The Effects of Complex Stapes Motion on the Response of the Cochlea

Alexander M. Huber, Damien Sequeira, Christian Breuninger, and Albrecht Eiber

Otol Neurotol. 2008 Dec;29(8):1187-92

Department of Otorhinolaryngology, Head and Neck Surgery, University Hospital of Zurich, Zurich, Switzerland (1-2); and Institute of Engineering and Computational Mechanics, University of Stuttgart, Stuttgart, Germany (3-4)


HYPOTHESIS: 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.
BACKGROUND: 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.
METHODS: 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.
RESULTS: The collected data of the presented study cannot be explained by the current theory of hearing.
CONCLUSION: 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. (Bold text emphasis by Martin Braun)


It has been established for several years that the stapes moves both piston-like and rocking-like, the first type being dominant at low frequencies and the second type being more important at middle and high frequencies.

Interestingly, only the piston mode can produce a pressure difference between scala vestibuli and scala tympany and thus generate a traveling wave along the basilar membrane. The rocking mode only shifts fluid locally from one side of the oval window to the other, without influencing the pressure in the scala vestibuli. This mode thus cannot generate a traveling wave. However, it can generate a compressional sound wave in the scala vestibuli that can reach the hair cells directly.

Until now it was unknown if the rocking mode has any influence on the excitation of hair cells. Huber et al. (2008) have now answered this question by a series of well-designed experiments. They succeeded in sufficiently separating the two modes, and they observed that the rocking mode indeed strongly excites the hair cells.

In conclusion, this study adds an important further piece to the huge bulk of evidence (link A, link B) that a cochlear traveling wave is no necessary element in hair cell excitation and that this excitation is most probably caused directly by compressional sound waves in the cochlear fluids. (Comment Martin Braun)

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