Influence of action and expectation on visual control of posture

Previous studies have shown that human subjects presented with a moving vis ual environment initiate a postural re-adjustment in the direction of motio n. The present study investigated how active control or expectation of the displacement of a visual scene affects this postural response. Center of fo ot pressure (COP) and head displacement were recorded using a sway platform and a tracking system, respectively. The subjects faced a visual scene (1x 1 m, at a distance of 45 cm) which moved transiently (with a velocity of 1 cm/s) in a direction parallel to the interaural axis. When the displacement of the visual scene was under the active control of the subjects, visually induced body sway was strongly inhibited, in comparison with the response to unexpected stimuli. Prior knowledge of the characteristics of the forthc oming displacement was sufficient, in most subjects, to reduce postural re- adjustment, even when subjects did not exert active control. Finally, the v isually induced postural response was strongly reduced even when subjects o nly triggered the stimulus, without any knowledge about the direction of mo tion. In conclusion, it appears that although vision is of primary importan ce in the control of postural orientation, high level processes such as exp ectation can modulate its impact by providing cues as to whether forthcomin g visual flow is the consequence of self-motion or object-motion. (C) 2001 Elsevier Science B.V. All rights reserved.