Perception: Interpreting
facing forward and downward. Which image you see depends on how the visual infor- mation is processed.
Our discussion of perception focuses on visual perception, rather than on all of the perceptual systems, for several reasons: Visual perception is a highly important sens- ing system; scientists understand how it works better than they do other systems; and it is representative enough of other systems to tell us something about the process of perception in general.
Perceptual Organization. Raw visual sensations are like the unassembled parts of a washing machine: they must be put together in an organized way before they are useful to us. Some of the fundamental ways in which the eye and the brain organize visual sensations were described about 75 years ago by Gestalt psychologists in their pioneering writings on perception (see chapter 1). These principles of perceptual orga- nization are still worthy of our attention (Palmer, 2002; Prinzmetal, 1995). The fol- lowing are fi ve of the so-called Gestalt principles of perception.
1. Figure-ground . When we perceive a visual stimulus, part of what we see is the center of our attention, the fi gure, and the rest is the indistinct ground. The vase in fi gure 5.23 shows that this way of seeing can reorganize the nature of reality. The fi gure and ground of this photo can be reversed to perceive either a vase or two opposing faces. This principle of perception is very useful in showing us that what we perceive is often based more on what goes on in our brains than what is in front of our eyes. The remaining Gestalt principles amplify this point.
2. Continuity . We tend to perceive lines or patterns that follow a smooth contour as being part of a single unit. In fi gure 5.24 , at which point did child B start bouncing her pogo stick? We tend to organize our perceptions of the tracks so that it appears that girl B started at point 1, but both girls could have made sharp turns in the center and headed off at right angles. We do not naturally organize sensations in this way, however; we tend to perceive continuity in lines and patterns.
3. Proximity . Things that are proximal (close together) are usually perceived as belonging together. In fi gure 5.25 , we see three vertical columns of blocks on the left side and three horizontal rows on the right side, owing to proximity.
4. Similarity . On the left side of fi gure 5.26 , we perceive two vertical columns of apples and two vertical columns of pears, even though they are evenly spaced.
On the right side, in contrast, a different arrangement results in the perception of two horizontal rows of each fruit. Similar things are perceived as being related.
5. Closure . Incomplete fi gures of familiar things, such as in fi gure 5.27 , tend to be perceived as complete wholes. Again, missing sensory information is auto- matically “fi lled in” in the process of perception to create complete and whole perceptions (Kellman & others, 2005).
Our perceptions are actively organized according to these and other, similar inborn principles.
Perceptual Constancy. We perceive the world as a fairly constant and unchanging place. Tables, lamps, and people do not change in size, shape, or color from moment to moment. Yet, the sensations that tell us about these things do change considerably from moment to moment. The size of the image that falls on the retina changes as a person walks away from us, but we do not perceive the person as shrinking in size.
The shape of a pot seen from different angles is different on the retina, but we do Figure 5.23
The distinction between fi gure and ground in visual perception is clearly illustrated by this vase prepared for Queen Elizabeth of England. Do you see a vase, or do you see the profi les of Queen Elizabeth and Prince Philip?
It depends on whether the dark space on each side of the vase is the fi gure or the ground in your perceptual organization of the stimulus.
Figure 5.24
At which point did girl B start jumping on her pogo stick? According to the principle of continuity, we would tend to perceive point 1 as her starting point, although either 1 or 2 would be equally possible.
A
1
B
2
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not believe that the pot is changing shape. This characteristic of perception is called perceptual constancy .
There are several types of perceptual constancy:
1. Brightness constancy. A piece of white paper does not change in perceived brightness when it moves from a dimly lit room to a brightly lit room, even though the intensity of the light reaching the eye changes considerably. For- tunately for our ability to cope with the world, our perception corresponds to the unchanging physical properties of the paper rather than to the changing sensory information about its brightness. When you stop to think about it, this is a remarkable accomplishment, but one that we take so much for granted that you may not have been aware that it was happening until you read this paragraph.
2. Color constancy. Colors do not appear to change much in spite of different conditions of light and surroundings that change incoming visual information.
3. Size constancy. A dollar bill seen from distances of 1 foot and 10 feet casts different-sized images on the retina, but we do not perceive it as changing in size. Familiar objects do not change in perceived size at different distances.
perceptual constancy Tendency for perceptions of objects to remain relatively unchanged in spite of changes in raw sensations.
Figure 5.25 Do you see vertical columns or horizontal rows? The principle of proximity determines how these stimuli are organized perceptually.
Figure 5.26 Do you see vertical columns or horizontal rows? The principle of similarity suggests that we organize the fi gure on the left into vertical columns and the one on the right into horizontal rows, even though the objects are equally spaced.
Figure 5.27
We see a face rather than unrelated lines because of the perceptual principle of closure.
fi gure-ground principle Gestalt principle of perception that states that part of a visual stimulus will be the center of our attention (fi gure) and the rest will be the indistinct ground. In many cases, the fi gure and ground can be reversed in our perception of the same stimulus.
continuity principle
(kon ´ ti-noo´´i-tee) Gestalt principle of perception that states that lines or patterns that follow a smooth contour are perceived as part of a single unit.
proximity principle (prok ´ sim´´-i-tee) Gestalt principle of perception that states that parts of a visual stimulus that are close together are perceived as belonging together.
similarity principle Gestalt principle of perception that states that parts of a visual stimulus that are similar are perceived as belonging together.
closure principle (klo ´ zhur) Gestalt principle of perception that states that incomplete fi gures of familiar objects tend to be perceived as wholes.
