Résumé
In several experiments, participants have been asked to move their head intentionally so as to track the motion of a visual target. The motion of different body segments underlying the movement of the head has been analyzed by focusing on the relative phase between rotations around the ankles and hips. Interestingly, transitions from one postural mode to the other, evoked by a gradual change in target frequency, exhibited typical signatures of self-organized systems: differential stability, critical fluctuations, phase transitions, critical slowing and hysteresis. For the looking and the tracking tasks, the in-phase and the antiphase postural coordination modes were adopted for low and high frequencies of the moving room, respectively. The coordination dynamics underlying the maintenance of upright stance appears similar regardless of the intention to sway, therefore challenging the distinction between quiet stance and sway.