## 4 Discussion  
In accordance with our hypotheses, the present results confirmed an effect of real-time CoP feedback on visuo-motor coupling during active postural tracking of a vertically moving visual target. The effect was stronger when tracking the more variable (i.e., chaotic) target motion. On the other hand, the presence/removal of feedback did not affect the dynamic stability of voluntary sway. * Feedback improved spatial accuracy at the cost of time delays. When real-time visual feedback of the CoP trajectory relative to the target motion was available during active target tracking, the CoP-target gain was close to one. This suggests that sway amplitude closely matched the target oscillation amplitude that was set to the individual stability boundaries in the antero-posterior direction (60% of foot length). We argue that real-time feedback regarding the instantaneous CoP position provided critical spatial information for actively controlling sway amplitude through an error correction process, which allowed performers to make corrective postural adjustments before horizontal CoP accelerations led to large deviations in postural sway. On the other hand, removing CoP feedback during tracking of the vertical target motion increased the CoP-target gain, suggesting target overshooting. Our results are concordant with those of a previous study showing that augmented CoP feedback improved the spatial accuracy of lateral weight shifting [7]. However, instead of translating the body toward a stationary target, in our protocol, the task goal required active and dynamic tracking of the moving target. Target overshooting in the absence of feedback in this case implies larger antero-posterior CoP acceleration and, consequently, increased difficulty and greater muscular effort in controlling the reversal in sway direction [28; 29]. In this case, the control of active sway relied on an internal representation of the body schema in space, which tended to overestimate the sensory consequences of the actual tracking task relative to the boundaries of the target's motion, resulting in erroneously larger sway movements [30]. Internal representations of the body schema are based on prior knowledge and appear to be central in the planning and control of goal-directed actions [31; 32]. The absence of error information in this case is related with exploratory behavior of the spatial task constraints and over-corrections [33; 34]. Greater reliance on prediction is also associated with greater perceptual bias [30], which may explain the overestimation of the target boundaries and the target overshooting observed in the absence of feedback. On the other hand, availability of feedback increased the phase lag between the target and the CoP motion when tracking the chaotic target, resulting in slower antero-posterior weight shifting. This finding contradicts previous studies showing that provision of visual feedback resulted in faster lateral weight shifting [10; 35]. The discrepancy could be due to the stationary nature of the target used in the previous weight shifting paradigms as opposed to the moving target used in the present study. When the target was stationary, information about the target's position was picked up prior to the onset of movement, which allowed for faster lateral weight transfer. However, in the case of a moving target, the target's position was continuously updated during task performance, while real-time feedback about the instantaneous CoP position imposed a greater need for online visuo-motor integration, resulting in time delays when making the appropriate postural adjustments. In this case, feedback may have acted as additional sensory noise that increased the time delay (phase lag) between the target and sway motion [36]. On the other hand, periodic target tracking did not increase the phase lag between postural sway and the target motion due to the predictable nature of the target motion.