity was observed for intense stimuli (see Fig. 3). The ratio of maximal and minimal sensitivities determines the gain of the amplificatory mechanism. (B) Spectral density of spontaneous movements along the line P_o = 0.5 for the different values of f_max indicated in red.