4. Shape constancy. A penny seen from straight ahead casts a circular image on the retina. When seen from a slight angle, however, the image it casts is oval, yet we continue to perceive it as circular.
The process of perceptual constancy means our perceptions are automatically adjusted to correspond with what we have learned about the physical world, rather than relying solely on changing stimulus input (Graf, 2006).
Depth Perception. The retina has a two-dimensional surface. It has only an up and a down, and a left and a right. How are we able to perceive a three-dimensional world with depth using a two-dimensional retina? The eye and the brain accomplish this remarkable feat by using a number of two-dimensional cues to create a perceptual distance.
The monocular cues to depth perception can be perceived by one eye (see fi gure 5.28) . We use these monocular cues in everyday life and artists manipulate them to create images in art that appear to have depth on fl at surfaces and to bring Shrek and other computer-animated fi gures to life. The eight monocular cues are these:
1. Texture gradient. The texture of objects is larger and more visible up close and smaller when far away. On curved surfaces, the elements of texture are also more slanted when the surface does not squarely face us. For example, we see fi gure 5.29 (p. 152) as three-dimensional even though it is a fl at image on the page, because we perceive the black circles to be slanted when the surface curves away from us (Todd & others, 2004).
2. Linear perspective. Objects cast smaller images on the retina when they are more distant. As a result, parallel lines, such as railroad tracks, appear to grow closer together the farther away they are from us. In paintings, objects with larger relative size appear to be closer than objects with smaller relative size.
3. Superposition. Closer objects tend to be partially in front of, or partially cover up, more distant objects.
4. Shadowing. The shadows cast by objects and highlights of refl ected light suggest their depth. For example, fi gure 5.30 (p. 152) has no real depth on this fl at page, but you perceive it as three-dimensional because of the shadowing and highlighting (Norman, Todd, & Orban, 2004).
5. Speed of movement. Objects farther away appear to move across the fi eld of vision more slowly than do closer objects. A dog running through a distant fi eld appears to move slowly, but it moves more quickly when the dog runs right in front of us.
6. Aerial perspective. Water vapor and pollution in the air scatter light waves, giving distant objects a bluish, hazy appearance compared with nearby objects.
7. Accommodation. As discussed earlier in the chapter, the shape of the lens of the eye must change to focus the visual image on the retina from stimuli that are different distances from the eye. This process is called accommoda- tion. Kinesthetic receptors in the ciliary muscle, therefore, provide a source of information about the distance of different objects. This information is useful, however, only for short distances up to about 4 feet.
8. Vertical position. When objects are on the ground, the farther they appear to be below the horizon, the closer they appear to be to us. For objects in the air, however, the farther they appear to be above the horizon, the closer they appear to be to us.
monocular cues (mon-ok ´ uˉ -lar) Eight visual cues that can be seen with one eye and that allow us to perceive depth.
Perceptual constancy helps us recognize this vase as unchanging, even though we are viewing it from different angles and from different distances.
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Binocular cues in depth perception can only be perceived using two eyes. The two binocular cues:
1. Convergence. When both eyes are looking at an object in the center of the visual fi eld, they must angle inward more sharply for a near object than for a distant object (see fi gure 5.31 , p. 152). Information from the muscles that move the eyes thus provides a clue as to the distance of an object from the viewer.
2. Retinal disparity. Because our two eyes are a couple of inches apart, they do not see the same view of three-dimensional objects, especially when the object is close. This disparity, or difference, between the images on the two retinas is a key factor in depth perception. Retinal disparity is the principle behind the old-fashioned stereopticon. As shown in fi gure 5.32 (p. 153), the individual looks at two pictures of the same scene in a viewer that lets each
binocular cues (bˉn-ok ´ uˉ-lar) Two visual cues that require both eyes to allow us to perceive depth.
Figure 5.28 Texture gradient, linear perspective, shadowing, superposition, and aerial perspective are monocular cues used in depth perception.
Texture gradient Linear perspective Shadowing
Superposition Aerial perspective
eye see only one of the two images. The images have been photographed from two slightly different spots to duplicate the disparity between two retinal images. When seen in the stereopticon, the two images fuse into a single scene perceived in startlingly good three dimension. Try placing your hand edgewise between the two pictures and the bridge of your nose to allow each eye to see only one of the pictures. Look at them for a while to see if they fuse into a single, three-dimensional scene. Recent studies have revealed that there are specifi c neurons in the visual areas of the cerebral cortex that process cues from the left and right eyes that contribute to depth perception (Parker, 2007).
Through a combination of these monocular and binocular cues, we are able to perceive our three-dimensional world using only two-dimensional information.
Figure 5.29
This fi gure creates an illusion of depth even though it is a fl at image on a two-dimensional page. We perceive it as a three-dimensional fi gure with depth because we interpret the black circles as being slanted because they are on a surface that curves away from us.
