We investigated the effect of visual information on an observer's body control. We measured observers' body sway and perceived magnitude of motion in depth while observing a stimulus with changing disparity and/or changing size. The stimulus variable was the size of the moving area. For the stimuli with changing disparity, local peaks in the power spectrum for the body sway data were clearly observed at the frequencies of the image motion when the size of the moving area was large. For the stimuli with changing size, no local peaks were not found. Psychophysical results showed that the magnitude of perceived depth for the stimulus with changing disparity decreased as the size of the moving area increased. However, changing size produced a strong sensation of motion in depth for the stimuli irrespective of the size of the moving area. These results indicate that changing disparity in a large field contributes to body posture control and supports the idea that visual perception and motor control are mediated by different pathways.
