Description
Human hearing is enhanced by an active process that amplifies the ear's mechanical inputs, sharpens frequency discrimination, and compresses six orders of magnitude in the amplitude of sounds into only two orders of magnitude in output. In addition, spontaneous otoacoustic emissions emerge from ears in a very quiet environment, an indication that the active process can be so exuberant as to become unstable. Cooperativity between mechanoelectrical-transduction channels confers negative stiffness on the hair bundle, eliciting a dynamical instability that underlies the active process. Experiments on individual hair bundles have shown that the bundle's operation near this instability, the Hopf bifurcation, accounts for the four characteristics of the active process.