## 4 Discussion
### The Effect of Vision in the Post-Stimulation Period  
Contrary to our initial hypothesis, we observed no major effect of vision on the occurrence, amplitude or duration of the post-stimulation effect on the ML position of the CoP. This consistent trend is supported by the qualitatively and quantitatively similar post-effects present when the stimulation was administered with both EO and EC. Incidentally, the well known decrement of ML body sway with vision was not remarkable in the present study, probably because of the wide support base with feet apart [62]. The ML sway about the mean CoP position post-stimulation showed a modest increment with both EO and EC. This can be explained by the post-stimulation displacement in the position of the CoP in the frontal plane and by its slow drift to the pre-stimulation values. The AP sway was greater with EC than with EO and greater in the post-stimulation period, as a non-specific vision-dependent adjustment to the termination of the perturbation cycles. The absence of a significant effect of vision on the post-effect ML CoP displacement is supported by the trials in which the participants were invited to close their eyes simultaneously with the cessation of the stimulation. Clear vision of the stationary environment did not correct the displacement of the CoP 'adapted' during the EO translations. This was unexpected, not least because vision during tilts of a support base can reduce sway variability by reducing the contribution of vestibular noise and improving the disturbance compensation [35]. Furthermore, our feet-apart condition would have favoured a symmetric position of the CoP with EO [63], but this did not occur. The effect of vision was not observed at the steady state of the adapted behaviour to the sequence of perturbations either. Therefore, we would conjecture that the visual flow from the substantially stationary visual scene from the environment played no or a minor role in the process of adaptation to the continuous perturbations, except for the delay in reaching the steady state, as discussed above. This shows that the CoP displacement and the responsible process dependent on the reaction to the continuous platform translation and on the anticipatory reaction to it, is robust to the rectifying effect of vision. Therefore, the post-effect is not prevented by either a continuous or a post-stimulation visual reference. In this light, the present post-effect would be related to that elicited on body AP orientation by a period of brisk walking on a treadmill, where the post-effect tilt is present when standing both with eyes open and eyes closed, but is larger without vision [31; 64]. Another study has shown that the effect of sensory manipulation producing a sway of the body in the frontal plane (by vestibular or cutaneous foot stimulation or the vibration of hip abductors) is not affected by the absence of normal visual information [65]. On the contrary, after the podokinetic stimulation, the continuing unwitting yaw rotation with EC, after the prolonged stepping-in-place, is temporarily reduced or actually stopped when the eyes are opened [30], to start again with the closing the eyes [66]. Standing on an inclined surface produces a post-effect as well, whereby subjects show a post-nicline leaning post-effect in which they maintain a similar trunk to support the surface orientation as during the period of standing upright on the inclined surface [67; 68]. However, in that case, participants immediately return to upright when they open their eyes briefly during the post-nicline period. Our findings are different from those obtained during continuous AP translations of the supporting platform, since in the latter case, the upper body was greatly stabilized by vision but goes through large oscillations with eyes closed or blindfolded [69; 70; 71].