Nature 349, 413 - 414 (31 January 1991); doi:10.1038/349413a0

Two-tone distortion in the basilar membrane of the cochlea

Luis Robles^*, Mario A. Ruggero & Nola C. Rich

Department of Otolaryngology, University of Minnesota, 2630 University Avenue S.E., Minneapolis, Minnesota 55414, USA
^* Permanent address: Depto. de Fisiología y Biofísica, Fac. de Medicina, Universidad de Chile, Santiago, Chile.

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 (f ₁ and f ₂, f ₂>f ₁), such as f ₂–f _l, (n+1)f ₁ – nf ₂ and (n +1)f ₂-nf ₁ (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.

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