■ OBJECTIVES: The motion of the semicircular canal cupula had been observed under mechanical endolymphatic flow or compression to the canal wall. In this study, the cupula motion was analyzed quantitatively by applying sinusoidal rotatory stimuli which were within the physiological range.

■ METHODS: The frog posterior semicircular canal (PSC) was carefully dissected in Ringer’s solution. The PSC was mounted on the turn table. India ink was gently injected into the canal in order to delineate the cupula contour. The turn table was sinusoidally rotated. The amount of the cupula deflection was measured during sinusoidal stimuli. Different frequencies, 0.4Hz and 0.7Hz were used. We performed the same experiments on whole membranous labyrinths.

■ RESULTS: The cupula motion appeared to be “diaphragma” when the stimuli were weak. The motion became “swing door” with the base of the cupula hinged on the crista when the stimuli were greater.

■ CONCLUSION: It was suggested that the cupula displacement was “diaphragma” under normal head movements. The cupula starts to move as “swing door” with increase of the stimulus.