Hockenbury Discovering Psychology 5th txtbk

124 CHAPTER 3 Sensation and PerceptionPerceptual IllusionsKey Theme• Perceptual illusions underscore the idea that we actively construct our perceptualrepresentations of the world according to psychological principles.Key Questions• How can the Müller-Lyer and moon illusions be explained?• What do perceptual illusions reveal about perceptual processes?• What roles do perceptual sets, learning experiences, and culture play inperception?Figure 3.18 Illusory Contours: HowMany Triangles Do You See? The Gestaltprinciples of perceptual organization contributeto the illusion of triangular contoursin this image. The second you look at thisambiguous image, you instantly reverse figureand ground so that the black circular regionsbecome the ground, while the whiteregion is visually favored as the figure. In organizingthese visual fragments, which arelined up very precisely, the Gestalt principlesof closure and good continuation contributeto the perceptual construction of a solidwhite triangle covering three black disksand an inverted triangle. The images producea second intriguing illusion: The purewhite illusory triangle seems brighter thanthe surrounding white paper.Figure 3.19 The Müller-Lyer IllusionCompare the two photographs. Which cornerline is longer? Now compare the twoline drawings. Which center line is longer?In reality, the center lines in the photographsand the line drawings are all exactlythe same length, which you canprove to yourself with a ruler.Our perceptual processes are largely automatic and unconscious. On the one hand,this arrangement is mentally efficient. With a minimum of cognitive effort, wedecipher our surroundings, answering important perceptual questions and makingsense of the environment. On the other hand, because perceptual processing islargely automatic, we can inadvertently arrive at the wrong perceptual conclusion.When we misperceive the true characteristics of an object or an image, we experiencea perceptual illusion.During the past century, well over 200 perceptual illusions have been discovered.One famous perceptual illusion is shown in Figure 3.18. The perceptual contradictionsof illusions are not only fascinating but can also shed light on how the normalprocesses of perception guide us to perceptual conclusions. Given the basics of perceptionthat we’ve covered thus far, you’re in a good position to understand how andwhy some famous illusions seem to occur.The Müller-Lyer IllusionLook at the center line made by the corners of the glass walls in photographs (a) and(c) in Figure 3.19. Which line is longer? If you said photograph (c), then you’ve justexperienced the Müller-Lyer illusion. In fact, the two center lines are the same length,even though they appear to have different lengths. You can confirm that they are thesame length by measuring them. The same illusion occurs when you look at a simpleline drawing of the Müller-Lyer illusion, shown in parts (b) and (d) of Figure 3.19.(a)(b)(c)(d)