## 4 Discussion
Once a particular visuo-motor transformation was learned after a couple of sway cycles, the participant was able to predict the target's motion, thereby reducing the load of online visual information processing during active target tracking [16]. Overall, our results suggest a trade-off in the control of the spatial and temporal properties of sway when tracking a vertically moving target which was modulated by the availability of visual CoP feedback. Feedback about the spatial error between the target and the CoP motion induced a prioritization of spatial over temporal coupling [37], while the latter decreased due to the speed accuracy trade-off [38]. * Feedback did not affect the dynamic stability of voluntary sway. Removal of feedback did not affect the local dynamic stability of sway. This suggests that the additional visuo-motor processing load needed for matching the feedback (yellow dot) to the target signal (red dot) in the feedback condition did not challenge the overall dynamic stability of the voluntary sway task any more than the target tracking alone with no feedback. Along the same line of evidence, in an optic flow stimulation study, participants experienced less postural instability during the actual saccadic tracking of the directional stimuli compared to the preceding perceptual phase of the heading direction [39]. This suggests that when the visual motion of the target was congruent with the eye and/or body motion, the postural stability was not challenged. On the other hand, provision of real-time visual feedback during gait decreased the local dynamic stability and induced instability of the trunk and the pelvis in the frontal plane [15]. Several reasons could explain this disagreement. Actively tracking a moving target by shifting the body weight in the sagittal plane is a less automatic and slower task compared to gait. Provision of augmented feedback during gait could therefore lead to a disruption of automaticity and impose greater demand for visuo-motor processing. Weight-shifting, on the other hand, is a less automatized task that requires higher precision control, particularly when reaching close to the stability limits. The plane of motion could also be an important task constraint determining the impact of dynamic visual feedback cues on dynamic stability [40]. In the gait study, feedback was represented by a horizontal visual motion to inform about the instantaneous position of the pelvis and the trunk in the frontal plane, whereas in our paradigm, CoP feedback, provided by a vertically moving dot, did not seem to affect the stability of sway in the sagittal plane. It is known that non-gravity (horizontal)-related dynamic visual cues induce greater instability when compared to gravity-relevant visual cues [40]
* The contribution of feedback increased when tracking the less predictable target. Tracking of the chaotic target resulted in lower coherence, greater target overshooting, and a longer phase lag between the target and the CoP motion when compared to periodic target tracking. These results confirm previous findings from our laboratory showing that voluntary, sagittal plane sway synchronizes better with predictable (i.e., periodic) target motion than less predictable (i.e., chaotic) target motion when actively tracking a vertically moving target [12; 16]. It could be suggested that postural tracking of the chaotic target imposed more extensive visuo-motor processing due to the less predictable nature of the target's motion, which may have kept participants more actively engaged in the tracking task due to the continuous need to attend to the target motion. Tracking of the sinusoidal target motion, on the other hand, gradually decreased the need to attend to the target, as participants were able to predict it and adjust their sway, employing an open-loop type of control [32]. This was confirmed by the consistency of the target-CoP phase lag across the two feedback conditions when tracking the sinusoidal target motion.