Two-Tone Distortion Products in the Basilar Membrane of the Chinchilla Cochlea

Abstract

It has been known since the early 18th. century that distortion products (DPs) exist in the auditory sensation evoked by pairs of tones [Jones, 1935]. Such DPs, also known as combination tones, can be heard at frequencies that are combinations of the primary frequencies, such as f₂-f₁, (n+1)f₁-nf₂ and (n+1)f₂-nf₁ , where f₁ and f₂ are the primary frequencies (f₂ > f₁, n = 1,2,3 ...). The most prominent and the best studied DPs are those with frequencies lower than those of the primary tones, particularly the “cubic difference tone” with frequency equal to 2f₁-f₂ [Zwicker, 1955; Goldstein, 1967; Smoorenburg, 1972], but DPs at frequencies greater than those of the primary tones (i.e., at 2f2-fJ and f2 + f ) have also been reported [Zurek and Sachs, 1979). Psychophysical studies on the frequency dependence of the 2fl -f2 distortion component suggested that this DP is generated in a mechanical nonlinearity located at the basilar membrane (BM) or in a structure closely coupled to it [Goldstein, 1967; Smoorenburg, 1972]. This was supported by recordings of cochlear microphonics [Gibian and Kim, 1982] and of responses of auditory-nerve fibers [Goldstein and Kiang, 1968; Buunen and Rhode, 1978; Kim et al., 1980; Siegel et al., 1982], cochlearnucleus neurons [Smoorenburg et al., 1976] and inner hair cells [Nuttall and Dolan, 1990), showing DPs at levels consistent with psychophysical data. Since the middle ear responds linearly to sound [Guinan and Peake, 1967; Ruggero et al., 1990] and since neural responses to DPs can be abolished by hair cell damage at cochlear sites preferentially tuned to the frequencies of the primary tones [Siegel ct al., 1982], it was concluded that DPs are generated at these sites and propagate mechanically along the cochlea to the basilar membrane location tuned to the DP frequency [Kim et al., 1980; Siegel et al., 1982).