The location of the cochlear amplifier: Spatial representation of a single tone on the guinea pig basilar membrane

Acoustic stimulation vibrates the cochlear basilar membrane, initiating a wave of displacement that travels toward the apex and reaches a peak over a restricted region according to the stimulus frequency. In this characteristic frequency region, a tone at the characteristic frequency maximally excites the sensory hair cells of the organ of Corti, which transduce it into electrical signals to produce maximum activity in the auditory nerve. Saturating, nonlinear, feedback from the motile outer hair cells is thought to provide electromechanical amplification of the travelling wave. However, neither the location nor the extent of the source of amplification, in relation to the characteristic frequency, are known. We have used a laser- diode interferometer to measure in vivo the distribution along the basilar membrane of nonlinear, saturating vibrations to 15 kHz tones. We estimate that the site of amplification for the 15 kHz region is restricted to a 1.25 mm length of basilar membrane centered on the 15 kHz place.