Like our other senses, the auditory system can produce illusions. Prominent among these are distortion products1-5: when listening to two tones, one of frequency f1 and the second of a higher frequency f2, an individual may hear not only these primary tones, but also a difference tone of frequency f2-f1, a sum tone of frequency f2+f1, and combination tones of frequencies such as 2f1-f2 and 2f2-f1. Discovered by Tartini early in the eighteenth century6,7, these illusory sounds are sufficiently conspicuous that they were employed to carry melodies in classical compositions. Distortion products originate within the cochlea, where they manifest themselves in the basilar membrane's vibration8. Here we demonstrate distortion products in individual hair cells of the bullfrog's sacculus, where they emerge from a nonlinearity inherent in the mechanoelectrical transduction process. In addition to offering an explanation for cochlear distortion products, our results suggest that the mechanical properties of hair bundles significantly influence the basilar membrane's motion.
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