Two-tone suppression of inner hair cell and basilar membrane responses in the guinea pig

Recordings of receptor potentials from inner hair cells (IHCs) and the basilar membrane (BM) motion were made in pigmented guinea pigs. The acoustic stimuli were single tones near best frequency (BF) and two-tone complexes. Single tone input/output (I/O) functions had a saturating growth for the magnitude and their phase shifts were strongly dependent on the tone frequency relative to BF. For IHCs, a BF tone stimulus produced no phase shift in the ac receptor potential response. Phase lag or lead occurred for tones below or above BF, respectively. BM velocity I/O functions were not as compressively saturating as IHC ac I/O curves. BM phase shifts (in relation to BF) were similar to those of the IHCs. Two-tone suppression was observed in both IHC and BM response measures. Suppressor tones on the low-frequency side of BF produced complex suppression results, which were inconsistent with a simple attenuation model for suppression. The growth of suppression was faster than the attenuation from equivalent level reductions of the probe tone, and phase shifts were phase lead. Depending upon experimental conditions, phase change with suppression may be in the opposite direction from phase change observed from pure attenuation of the probe tone. High-frequency suppressors (relative to BF) are consistent with an attenuation model of suppression for the IHCs of the current study. High side suppression of basilar membrane velocity, however, differed from the IHCs in a systematic way. The phase change caused by suppression of BM velocity was always smaller than that of an equivalent reduction in the probe tone level.