## 1 Introduction  
An integral component of balance rehabilitation exercises is real time visual feedback regarding the instantaneous center of pressure (CoP) or center of mass (CoM) trajectory in relation to a stationary or moving visual target position. Although this visualization increases the role of visual information in the multisensory integration process of controlling posture, its role in standing and dynamic balance control is not well understood. Experimental evidence on the role of visual feedback in controlling standing balance when the goal of the task is to minimize sway around a reference position is conflicting. On one hand, real time CoP or CoM feedback decreased sway amplitude [1] and the instability induced by proprioceptive, somatosensory, and vestibular perturbations [2]. On the other hand, augmented feedback of the two-dimensional CoP or CoM position did not improve postural stability of a two-legged quiet standing posture [3], whereas, when standing participants were exposed to a complex visual room oscillation, provision of CoP feedback further destabilized standing sway [4]. Furthermore, the spatial CoP variability of visually controlled whole-body leaning did not decrease when performance feedback was provided in addition to only target information [5]. A possible explanation could be the fact that visual feedback operates at a relatively low frequency scale and therefore it is not appropriate for correcting the high-frequency, small-amplitude CoP oscillations of standing sway [6]. On the other hand, feedback may have a more prominent impact on dynamic equilibrium control which involves greater, longer, and lower frequency CoP fluctuations (drifts) over the base of support. Provision of augmented visual feedback during performance of dynamic balancing tasks revealed more systematic effects. Removing visual CoP feedback during lateral weight shifting decreased the speed and spatial accuracy of performance [7]. However, the same removal did not have an impact on performance when participants swayed voluntary while trying to produce different visually imposed ankle-hip coordination patterns represented in a complex Lissajous figure [8]. This suggests that more simple and direct forms of visual feedback are needed in order to improve performance. Indeed, the provision of two-dimensional instead of one-dimensional information resulted in faster lateral weight-shifting [9], while augmentation of the error feedback signal drove subjects to their steady-state performance faster than unaltered visual feedback [10]. Horizontally biased visual feedback induced horizontal compensatory postural adjustments, which increased CoP asymmetry even in quiet standing after practice [11]. This evidence suggests that augmented, real-time visual feedback regarding sway improves spatial accuracy and speed of performance when the goal of the task is to transfer the body toward a stationary visual target, such as in lateral weight shifting. The role of visual feedback when actively tracking a moving visual target with the whole body is less studied. Work from our laboratory suggests that humans, regardless of their age, can couple their voluntary sway to a moving visual target regardless of whether this oscillates in a stereotypical (i.e., periodically) or in a more complex (i.e., chaotically) fashion [12]. However, the role of CoP feedback in visuo-postural entrainment and its dependence on target predictability is still not known. The extent to which the body's stability is challenged by the availability of feedback and target predictability during active target tracking is another unresolved issue. Local dynamic stability represents the ability of a system to maintain its movement pattern despite intrinsic and extrinsic perturbations [13; 14].