When observers are presented with two immobile stimuli, that are presented in a sequential fashion at two differing locations, the stimuli will often be perceived as a solitary object that is simply moving from a starting location to another position. This apparent motion or apparent movement is of great interest to researchers because the perceived movement does not derive strictly from the physical aspect of vision such as the stimulation caused by impingement on the retina. Instead, apparent motion appears to arise from the visual system's processing of the physical properties of the percept. It is for this reason that apparent motion is a key area of research in the domain of vision research. The Ternus illusion is perhaps one of the best examples of such an effect.