Figure 5.30
This fi gure has no depth on this fl at two-dimensional page, but you perceive it as three-dimensional because the shadowing and highlighting create an illusion of depth.
Figure 5.31 The degree to which the eyes must look inward (convergence) to focus on objects at different distances provides information on the distance of that object. It’s a binocular cue in depth perception because it requires the use of both eyes.
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Visual Illusions. Instructors of introductory psychology have long enjoyed amaz- ing their classes with visual illusions . These illusions intentionally manipulate the cues that we use in visual perception to create a false or illusory perception. They are instructive, therefore, in showing us more about the process of perception and for showing us in yet another way that what we see is not always the same as the visual information that enters the eyes. For example, are the two horizontal lines in fi gure 5.33 (the Ponzo illusion) the same size? (They are, even though the upper line looks lon- ger.) How about the two lines in fi gure 5.34 (the vertical-horizontal illusion)—most people see the vertical line as longer, even though they are the same length. The white square that you see in fi gure 5.35 (p. 154) does not exist in the drawing, which is just four circles with missing quarters. My personal favorite is the Zollner illusion, shown in fi gure 5.36 (p. 154). Believe it or not, the diagonal lines are parallel. Even after you cover all but two lines or measure the distance between the diagonal lines for yourself, this illusion is amazing.
How do these illusions fool us? They do so by using monocular depth cues to create an illusion. Consider the Müller-Lyer illusion: the two vertical lines on the left
visual illusion Visual stimuli in which the cues used in visual perception create a false perception.
Figure 5.32 Two photos taken from slightly different angles are used in a stereopticon to create an illusion of depth through retinal disparity.
Figure 5.33
The Ponzo illusion. Are the horizontal lines the same length?
Figure 5.34
This fi gure often produces an illusory judgment of length. Which line is longer, the horizontal or the vertical line? Actually, they are the same length.
of fi gure 5.37 (p. 154) are of different lengths—or are they? Actually, they just look different because of the context they are in. Ordinarily, the short lines at the end of the longer lines would be cues to depth, as in the two booklets shown on the right side of fi gure 5.37 . We see the vertical line as longer when the cues suggest that it is farther away. In the Ponzo illusion ( fi gure 5.33 ), the two vertical lines appear to be converging in the distance, like railroad tracks, suggesting that the horizontal line at the top is farther away, so we see it as longer. Perhaps the most impressive visual illusion ever created in a psychology laboratory is the Ames room. When this room is viewed through a peep-hole made in one wall (used to restrict the availability of binocular cues), the room appears to be a normal square. Actually, however, the room is much deeper on one side than the other, but many cues of depth perception have been altered to give the illusion of equal depth for all sides of the back wall.
The effect this room has on perception is startling when people are in the room (see fi gures 5.38 and 5.39 ).
Not all visual illusions are laboratory demonstrations, however. They are common in everyday life. Few sights are more beautiful than a huge full moon on the horizon.
Have you ever stopped to wonder why it always looks bigger on the horizon than overhead? It doesn’t really grow, you know; it’s an illusion. In fact, it’s an illusion that still puzzles scientists. There is no widely accepted theory of the moon illusion (Reed, 1984; Rock & Kaufman, 1972), but it is based partly on the misperception of depth.
As shown in fi gure 5.40 (p. 156), an object that our senses tell us is farther away is perceived as being larger than an object that casts the same-size image on the retina but appears to be closer. The two triangles in this fi gure are the same size, but the one at the top is perceived as larger because it appears to be farther away. Ordinarily, the top triangle would be larger if it were farther away, but it could still cast as large a retinal image as a closer object.
The moon illusion is based partly on the same principle. When the moon is over- head, not only does it appear closer owing to its vertical position, but we have no distance cues, so depth cues do not accurately infl uence our perception of the moon’s size. When it’s near the horizon, however, it appears to be farther away because of its vertical position. In addition, we can see the moon is farther away than objects such as distant trees and buildings, which we know to be large but which cast a small image on the retina. When the size of the moon is perceived in comparison with these objects, it looks much bigger.
And then there is the dreaded Poggendorf illusion! Look at the diagonal line that appears to pass behind the blue bar in fi gure 5.41 (p. 156). Which line on the right is the continuation of the diagonal line? Most persons choose the middle line. Now place the edge of a piece of white paper along the line. Which line on the right do you think is the continuation of the line on the left now? In the Poggendorf illusion, lines that appear to pass behind solid objects at an angle appear to be “moved over” when they emerge. You can demonstrate this phenomenon again by drawing a straight line with Figure 5.36
The Zollner illusion. Are the diagonal lines parallel?
Figure 5.35
Do you see a white square? Most of us perceive the illusory “Kanizsa square”
in front of four orange circles, in spite of the fact that there is no square actually depicted in the drawing—just four circles with missing quarters.
What we see is often not literally what is “out there.”
Figure 5.37
The Müller-Lyer illusion. Most people see the vertical line on the right as being longer, even though they are the same length. The shorter lines give an illusion of depth, as in the two books on the right.
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