Two-tone distortion in the basilar membrane of the cochlea

WHEN humans listen to pairs of thnes they hear additional tones, or distortion products, that are not present in the stimulus¹. Two-tone distortion products are also known as combination tones, because their pitches match combinations of the primary frequencies (f₁ and f₂, f₂>f₁), such as f₂-f_l, (n+1)f₁ - nf₂ and (n +1)f₂-nf₁ (n = 1, 2,3 ...) (refs 2-4). Physiological correlates of the perceived distortion products exist in responses of auditory-nerve fibres^5-8 and inner hair cells⁹ and in otoacoustic emissions (sounds generated by the cochlea, recordable at the ear canal)^7,10-12. Because the middle ear responds linearly to sound^13,14 and neural responses to distortion products can be abolished by damage to hair cells at cochlear sites preferentially tuned to the frequencies of the primary tones⁸, it was hypothesized that distortion products are generated at these sites and propagate mechanically along the basilar membrane to the location tuned to the distortion-product frequency^7,8. But until now, efforts to confirm this hypothesis have failed^15,16. Here we report the use of a new laser-velocimetry technique¹⁷ to demonstrate two-tone distortion in basilar-membrane motion at low and moderate stimulus intensities.