## 4 Vision  
During quiet stance, unsteadiness in the dark is a sign of impaired proprioception (see [34] for a good review article, with historic hints). For instance, closing the eyes increased the body's sway in all the subjects, as detected by instrumented platforms [35]. The increase in sway can be inconspicuous in healthy subjects, whereas in patients with neuropathy or deficits of the vestibular system, the postural imbalance when standing or walking with EC can be remarkable [36; 37; 38]. An investigation based on continuous random platform tilts [39] suggested that vision modulates the vestibular noise, thereby reducing the threshold for position detection. When healthy subjects stand with EC on the antero-posterior translating platform, the balancing behaviour consisting of large head antero-posterior displacements (compared to those of the platform and head with EO) is obvious [40]. The large head displacements when balancing on that platform with EC occur therefore in the absence of any proprioceptive malfunction. This leads to two easy conclusions: (1) vision does increase stability when standing, both under quiet conditions and when balancing on the translating platform; (2) standing on the translating platform is quite different from a quiet stance in that, even with full somato-sensation, unsteadiness becomes an issue without vision. It is perhaps appropriate here to mention that healthy subjects never fall and rarely take a compensatory step during the trials, which commonly comprise numerous perturbation cycles [41]. The translating platform is not just a means for evaluating the role of vision in stance by enhancing the likelihood to detect clear-cut differences between eyes open/eyes closed conditions. It is an interesting protocol to address the role of vision and its disorders in the control of dynamic balance as well. It has been shown that the head antero-posterior oscillations, normally limited with EO, increase steadily with the severity of poor vision. When subjects balance on the mobile platform wearing test lenses to modify visual acuity from clear vision to blurred vision, head stabilization in space progressively worsens with the decrease in visual acuity and becomes similar to the EC behaviour when vision is severely blurred [42]. Thus, a simple light/dark dichotomy hypothesis for the visual control of dynamic stance is not supported, because the balancing pattern changes gradually from  
Figure 2: Top trace shows the tibialis anterior (TA) EMG at the beginning of a series of perturbations delivered with eyes open at a 0.2 Hz frequency. The bottom trace is the platform antero-posterior translation. head-fixed-in-space to large head oscillation as a function of visual acuity. This suggests that the visual input is integrated as is the general somaesthetic feedback and that we have no capacity of extrapolating simple information from the fixed environment from an indistinct image of it. It is notable that, in this case, head stability gradually diminishes progressively with visual acuity deterioration even when all balancing trials are performed within a short period of time and within the same laboratory, i.e., when subjects are well accustomed to both the environment and the continuous perturbation protocol. Hence, clear vision is a powerful player in the control of balance, particularly under critical dynamic conditions. However, what is the actual purpose of vision under these conditions? Is it necessary for minimizing body segments' oscillations? Two questions deserve an answer. (1) Are there conditions in which we can do without vision because other sensory receptors firing under unstable conditions come to the rescue? (2) Are supra-postural visual tasks able to modulate the stabilizing effect of vision? The former question receives an explicit answer: no. We have already mentioned that reduced vision is not enough to stabilize body oscillations on the platform.