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-fl, (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 fibres5-8 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 sound13,14 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 frequency7,8. But until now, efforts to confirm this hypothesis have failed15,16. 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.
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