The purpose of the present study was to verify our hypothesis that perception of the outer world determines sensations, eye movements and bodily movements. We did so by assessing the effects of Coriolis stimulation under a μG condition during parabolic flight. Using a suspended rotary chair, we subjected five young men to Coriolis forces under μG and 1-G conditions. Wearing a linear accelerometer on their chest, and goggles with a CCDcamera, which recorded eye movements, each subject was rotated clockwise at speeds of 50, 100, 150 deg/s under 1 G and at 100 deg/s under μG. Under the 1-G condition, the subject's body was displaced in response to inertial inputs provoked by Coriolis stimulation, i.e., momentarily accelerated leftwards while bending during clockwise rotations. Torsional nystagmus, moving sensation, and motion sickness were produced under 1 G. Under the μG condition, subjects shifted slightly to the right, but nystagmus was as pronounced as that under 1 G. Moving sensation and motion sickness were far less pronounced under μG than under 1 G. The results can be explained by our hypothesis. When gravity provides the Z-axis of the three-dimensional outer world, inertial inputs from the labyrinths displace the coordi-nates in the head, producing body sways, nystagmus and moving sensations. Because the Z-axis of the outer world is not ordained under μG, inertial inputs do not influence spatial orientation. Thus, neither body movements nor moving sensations are produced. However, because gaze is controlled by both head and eye movements, eye movements in the head are evoked directly by head movements, whether or not orientation in space is based on gravity. Space motion sickness is likely provoked by a failure in posture control depending on the outer world under 1 G.
