Elements that move in the same direction are perceived to be more related than elements that move in different directions or are stationary.
Perceptual organization of movement. In perception: Gestalt principles. One Gestalt principle, that of common fate, depends on movement and is quite striking when observed. According to the principle of common fate, Humans perceive visual elements that move in the same speed and/or direction as parts of a single stimulus.
A common example of this is a flock of birds. When several birds fly in the same direction, we normally assume that they belong to a single group. Birds that fly in a different direction do not appear to be included in the said group. A marching band is another example that usually exhibits the gestalt law of common fate.
Consider common fate as a grouping strategy when displaying information with moving or flickering elements. Related elements should move at the same time, velocity, and direction, or flicker at the same time, frequency, and intensity. It is possible to group elements when these variables are dissimilar, but only if the motion or flicker forms a recognizable pattern. When moving elements within bounded regions, move the edges of the region in the same direction as the elements to achieve a figure relationship or in the opposite direction as the elements to achieve a ground relationship.