Spatial Patterns of Cochlear Difference Tones

First-order intermodulation components in cochlear-microphonic potentials were measured with the differential electrode technique from all four turns of the guinea pig cochlea. Measurements were made with six pairs of primary frequencies and a wide range of primary signal intensities. The spatial patterns of the first-order difference tones were compared with those of the primaries, and with pure tones whose frequency was the same as that of the difference tone. The results indicated that at low and moderate primary intensity levels the distortion component was localized in the cochlea somewhat apical from the region of maximum excitation by the higher-frequency primary. With increasing stimulus intensity, a general shift of the distribution pattern was observed, accompanied by the development of a second region of maximal difference-tone activity. This second region, where the difference tone became more prominent as the intensity was increased, corresponded to the location of maximal microphonic elicited by a pure tone whose frequency was the same as that of the difference tone. These results further confirm that distortion of the cochlear microphonic is a two-stage process.