In 2 experiments, the authors independently varied the degree of cognitive and perceptual difficulty of suprapostural tasks. Participants were 23 students in Experiment 1 and 15 in Experiment 2. Increases in perceptual difficulty tended to be correlated with decreases in the variability of postural sway, consistent with the hypothesized functional integration of postural control with suprapostural tasks. Sway variability was not influenced by changes in the cognitive difficulty of tasks when perceptual difficulty was held constant, contrary to predictions derived from the perspective that postural and suprapostural activities compete for a limited pool of central processing resources. The results underscore the need for researchers to differentiate between suprapostural tasks that require perceptual contact with the environment and those that do not.

We demonstrated that postural responses to imposed optic flow are to some extent voluntary. In a moving room, participants either stood normally or were instructed to resist any influence of visible motion on their stance. When participants attempted to resist, coupling of body sway with motion of the room was significantly greater than when the eyes were closed, but was significantly reduced relative to coupling in the normal stance condition. The results indicate that the use of imposed optic flow for postural control is not entirely automatic or involuntary. This conclusion motivates a search for non-perceptual factors that may influence the degree to which body sway is coupled to imposed optic flow.

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Adults who are blind stood in a room that could be moved around them. A sound source moved with the room, simulating the acoustic consequences of body sway. Body sway was greater when the room moved than when it was stationary, suggesting that sound may have been used to control stance.

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