Intermodulation components in inner hair cell and organ of Corti responses

Two-tone responses are recorded from inner hair cells and from the organ of Corti fluid space in second and third turns of the guinea pig cochlea where best frequencies (BF) are approximately 4000 and 1000 Hz, respectively. This allows both ac and dc response components to be obtained and facilitates comparisons with psychophysical investigations that have traditionally been conducted at low and moderate frequencies. The measurements of ac responses in the organ of Corti fluid space also allow comparisons with mechanical results because the cochlear microphonic is proportional to basilar membrane displacement. By using a constant frequency ratio (f2/f1) of 1.4, local distortion products generated at the recording location are prominent when the two primaries are near the BF of the cell. However, when the primary pairs increase above BF, quadratic and cubic difference tones are recorded even when responses to the primaries are not measurable. The presence of these traveling distortion products is consistent with the idea that both f2-f1 and 2f1-f2 have their own traveling waves. Notches in the existence regions of quadratic and cubic difference tones were also observed and found to be influenced by mutual suppression between the two inputs.