Postural costs of suprapostural task load.

In an immersive visualization experiment, participants performed a conjunction search task while standing either in open (heels 10 cm apart, feet at a comfortable angle) or closed stance (feet pressed together). In the world-frame condition, the search display maintained its position in space as the participant swayed, generating optic flow informative about sway. In the head-frame condition, the display maintained constant distance and orientation with respect to the participant's head, providing no visual information about sway. In both conditions, participants (surprisingly) searched faster while in the more difficult closed stance. Interpretation of this result is unclear. Participants also swayed more as search-load increased, and made more errors in the high search-load condition. It is suggested that this performance tradeoff is a result of the sharing of a limited-capacity, modality-non-specific spatial-attentional resource between postural and suprapostural tasks.