## 2 Materials and Methods
### Task and Procedures  
Participants stood with bare feet apart at pelvis width and with the arms by their side on a force platform firmly fixed on the top of a motorized platform (Officina Lomazzi, Legnano, Italy). The location of the feet of each participant was marked on a thin (0.5 mm) rubber sheet of the same size as the force platform (see below), onto which it was firmly attached, to assure consistent placement between testing trials. Both at rest and during the platform translations, the mean distance between the parallel feet was 16.8 cm \(\pm\) 3.4 (intermalleolar distance). The motorized platform moved periodically from side to side for a 10 cm distance in the horizontal plane. The profile of the continuous translation cycles was asymmetric, each cycle being made of a half cycle with a fast phase (0.5 Hz) followed by a half cycle with a slow return phase (0.18 Hz). The profile was switched from fast right to fast left in different trials. A trial consisted in one hundred continuous perturbation cycles (henceforth, the stimulation). All trials always started with the fast half-cycle and ended with the slow half-cycle. These translations produced a perturbation of the standing body in the frontal plane and a medio-lateral displacement of CoP from right to left and vice versa. There were no still periods between the cycles but the stimulation was preceded and followed by two extended periods of quiet stance. Each trial lasted 14 min and was composed by a quiet-stance period (3 min), the stimulation (5 min) and another stance period (6 min). The first 2 min of the trial were considered a familiarization period and not analysed.  
Participants performed one trial in each of these five different conditions: (1) eyes closed (EC) during the entire trial, with the fast platform translation phase toward the right (i.e., fast right, FR-EC): the platform made a fast half-cycle to the right side of the participant, followed by a slow half-cycle to the left side; (2) EC, with the fast phase towards the left (fast left, FL-EC), followed by a slow half-cycle to the right; (3) eyes open (EO) with the fast phase to the right side (FR-EO); (4) EO with the fast phase to the left side (FL-EO); and (5) like 3, but subjects kept their eyes closed during the 6 min post-stimulation period of quiet stance (FR-EO/EC). At the very end of the first trial, participants were invited to slowly tilt their body to the right and to the left as much as possible, without moving their feet, and keep those positions for 1-2 s. When vision was allowed, participants were invited not to focus on one particular point of the patterned laboratory wall (at 8 m distance) and were allowed to freely explore the scene in front of them, without moving the head. Thus, no precise visual task was required [37]. The control stimulation was a symmetric translation sequence (10 cm, 0.5 Hz) performed with EC, again preceded and followed by quiet stance periods.  
The various trials with different vision conditions and asymmetry profiles were randomized across participants and conditions and separated by one or more days. During all trials, participants wore earphones with music to screen the noise produced by the moving platform. They were instructed to stand in a relaxed mode but not to move their feet during the trial. They were aware of the onset of the acquisition and were warned of the onset of the platform perturbation sequence in order to minimize any startle reaction [38]. Nausea and other unpleasant feelings were never reported by any participant either during or after the recording sessions. Some subjects (5/20) spontaneously reported a mild sense of heaviness of the thigh muscles of the side toward which the platform translation moved slowly.