Non-linear response to amplitude-modulated waves in the apical turn of the guinea pig cochlea.

Mechanical vibrations of the Hensen's cells were measured in the apical turn of the cochlea in living guinea pigs, in response to amplitude-modulated (AM) sound. The FFT of the input wave consisted of spectral components at the carrier frequency C and two sidebands (C+/-M) separated from the carrier by the modulation frequency M. The FFT of the velocity response consisted of components at: (i) the modulation frequency M, and harmonics n M; (ii) Carrier frequency C and sidebands (C+/-n M); (iii) harmonics of the carrier frequency and their side bands (2C+/-n M); (3C+/-n M); (4C+/-n M); em leader n=1,2,3, em leader,10. The carrier and the first pair of side bands were broadly tuned and nearly linear. Other components were sharply tuned and highly non-linear, suggesting a different origin. Evidence is presented that these components are generated in the non-linear stereocilia dynamics. An important function of this non-linearity is to demodulate the AM wave to extract information contained in the modulation.