Abstract
Vection induced by optic flow simulating body rotation might be affected by the rotation axes relative to their observer. Moreover, the observer?s posture might influence the vection according to whether the rotation axis coincides with the gravity direction. We measured the strength of vection and eye movements under nine combination conditions of the visual rotation axes (pitch, yaw, roll) and posture (upright, supine, and left-side-down) to investigate the hypothesis. The stimulus period in each trial was two minutes. The vection strength was measured by subjective evaluation using an 11-point scale after each trial and real-time evaluation using a linear potentiometer during stimulus observation. Four trials were carried out for each observer for each condition. For roll rotation, vection was significantly weaker and shorter in the supine posture than in other postures. For pitch rotation, vection was significantly weaker in the left-side-down posture than in other postures. These results indicate that vection is ineffective when the body rotation axis simulated by optic flow coincides with the gravity direction.