Students of perception have 
not done much better in their 
attempts to explain this 
illusion of motion. In the 
laboratory, apparent motion 
(also referred to as stroboscopic 
motion or the phi 
phenomenon) is studied in its 
utmost simplicity by flashing a 
single object or line in one 
place, and then, a short time 
later, flashing a similar object 
or line in another place. If the 
spacing and timing are just 
right, the observer will see the 
object or line moving from the 
first location to the second 
(from A to B in the illustration 
at left). Usually, the cycle is 
repeated and the observer sees 
the object moving back and 
forth. Why do we tend to see 
movement when the stimulus 
consists of one stationary 
object followed by another? The 
presumption is that, if we can 
unravel this problem, it will 
provide a key for understanding 
motion perception in general.