## 3 The Important Role of Optic Flow in Postural Control
### The Functional Role of the Foveal and Peripheral Visual Field
#### 3.1.1 First Perspective: Peripheral Vision Plays the Most Important Role in Postural Control  
The results of several papers seem to support the hypothesis that peripheral vision is more important than central vision in the control of posture. [PERSON] and [PERSON] stimulated the participant's visual field (central and peripheral) with either a horizontal or a vertical rectangular grating. In their protocol, the lateral acceleration was measured at the level of the ankles, hips, and head. The results of their experiment showed that the peripheral vision played the most important role in postural control, while the central vision only played a supplementary role [41]. [PERSON] et al. presented the participants with random patterns of circular dots that were either static or moving [42], showing that the peripheral vision had a stabilizing effect in the direction of the stimulus observation. These results suggested that peripheral vision makes a strong contribution to postural control, and that the peripheral visual field controlled posture in a head-centered rather than in a body-centered frame of reference. [PERSON] et al. showed the participants random patterns of dots that were moving at a constant spatial frequency and were sinusoidal in depth [43]. The authors performed both central and peripheral occlusions of the visual field to find that self-motion perception and body sway were more dependent on the stimulation of the peripheral visual field. [PERSON] et al. examined the roles of central and peripheral vision in the control of posture by using a 3D tunnel stimulus that was either static or moving sinusoidally in the anterior-posterior direction [44]. The authors used peripheral and central occlusions at different degrees and a full visual field condition. In line with previous studies, the results showed that in the presence of an optic flow, the peripheral vision played a crucial role in postural control, being responsible for a compensatory sway, while central vision played an accessory role that was likely related to spatial orientation. [PERSON] et al. [6], using the simultaneous presentation of conflicting central and peripheral optokinetic stimuli, showed that the exocentric orientation was dependent on the peripheral stimuli. [PERSON] and [PERSON] examined how the size and eccentricity of a moving visual surround influenced the control of stance. Their results showed that postural control was determined primarily by visual motion in the peripheral visual field [45]. [PERSON] et al., using a set of visual stimuli made by expanding and contracting optic flow patterns, showed that the visual stimuli always evoked an excitatory input on postural muscles, but that the stimulus structures produced different postural effects [21]. The authors found that the stimuli presented in the peripheral visual field stabilized body sway, while stimuli presented in the central visual field evoked much larger body sways. In a recent study, [PERSON] et al. manipulated the central and peripheral visual fields and the occurrence of optical flow during quiet standing [46]. The results showed that stimuli presented in the peripheral visual field evoked smaller postural sway than stimuli presented in the central visual field, meaning that the optical flow occurring in the peripheral visual field was essential for stable quiet standing.