Effect of modulation of basilar membrane position on the cochlear microphonic.

Certain characteristics of the extracellular cochlear microphonic (CM) recorded by intracochlear electrodes change in a bimodal manner as a function of prior acoustic exposure, intensity of stimulation, or stimulus frequency. In the present study, it is shown that biasing the basilar membrane position toward scala tympani serves to enhance the CM amplitude when the cochlea is unfatigued, when low-intensity stimuli are used, or when frequencies below the best frequency of a differential electrode pair are used. Conversely, after acoustic fatigue, or during high-intensity or high-frequency stimulation, the microphonic potential is enhanced by a movement of the basilar membrane toward the scala vestibuli. The two populations of hair cells, whose responses are enhanced and diminished on opposing positions of the basilar membrane, are probably outer and inner hair cells.