So far, we have considered 
the case of a wagging finger or 
other single stimulus object. 
But in moving pictures several 
things are often moving or 
different parts of one object are 
moving simultaneously. For 
example, when we see a woman 
walking across the screen, we 
usually see feet, arms, and the 
entire body move. Consider the 
simplified situation in which 
three spots, 1, 2, and 3 are 
flashed at one time and a 
second set of three spots, A, B, 
and C are flashed slightly later 
in time at somewhat different 
positions.  Thus the flash at
Time 1 is:
1  2  3
and the flash at Time 2 is:
   A  B  C.
Thus the observer sees an array
of 3 spots which is abruptly 
replaced by an array of 
identical spots, except that the 
entire array is offset slightly to 
the right.  What spot presented 
at Time 1 should be seen as 
moving to what location of the 
array presented at Time 2, and 
why? This issue is referred to as 
the correspondence problem.