## 3 Results  
### The CoP Position Pre- and Post-Stimulation  
The CoP was recorded by the force platform firmly mounted onto the motorized platform translating in the frontal plane with two asymmetric cycle profiles (fast left, FL or fast right, FR). The mean ML position of the CoP during the initial quiet stance period was close to zero in platform coordinates (i.e., the centre of the force platform rested halfway between the feet) (mean \(\pm\) SD: \(-\)0.028 cm \(\pm\) 0.76). The CoP position of a representative participant during the entire trial epoch (standing quietly EC, platform stimulation EC, standing quietly EC) is shown in Figure 1A. Here, the fast half-cycle of the perturbation was directed toward the participant's right side (FR-EC). The panel B shows two trial segments with a large medio-lateral peak-to-peak CoP displacement during the fast phase followed by a less ample and slower shift during the slow phase. The corresponding profiles of the platform translations are shown in panel C, where the fast half-cycles are directed upwards (to the right side of the body). In all the cycles, the fast right translation produced a clear rapid displacement of the CoP to the opposite left side. This was followed by a displacement to the right during the subsequent slow half cycle. The red area in panel B indicates the medio-lateral shift to the right direction (upward in the Figure) of the ML CoP post-stimulation with respect to its pre-stimulation mean position (dotted red line). The opposite perturbation profile (FL, not shown) produced a very similar but opposite CoP displacement pattern. Figure 1: Medio-lateral (ML) centre of feet pressure (CoP) position in a representative participant during the fast right (FR) stimulation under the eyes closed condition (EC) (**A**,**B**) and platform translation trace (**C**). During the fast half-cycle to the right side, the CoP showed a large and fast ML displacement followed by a less ample and slower displacement during the slower half-cycle. In the central panel, the red area indicates the medio-lateral shift of the CoP post-stimulation, while the dotted red line indicates the pre-stimulation ML CoP position. Here and in the following Figures, right is upward. Across all participants, the ML CoP position during the quiet-stance period following the end of the stimulation sequence was normally different from the pre-stimulation position, constituting a post-effect of the continuous asymmetric platform translation cycles. The difference was significant, as assessed by the comparison of the mean difference between these positions to zero by means of the Student's \(t\) test, both under the EC and EO condition, for both profiles of the asymmetric platform translation (\(p<0.05\), d \(>0.85\) and \(\pi>0.83\) for the four conditions). The post-stimulation displacement (mean \(\pm\) SD: FL-EC \(-0.81\) cm \(\pm\) 0.89, FL-EO \(-0.49\) cm \(\pm\) 0.66, FR-EC 0.55 cm \(\pm\) 0.9, FR-EO 0.57 cm \(\pm\) 0.77) depended on the profile of the asymmetric platform translation cycles. On average, the CoP was displaced in the same direction as the fast phase of the cycles, i.e., to the left when the fast phase of the cycle was directed to the left (FL) and vice versa (FR). The bar graph in Figure 2A shows the mean displacement of the ML CoP, with respect to the pre-stimulation positions, during the post-effects in the four conditions (FL-EC, FL-EO; FR-EC, FR-EO). The opposite displacement after the stimulation with the reverse profile is obvious. These displacements were of moderate amplitude: for comparison, the mean absolute displacement of the CoP during the maximal voluntary body tilts to the right and to the left on the still platform was 9.85 cm \(\pm\) 2.06 (right) and 10.45 cm \(\pm\) 1.47 (left), respectively (right vs. left: paired \(t\)-test, \(p\) = 0.19). ANOVA showed that the main effect of the translation profile asymmetry (FR vs. FL) on the ML CoP post-stimulation position was significant (F(1,19) = 26.5, \(p<0.001\), d = 2.35, \(\pi=0.99\)).