The spatial pattern of cochlear amplification
Sensorineural hearing loss, which stems primarily from the failure of mechanosensory hair cells, changes the traveling waves that transmit acoustic signals along the cochlea. However, the connection between cochlear mechanics and the amplificatory function of hair cells remains unclear. Using an optical technique that permits the targeted inactivation of prestin, a protein of outer hair cells that generates forces on the basilar membrane, we demonstrated that these forces interact locally with cochlear traveling waves to achieve enormous mechanical amplification. By perturbing amplification in narrow segments of the basilar membrane, we further showed that a cochlear traveling wave accumulates gain as it approaches its peak. Analysis of these results indicates that cochlear amplification produces negative damping that counters the viscous drag impeding traveling waves; targeted photoinactivation locally interrupts this compensation. These results reveal the locus of amplification in cochlear traveling waves and connect the characteristics of normal hearing to molecular forces.
The upper panel shows a traveling wave on the basilar membrane of a normal chinchilla during sinuoidal stimulation at 9.4 kHz and 70 dB SPL. The wave propagates from the cochlear base (to the left of the image) toward the apex (to the right). The lower panel shows the diminished response under identical conditions after the ear has been rendered anoxic and the active process has been arrested.