Separate mechanical processes underlie fast and slow effects of medial olivocochlear efferent activity.

Sound-evoked vibrations of the basilar membrane (BM) in anaesthetised guinea-pigs are shown to be affected over two distinct time scales by electrical stimulation of the medial olivocochlear efferent system: one is fast (10-100 ms), the other much slower (10-100 s). For low and moderate level tones near the BM's characteristic frequency, both fast and slow effects inhibited BM motion. However, fast inhibition was accompanied by phase leads, while slow inhibition was accompanied by phase lags. These findings are consistent with a hypothesis that both fast and slow effects decrease sound amplification in the cochlea. However, the opposing directions of the phase changes indicate that separate mechanical processes must underlie fast and slow effects. One plausible interpretation of these findings is that efferent slow effects are caused by outer-hair-cell stiffness decreases, while efferent fast effects are caused by reductions in 'negative damping'.