We rely on our vision to perceive motion for many crucial activities. One use is to separate a moving figure from a stationary background. For example, a camouflaged animal in the forest might only become noticeable when moving. This is clearly useful whether humans are the hunter or the hunted. Motion also helps people to assess the 3D structure of an object. Imagine assessing the value of a piece of fruit in the market by rotating it around. Another use is to visually guide actions, such as walking down the street or hammering a nail. VR systems have the tall order of replicating these uses in a virtual world in spite of limited technology. Just as important as the perception of motion is the perception of non-motion, which we called perception of stationarity in Section 2.3. For example, if we apply the VOR by turning our heads, then do the virtual world objects move correctly on the display so that they appear to be stationary? Slight errors in time or image position might inadvertently trigger the perception of motion.