Invitation to Psychology

Chapter 6 Sensation and Perception 203

near edge of a building; the one on the right is like

the far corner of a room (see part b of the figure).

Although the two lines produce retinal images of

the same size, the one with the outward-facing

branches suggests greater distance. We are fooled

into perceiving it as longer because we automati-

cally apply a rule about the relationship between

size and distance that is normally useful: When

two objects produce the same-sized retinal image

and one is farther away, the farther one is larger.

The problem, in this case, is that the two lines are

the same distance away, so the rule is inappropri-

ate. For other illusions that occur when the brain

misinterprets sensory information, see Figure 6.7.

Simulate the Experiment Müller-Lyer Illusion at

MyPsychLab

Although illusions can occur in any sensory
modality, visual illusions have been the best stud-
ied. Visual illusions sometimes occur when the
strategies that normally lead to accurate percep-
tion are overextended to situations where they do
not apply. Compare the lengths of the two vertical
lines in Figure 6.6. You will probably perceive the
line on the right as slightly longer than the one on
the left, yet they are exactly the same. (Go ahead,
measure them; everyone does.) This is the Müller-
Lyer illusion, named after the German sociologist
who first described it in 1889.
The leading explanation for the Müller-Lyer
illusion is that the branches on the lines serve
as perspective cues that normally suggest depth
(Gregory, 1963). The line on the left is like the

(a) (b)

Figure 6.6 The Müller-Lyer illusion
The two lines in (a) are exactly the same length. We are probably fooled into perceiving them as different because the
brain interprets the one with the outward-facing branches as farther away, as if it were the far corner of a room, and
the one with the inward-facing branches as closer, as if it were the near edge of a building (b).

(a) (b) (c)

Figure 6.7 Fooling the eye
Although perception is usually accurate, we can be fooled. In (a), the cats as drawn are exactly the same size; in
(b), the diagonal lines are all parallel. To see the illusion depicted in (c), hold your index fingers 5 to 10 inches in
front of your eyes as shown and then focus straight ahead. Do you see a floating “fingertip frankfurter”? Can you
make it shrink or expand?