The effects of efferent activation on the acoustically and electrically evoked otoacoustic emission

The effects of efferent activation on the otoacoustic emission were measured in anesthetized guinea pigs. The otoacoustic emission (2F(1)-F(2)) was evoked by the conventional method of presenting either two continuous tones or a sinusoidal current to the round window (RW) of the cochlea. The efferent effects on the acoustically evoked emission are greatest at low stimulus levels and least for high levels. The efferent effects on the electrically evoked emission (EEOAE) are relatively constant across current levels. In each case, efferent activation resulted in an initial large reduction in the emission amplitude followed by a smaller and more constant reduction. Strychnine eliminated the efferent effects independent of the method of emission activation. Strychnine had no effect on the EEOAE, suggesting that the RW current did not evoke a local efferent effect. Slow versus fast efferent effects were observed in the recovery of the emission amplitude at the termination of efferent activation. Only a fast recovery in the emission amplitude was observed for stimuli below 10 kHz while the amplitude recovery had fast and slow components for stimuli presented above 10 kHz.