Changes in the 2f1-f2 acoustic emission and whole-nerve response following sound exposure: long-term effects

We examined the effects of 1-min tonal exposures on the amplitude of the 2f1-f2 distortion product in the ear canals of cats. These effects were compared with the changes in the whole-nerve action potential (AP) responses to tone bursts following similar exposures. Both the distortion product and AP amplitudes were reduced following exposure. The time courses of AP and acoustic emission (AE) recoveries were to a certain extent similar, and the post-exposure effects depended in similar ways upon the frequency and intensity of the exposure tone (adapter). Assuming that the 2f1-f2 AE is generated by nonlinearities in the motion of the cochlear partition, the reduction in its amplitude suggests that 1-min sound exposures can alter cochlear mechanics. The analogous aspects of the AP and AE post-exposure effects further suggests that components of long-term adaptation as measured by the AP are associated with changes in the mechanics of transduction. Finally, alterations of the middle ear cavities of the cat by opening the bulla or bony septum can result in marked changes in the effectiveness of an adapter in reducing AE amplitude.