Single cochlear nerve fiber recordings from unexposed chinchillas show spatial distributions of amplitude and phase of the distortion products f2 - f1 and 2f1 - f2 similar to those previously reported for the cat (35, 37, 42). Damaging the organ of Corti in the region corresponding to the frequencies of a two-tone stimulus substantially reduces the amplitude of these distortion products at their characteristic places. The distortion products f2 - f1 and 2f1 - f2 thus appear to be generated in the organ of Corti in the region of the primary-frequency places. The neural responses suggest that the distortion products are propagated in the motion of the cochlear partition like externally applied stimulus tones at the distortion frequencies with a similar spatial distribution of distortion product amplitude and phase. Models of the cochlear that assume nonlinear cochlear-partition dynamics can account for the similarity by demonstrating that distortion products generated by cochlear-partition nonlinearity can propagate apicalward in the motion of the cochlear partition. Models of the cochlea using a linear-system model for cochlear partition motion, in cascade with a nonlinear transduction stage and a subsequent sharp filter, are inadequate to account for present observations, unless two currently implausible assumptions are made: 1) stimulus tones near 4 kHz must be propagate in normal cochleas at least as far apically as the 300-Hz place with sufficient amplitude to generate f2 - f1 there, and b) damage to the organ of Corti must interfere with this propagation of 4 kHz stimulus tones to the 300-Hz place. Distortion generation in the cochlea is sensitive to delicate alterations of the organ of Corti. Short, moderate-intensity exposures to sound can reversibly reduce the amplitude of the distortion products f2 - f1 and 2f1 - f2 seen in responses from cochlear nerve fibers with characteristic frequencies (CF) near the distortion frequencies. Since such exposures do not produce permanent structural changes visible under light microscopy, it seems most reasonable to believe that subtle changes in the organ of Corti (most likely in the hair cells themselves) in the region most responsive to f1 and f2 reduct the generation of mechanically present distortion products.
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