No abstract

The authors studied relations between postural sway, optical flow, and constraints on posture imposed by a suprapostural looking task. Optical flow resulted from unperturbed sway and was not imposed by the experimenters. Participants fixated a distant target or a nearby target. In the key condition, participants looked past (i.e., ignored) a nearby target to fixate the distant target. The authors recorded the variability of head position as a measure of the amplitude of postural sway. In 5 of 7 experiments, sway variability was influenced by the location of the fixated target not by the distance of the nearest visible surface (the unfixated nearby target). Postural sway was modulated to facilitate the performance of suprapostural tasks and was not driven by optical flow in an autonomous (task-independent) manner. The authors concluded that posture can be understood only in the context of explicit manipulations of suprapostural tasks.Many studies have demonstrated that optical flow can influence dynamic spatial orientation (e.g., Andersen &

The coordination of multiple body segments (torso and legs) in the control of standing posture during a suprapostural task was studied. The analysis was motivated by dynamical theories of motor coordination. In 2 experiments it was found that multisegment postural coordination could be described by the relative phase of rotations around the hip and ankle joints. The effective length of the feet, the height of the center of mass, and the amplitude of head motions in a visual tracking task were varied. Across these variations, 2 modes of hip-ankle coordination were observed: in-phase and anti-phase. The emergence of these modes was influenced by constraints imposed by the suprapostural tracking task, supporting the idea that such tasks influence postural control in an adaptive manner. Results are interpreted in terms of a dynamical approach to coordination in which postural coordination modes can be viewed as emergent phenomena.