Effects of electrical biasing on electrically-evoked otoacoustic emissions

Electrically-evoked otoacoustic emissions were produced using a 10 microA, 750 Hz AC current plus a biasing DC current in the range of +/- 10 microA. Concurrently, a 1643 Hz tonal stimulation was delivered to the eardrum. At low sound levels, negative DC current increased the emission while positive DC current reduced the emission. Such findings are reasonably explained by a negative-feedback model of cochlear function. At high sound levels, negative DC current reduces the emission, while positive current has little effect. These data can be accounted for by voltage-dependent length changes shown to occur in isolated outer hair cells, with the additional requirement that voltage-dependent K+ channels in outer hair cells reduce the effectiveness of positive DC current in changing membrane potential.