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 stimulus. Two-tone distortion products are also known as combination tones, because their pitches match combinations of the primary frequencies (f1 and f2, f2 greater than f1), such as f2-f1, (n + 1)f1-nf2 and (n + 1)f2-nf1 (n = 1, 2, 3...). Physiological correlates of the perceived distortion products exist in responses of auditory-nerve fibres and inner hair cells and in otoacoustic emissions (sounds generated by the cochlea, recordable at the ear canal). Because the middle ear responds linearly to sound 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 tones, 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. But until now, efforts to confirm this hypothesis have failed. Here we report the use of a new laser-velocimetry technique to demonstrate two-tone distortion in basilar-membrane motion at low and moderate stimulus intensities.