## 3 The Important Role of Optic Flow in Postural Control
### Different Methodologies Used for Assessing the Role of the Central and Peripheral Visual Fields  
As introduced earlier, several studies aimed at uncovering the functional roles of the peripheral and central visual fields in postural control have reached different conclusions. According to the retinal distribution of photoreceptors, the definition of central vision ranges between \(2^{\circ}\) and \(4^{\circ}\) of the visual field [36]. However, considering the projections from the retina to the cortical visual areas, the central visual field has been defined as the \(7^{\circ}\) of the regions surrounding the fovea [39]. It has to be noted that the few papers that aimed at investigating the role of central vision used stimuli whose dimensions ranged from \(7^{\circ}\) to \(60^{\circ}\). [PERSON] et al. stimulated the central visual field with stimuli of \(4^{\circ}\) and \(7^{\circ}\)[42]; [PERSON] et al. used stimuli of \(7^{\circ}\)[18, 21]; [PERSON] et al. used stimuli of \(8^{\circ}\)[46]; [PERSON] et al. used stimuli of \(10^{\circ}\)[49]; [PERSON] and [PERSON] used stimuli that ranged from \(15^{\circ}\) to \(60^{\circ}\)[45]; and [PERSON] et al. used stimuli that ranged from \(4^{\circ}\) to \(30^{\circ}\)[44].  
The peripheral visual field is commonly considered to be the area adjacent to the central visual field. Thus, if the central visual field was not uniformly defined, the peripheral visual field would also be affected by the same issue. For this reason, [PERSON] et al. [18, 21] considered the periphery of the visual field outside the inner \(20^{\circ}\) of the central visual field, to ensure stimulation of a retinal region formed only by rod photoreceptors [50].  
Besides the different definitions of central and peripheral visual fields, the methodologies and the visual stimuli also differed between the studies. In few studies, stimuli were formed by random patterns of dots which only provided spatiotemporal changes in the visual field [42]. In other studies, stimuli were made of patterns of vertical or horizontal bands that alternated between black and white [41]. Undoubtedly, the use of different moving visual stimuli and/or the different retinal area of stimulation changed the evoked muscular responses. [PERSON] et al. [12, 14] stimulated the central visual field by presenting moving stimuli in front of the subjects, while stimulating the peripheral visual field by presenting moving stimuli to the right and left of the subjects. As [PERSON] [46] already pointed out, this type of stimulation might not be adequate for studying the functional role of the peripheral and central visual fields given that the dimensions of the stimulated visual field varied significantly and were not directly changed. Other authors used an occlusion method in their stimulation paradigm, which resulted in a more specific distinction between the stimulated retinal areas [18, 21, 42, 43, 46]. Table 1 reports the fundamental aspects of each paper including methodologies and conclusions.  
\begin{table}
\begin{tabular}{} \end{tabular} \\ \hline \end{tabular}
\end{table}
Table 1: Comparison of methodologies and definition criteria across studies.