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We have learned how the sensory systems absorb information and transform it into the electrical and chemical language of neurons. Now let’s look more closely at the next step: perception, which draws from experience to organize and interpret sensory information, turning sensory data into something meaningful. Perhaps the most important lesson you will learn in this section is that perception is far from foolproof. We don’t see, hear, smell, taste, or feel the world exactly as it is, but as our brains judge it to be.

What you see is not always what you get. Consider this experiment: Researchers asked a group of undergraduates studying oenology (the study of wine) to sample two beverages. One was a white wine, and the other was the exact same white wine—colored red with flavorless dye. When it came time to evaluate the aromas, the students used the standard white-wine adjectives (for example, honey and apple) to describe the white wine, and this came as no surprise. When asked to describe the “red wine,” which was really the same white wine, the tasters used the standard red-wine adjectives (for example, raspberry and peppery; Morrot, Brochet, & Dubourdieu, 2001). It appeared they were tricked by their eyes! What they saw was red wine, so what they perceived were red wine aromas, even though they were not actually smelling red wine aromas. Perception is essential to our functioning, but as the wine-tasting example illustrates, it is sometimes misleading.

A great way to detect distortions in perceptual processes is to study illusions. An illusion is a perception that is incongruent with real sensory data, conveying an inaccurate representation of reality. Looking at perception when it is working “incorrectly” allows us to examine knowledge-based processing, which draws upon past experience to make sense of incoming information. If a clock stops ticking, you can open it up to see what’s wrong and better understand how it works. Illusions serve a similar purpose. Let’s examine one.

Look at the Müller-Lyer illusion in Figure 3.7. It appears lines (b) and (d) are longer, but in fact all four lines (a–d) are equal in length. Why does this illusion occur? Our experiences looking at buildings tell us that the corner presented in line (c) is nearer because it is jutting toward us. The corner presented in line (d) seems to be farther away because it is jutting away. Two objects that are the same size will project different images on the retina if one is farther away; the farther object will project a smaller image on the retina. People living in “carpentered worlds” (that is, surrounded by structures constructed with corners, angles, and straight lines, as opposed to living in more “traditional” settings) are more likely to be tricked by this illusion because of their experience seeing manufactured structures. People in more traditional settings, without all the hard edges and straight lines, are less likely to fall prey to such an illusion (Segall, Campbell, & Herskovits, 1968).

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Another visual illusion is stroboscopic motion, the appearance of motion produced when a sequence of still images are shown in rapid succession. (Think of drawing stick figures on the edges of book pages and then flipping the pages to see your figure “move.”) The brain interprets this as movement. Even infants appear to perceive this kind of motion (Valenza, Leo, Gava, & Simion, 2006). Although illusions provide clues about how perception works, they cannot explain everything. Let’s take a look at principles of perceptual organization, which provide universal guidelines on how we perceive our surroundings.

LO 13 Identify the principles of perceptual organization.

Many psychologists turn to the Gestalt (gə-ˈstält) psychologists to understand how perception works. Gestalt psychologists, who were active in Germany in the late 1800s and early 1900s, became interested in perception after observing illusions of motion. They wondered how stationary objects could be perceived as moving and thus attempted to explain how the human mind organizes stimuli in the environment. Noting the tendency for perception to be organized and complete, they realized that the whole is greater than the sum of its parts; the brain naturally organizes stimuli in their entirety rather than perceiving the parts and pieces. Gestalt means “whole” or “form” in German. The Gestalt psychologists, and others who followed, studied the principles that explain how the brain perceives objects in the environment as wholes or groups (Infographic 3.4, below).

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One central idea illuminated by the Gestalt psychologists is the figure-ground principle (Fowlkes, Martin, & Malik, 2007). As you focus your attention on a figure (for example, the vase at right and in Infographic 3.4), all other features drop into the background. If, however, you direct your gaze onto the faces, the vase falls into the background with everything else. The figure and ground continually change as you shift your focus. Other gestalt organizational principles include the following:

Although these organizational principles have been demonstrated for vision, it is important to note that they apply to the other senses as well. Imagine a mother who can discern her child’s voice amid the clamor of a busy playground: Her child’s voice is the figure and the other noises are the ground.

LO 14 Identify concepts involved in depth perception.

That same mother can tell that the slide extends from the front of the jungle gym, thanks to her brain’s ability to pick up on cues about depth. How can a two-dimensional image projected on the retina be perceived in three dimensions? There appear to be inborn abilities and learned cues for perceiving depth and distance. Let’s start by examining one inborn ability to recognize depth and its potential for danger. Watch out, baby.

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VISUAL CLIFF In order to determine whether depth perception is innate or learned, researchers studied the behavior of babies approaching a “visual cliff,” a flat, clear glass surface with a checkered tablecloth-like pattern directly underneath that gives the illusion of a drop-off (Gibson & Walk,1960). The researchers placed infants ages 6 to 14 months at the edge of the glass. At the other end of the glass were the babies’ mothers, coaxing them to crawl over. For the most part, the children refused to move toward what they perceived to be a drop-off, suggesting that the perception of depth was innate. Without ever having explored the edge of an actual abyss, these infants seemed instinctively to know it should be avoided.

