By the same token, some cells 
in the visual system are most 
responsive to one direction of 
contour motion. If these are 
fatigued, the cells responsive to 
the opposite direction of 
contour motion are, by 
comparison, more active. 
Hence stationary contours 
stimulating that region of the 
retina will create an 
impression of motion in the 
opposite direction. These 
adaptation effects, of both color 
and motion, are clearly sensory 
in origin. In fact, the motion 
aftereffect is paradoxical in 
one respect. A sensation of 
motion (or of expansion or 
contraction) does exist, but the 
contours do not appear to be 
going anywhere. No doubt this 
occurs because other 
information continues to 
indicate no change in location.

    If this kind of explanation of 
the aftereffect of motion is 
correct, the phenomenon is 
different from all the other 
kinds of motion perception that 
were considered in this 
chapter. Whereas it can be 
explained in terms of localized 
sensory mechanisms and is not 
an aftereffect of motion 
perception but rather of a 
certain kind of sensory 
stimulation, the other 
phenomena cannot be 
explained simply in terms of 
sensory mechanisms and do 
concern the perception of 
motion. Contrary to what might 
seem to be a plausible 
explanation of motion 
perception, the displacement of 
an object’s image over the 
retina cannot account for such 
perception, although it is 
relevant information to be 
taken into account. We have 
seen that illusions of motion of 
various kinds--induced, 
directional, apparent, and 
autokinetic--as well as the 
perception of real motion, are 
based on more complex central 
processes entailing constancy 
operations, relativistic 
comparisons often based on 
structures serving as reference 
frames, and unconscious 
problem solving.

A number of new concepts and 
principles have thus emerged 
in connection with the various 
phenomena of perceived 
motion that were not relevant 
in the earlier discussions of 
static phenomena. In 
particular, we have seen for 
the first time how the body of 
the observer, the visible self, is 
another object in the field that 
conforms to the same lawful 
processes that govern the 
perception of other objects. 
Some of these same concepts 
will come up again in the next 
chapter, where we consider 
how we perceive the 
orientation of objects and 
ourselves in the world.