THE EFFECTS OF COMPLEX STAPES MOTION ON THE RESPONSE OF THE COCHLEA IN GUINEA PIGS

Studies into the vibration modes of the stapes in response to acoustic stimulation of the normal ear have revealed a complex movement pattern of its footplate. These complex vibrations can be expressed as one translational displacement and two rotational movements around the long and short axes of the stapes, known as the three elementary motions. According to the classical theory of hearing, the rotational motions induce no volume displacement of cochlear fluid and, therefore, no cochlear activity (i.e. hearing sensation). It is the goal of this study to verify this hypothesis. A custom-built, three-axis piezoelectric actuator, capable of eliciting any desired vibration mode, was coupled to the surgically prepared stapes superstructure of anesthetized guinea pigs. When producing different movement patterns, electrophysiological measurements of the cochlear potentials were simultaneously recorded. Mechanical stimulation of the stapes according to the three elementary motions delivered three cochlear potentials of different amplitude. The greatest potential resulted from translational motions. The results of the present study show a cochlear excitation in all performed movement patterns of the stapes. Hence, the prevailing hypothesis could not be verified.