coherent whole, the pieces just don’t fit together properly (image B). Image C
has two arms that somehow turn into three prongs right before your vigilant
gaze, and the perpetual staircase in image D forever ascends or descends.
Motion Perception
One type of perception that does require you to compare across different
glimpses of the world is motion perception. Consider the two images given in
figure 7.23. Suppose that this individual has stood still while you have walked
toward him. The size of his image on your retina has expanded as you have
drawn near. The rate at which this image has expanded gives you a sense of
how quickly you have been approaching (Gibson, 1979). You use this type of
information to navigate effectively in your world.
Suppose, however, you are still but other objects are in motion. The percep-
tion of motion, like the perception of shape and orientation, often depends on a
reference frame. If you sit in a darkened room and fixate on a stationary spot
of light inside a lighted rectangle that is moving very slowly back and forth,
you will perceive instead amovingdot going back and forth within astationary
rectangle. This illusion, calledinduced motion,occurs even when your eyes are
quite still and fixated on the dot. Your motion-detector cells are not firing at all
in response to the stationary dot but presumably are firing in response to the
Figure 7.22
Impossible figures.
164 Philip G. Zimbardo and Richard J. Gerrig