Physical basis of two-tone interference in hearing

The cochlea uses active amplification to capture faint sounds. It has been proposed that the amplifier comprises a set of self-tuned critical oscillat ors: each hair cell contains a force-generating dynamical system that is ma intained at the threshold of an oscillatory instability, or Hopf bifurcatio n. While the active response to a pure tone provides frequency selectivity, exquisite sensitivity, and wide dynamic range, its intrinsic nonlinearity causes tones of different frequency to interfere with one another in the co chlea. Here we determine the response to two tones, which provides a framew ork for understanding how the ear processes the more complex sounds of spee ch and music. Our calculations of two-tone suppression and the spectrum of distortion products generated by a critical oscillator accord with experime ntal observations of basilar membrane motion and the nervous response. We d iscuss how the response of a set of self-tuned oscillators, covering a rang e of characteristic frequencies, represents the structure of a complex soun d. The frequency components of the stimulus can be inferred from the timing of neural spikes elicited by the vibrating hair cells. Passive prefilterin g by the basilar membrane improves pitch discrimination by reducing interfe rence between tones. Our analysis provides a general framework for examinin g the relation between the physical nature of the peripheral detection appa ratus and psychophysical phenomena such as the sensation of dissonance and auditory illusions.