Understanding Psychology (10th Ed)

132 Chapter 
4 
Sensation and Perception
crooked and about to fall down. Instead, it appears to stand completely straight, at 
right angles to the ground. 
The fact that the Parthenon appears to be completely upright is the result of a 
series of visual illusions.
 Visual illusions 
are physical stimuli that consistently pro-
duce errors in perception. In the case of the Parthenon, the building appears to be 
completely square, as illustrated in Figure 5a. However, if it had been built that way, 
it would look to us as it does in Figure 5b. The reason for this is an illusion that makes 
right angles placed above a line appear as if they were bent. To offset the illusion, the 
Parthenon was constructed as in Figure 5c, with a slight upward curvature.
The Müller-Lyer illusion (illustrated in Figure 6) has fascinated psychologists for 
decades. Although the two lines are the same length, the one with the arrow tips 
pointing outward, away from the vertical line (Figure 6a, left) appears to be shorter 
than the one with the arrow tips pointing inward (Figure 6a, right). 
Although all kinds of explanations for visual illusions have been suggested, most 
concentrate either on the physical operation of the eye or on our misinterpretation 
of the visual stimulus. For example, one explanation for the Müller-Lyer illusion is 
that eye movements are greater when the arrow tips point inward, making us per-
ceive the line as longer than it is when the arrow tips face outward. In contrast, a 
different explanation for the illusion suggests that we unconsciously attribute par-
ticular signifi cance to each of the lines (Gregory, 1978; Redding & Hawley, 1993). 
When we see the left line in Figure 6a we tend to perceive it as if it were the relatively 
close outside corner of a rectangular object, such as the outside corner of the room 
illustrated in Figure 6b. In contrast, when we view the line on the right in Figure 6a, 
we perceive it as the relatively more distant inside corner of a rectangular object, 
such as the inside room corner in Figure 6c. Because previous experience leads us to 
assume that the outside corner is closer than the inside corner, we make the further 
assumption that the inside corner must therefore be longer. 
Despite the complexity of the latter explanation, a good deal of evidence sup-
ports it. For instance, cross-cultural studies show that people raised in areas where 
there are few right angles—such as the Zulu in Africa—are much less susceptible to