## 2 Materials and Methods
### Apparatus, Stimuli, and Task  
Postural performance was recorded using a force platform (60 x 90 cm, Kistler, 1000 Hz, Winterthur, Switzerland). Visual stimuli were displayed on a large TV Screen (47 inch, HD LG), located 1.5 m in front of the participant at eye level (Figure 1A).  
The target's motion was constructed in MATLAB (version R2014b, Math Works Inc, Natick, MA, USA) using two signals of different degrees of predictability. The _predictable signal_ was a sinewave with a single frequency (f) set at 0.25 Hz that was generated using the \(\sin\) function [\(\mathit{sine}\ =\ \sin(2 pi\times\mathrm{f}\times\mathrm{f})\)]. This particular frequency was selected because it was the dominant frequency of intuitive, self-paced voluntary sway based on prior studies [12; 16] and pilot testing. The _less predictable signal_ was derived from a Lorenz attractor according to the parameters: \(\sigma\) = 10, \(\beta\) = 8/3, and r = 28 and the initial conditions: x0 = 0.1, y0 = 0.1, and z0 = 0.1. The signal characteristics were h (time resolution) = 0.0040, steps (number of points) = 10,000 (we choose 6000 data points from the y-axis [y (4000: 10,000)], and noise flag = 0 [17; 18]. The frequency range for the Lorenz signal was confined between 0 and 1 Hz, which was an ecologically valid spectrum of frequencies for voluntary sway [19], and its power spectrum revealed a dominant frequency around 0.25 Hz.  
Prior to the experiment, participants were asked to step on the platform and adopt the testing position with their arms freely hanging by their sides. The distance between the internal malleoli was set at 10% of body height. The antero-posterior component of the base of support (foot length) was measured using the distance of the most anterior point (toe) to the calcaneus in order to normalize the amplitude of target motion. The maximum (peak to peak) amplitude of target motion was set to 60% of the foot length (Figure 1B).  
During the experiment, participants tracked the motion of the visual target, which moved vertically on the screen, with their bodies by shifting their CoP in the antero-posterior direction. Two different-colored dots were shown on the screen, specifically, a red dot, which illustrated the target to be followed, and a yellow dot, which depicted the participant's antero-posterior CoP component serving as the instantaneous performance feedback signal. The only instruction given to participants was to follow the motion of the red dot by controlling the motion of the yellow one, which was achieved by shifting their weight antero-posteriorly. The two stimuli (target and feedback) were synchronously represented on the TV monitor using custom build software (developed in MATLAB) while their position was updated at a rate of 50 Hz, providing 120 s of continuous target stimulus motion and resulting in 6000 data points for each signal. Participants performed one 120 s long tracking trial in each of the following conditions: (a) Tracking of the sinusoidal target with (yellow dot was visible) and without (yellow dot was not visible) visual feedback, and (b) tracking of the chaotic (Lorenz) target with and without feedback. Task conditions were randomized to avoid possible order effects. Participants were provided ample time to familiarize themselves with the task prior to actual testing. Prior to each trial, the room lights were dimmed. Between each trial, participants rested for about 2 min. The experimental session, including subject preparation lasted no longer than 60 min.