Implicit in the sensory 
theory of apparent motion is 
the assumption that what 
distinguishes apparent from 
real motion is the extent of 
separation between A and B on 
the retina. But that assumption 
may be incorrect. A and B are 
separate from one another in 
perceived space, whereas an 
object in real motion is seen to 
be located in a series of 
adjacent positions in space. If 
we track a really moving object, 
we perceive it to be moving 
even though its image remains 
stationary on the retina. 
Perhaps, then, an analogous 
situation prevails in the case of 
apparent motion. In an 
experiment Sheldon Ebenholtz 
and I performed some years ago, 
observers had to synchronize 
their eye movements with the 
flashing on and off first of A 
and then of B. As A appeared, 
observers looked directly at it. 
Thus the image of A fell in the 
central region of the retina, 
the fovea. As A disappeared, 
observers rapidly shifted their 
eyes to point B; just as the eyes 
reached that position, B 
flashed. It, too, then projected 
onto the fovea. The observers 
perceived apparent motion from 
A to B. In this case, a simple 
sensory explanation will not 
suffice because only one region 
of the retina was stimulated, 
not two. A single retinal region 
can represent two locations in 
perceived space because the 
direction of stimulation is 
interpreted differently on the 
basis of the two different 
positions of the eyes. In that 
respect, this experiment is 
analogous to the one in which 
we track a really moving object.