The effects of complex stapes motion on the response of the cochlea

The piston-like motion of the stapes footplate is the only effective stimulus to the cochlea, and rocking-like stapes motions have no effect on hearing. Studies of the vibration of the stapes in response to acoustic stimulation of the normal ear have revealed a complex movement pattern of its footplate. At low frequencies, the vibrations are predominantly piston-like, but they become increasingly rocking-like at middle and high frequencies. These complex vibrations can be decomposed into a translational, piston-like displacement and 2 rotational movements around the long and short axes of the stapes. The rotational components produce no net volume displacement of the cochlear fluid at some distance from the footplate. Therefore, according to the classic theory of hearing, the rotational motion is not transformed into cochlear activity and a hearing sensation. It was the goal of this study to test this hypothesis experimentally in guinea pigs. A piezoelectric 3-axis device was used to vibrate the stapes in various desired directions while simultaneously monitoring the actual motion of the stapes by a 3-dimensional laser Doppler interferometer and the cochlear activity by recording the compound action potential. The collected data of the presented study cannot be explained by the current theory of hearing. The qualitative results provide supportive evidence that complex movements of the stapes footplate may lead to cochlear activity. Further experiments are necessary to confirm and quantify these effects.