BINOCULAR CUES Some cues for perceiving depth are the result of information gathered by both eyes. Binocular cues provide information from the right and left eyes to help judge depth and distance. For example, convergence is the brain’s interpretation of the tension (or lack thereof) in the eye muscles that direct where both eyes focus. If an object is close, there is more muscular tension (as the muscles turn our eyes inward toward our nose), and the object is perceived as near. This perception of distance is based on experience; throughout life, we have learned that the more muscular tension, the closer an object. Let’s take a moment and feel that tension.

Another binocular cue is retinal disparity, which is the difference between the images seen by the right and left eyes. The greater the difference, the closer the object. The more similar the two images, the farther the object. With experience, our brains begin using these image disparities to judge distance. The following Try This provides an example of retinal disparity.

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MONOCULAR CUES Judgments about depth and distance can also be informed by monocular cues, which do not necessitate the use of both eyes. An artist who paints pictures can transform a white canvas into a three-dimensional perceptual experience by using techniques that take advantage of monocular cues. The monocular cues include (but are not limited to) the following:

All these perceptual skills are great—if you are standing still—but the world around us is constantly moving. How do our perceptual systems adapt to changes? We possess the ability to perceive objects as having constant properties, although our environments are constantly changing. Perceptual constancy refers to the tendency to perceive objects as maintaining their shape, size, and color even when the angle, lighting, and distance change. A door is shaped like a rectangle, but when it opens, the image projected on our retina is not a rectangle. Yet, we still perceive the door as having a rectangular shape. This is called shape constancy. As you gaze on cars and trucks from the window of an airplane, they may look like bugs scurrying around, but you know some are as big as elephants because your perceptual toolkit includes size constancy. Through experience, we get to know the size of everyday objects and perceive them accordingly, regardless of whether they are far or near. Similarly, color constancy allows us to see the world in stable colors, even when the sensory data arriving at our photoreceptors change. A bright red backpack appears bright red outside in the sunshine or inside a house. The light waves bouncing off of the backpack have changed, but your understanding of the color has not been altered.

Although examples of these organizational tendencies occur with robust regularity, some of these perceptual skills are not necessarily universal, as the Müller-Lyer illusion demonstrates. Some studies suggest that children learn to perceive size constancy (Granrud, 2009), and this skill may not develop properly when children are deprived of certain types of stimuli.

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Learning also plays a role in the phenomenon of perceptual set—the tendency to perceive stimuli in a specific manner based on past experiences and expectations. If someone handed you a picture of two women and said, “This is a mother with her daughter,” you would be more likely to rate them as looking alike than if you had been given the exact same picture and told, “These women are unrelated.” Indeed, research shows that people who believe they are looking at parent–child pairs are more likely to rate the pairs as similar than those who believe the members of the pairs are not related (even though they are looking at the same adult–child pairs; Oda, Matsumoto-Oda, & Kurashima, 2005). In short, we tend to see what we are looking for.

Perceptual sets are molded by the context of the moment. If you hear the word “sects” in a TV news report about feuding religious groups, you are unlikely to think the reporter is saying “sex” even though “sects” and “sex” sound the same. Similarly, if you see a baby swaddled in a blue blanket, you are probably more likely to assume it’s a boy than a girl. Studies show that expectancies about male or female infants influence viewers’ perceptions of gender (Stern & Karraker, 1989). Simply labeling infants as premature or full-term will shape people’s attitudes about them (Porter, Stern, & Zak-Place, 2009).

Although our perceptual sets may not always lead to the right conclusions, they do help us organize the vast amount of information flooding our senses. But can perception exist in the absence of sensation? In other words, is it possible to perceive something without seeing, hearing, smelling, tasting, or feeling it?

LO 15 Define extrasensory perception and explain why psychologists dismiss claims of its legitimacy.

Have you ever had a dream that actually came to pass? Perhaps you have heard of psychics who claim that they can read people’s “auras” and decipher their thoughts. Extrasensory perception (ESP) is this purported ability to obtain information about the world without any sensory stimuli. And when we say “no sensory input,” we are not referring to sights, sounds, smells, tastes, and feelings occurring subliminally, or below absolute thresholds. We mean absolutely no measurable sensory data. The study of these kinds of phenomena is called parapsychology.

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More interesting than ESP, we would argue, are the remarkable (and real) stories of people who thrive despite difficulties with sensation and perception. Are you wondering what became of Emma, Zoe, and Sophie?

Question 1

Question 2

1. convergence

2. retinal disparity

3. interposition

4. relative size

Question 3

Question 4

1. perceptual set

2. perceptual constancy

3. convergence

4. texture-gradient