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How do we create meaning from the blizzard of sensory stimuli that bombards our body 24 hours a day? Meanwhile, in a silent, cushioned, inner world, our brain floats in utter darkness. By itself, it sees nothing. It hears nothing. It feels nothing. So, how does the world out there get in? To phrase the question scientifically: How do we construct our representations of the external world? How do a campfire’s flicker, crackle, and smoky scent activate neural connections? And how, from this living neurochemistry, do we create our conscious experience of the fire’s motion and temperature, its aroma and beauty? In search of answers, let’s examine the basics of sensation and perception, and look at some processes that cut across all our sensory systems.

6-1 What are sensation and perception? What do we mean by bottom-up processing and top-down processing?

Heather Sellers’ curious mix of “perfect vision” and face blindness illustrates the distinction between sensation and perception. When she looks at a friend, her sensation is normal: Her sensory receptors detect the same information yours would, and her nervous system transmits that information to her brain. Her perception—the processes by which her brain organizes and interprets sensory input—is almost normal. Thus, she may recognize people from their hair, gait, voice, or particular physique, just not their face. Her experience is much like the struggle any person would have trying to recognize a specific penguin.

In our everyday experiences, sensation and perception blend into one continuous process.

As our brain absorbs the information in FIGURE 6.1, bottom-up processing enables our sensory systems to detect the lines, angles, and colors that form the flower and leaves. Sensation is the world’s gateway to the brain. Using top-down processing we then interpret what our senses detect.

6-2 What three steps are basic to all our sensory systems?

Every second of every day, our sensory systems perform an amazing feat: They convert one form of energy into another. Vision processes light energy. Hearing processes sound waves. All our senses

The process of converting one form of energy into another that our brain can use is called transduction. Later in this chapter, we’ll focus on individual sensory systems. How do we see? Hear? Feel pain? Taste? Smell? Keep our balance? In each case, one of our sensory systems receives, transforms, and delivers the information to our brain.

Let’s explore some strengths and weaknesses in our ability to detect and interpret stimuli in the vast sea of energy around us.

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6-3 How do absolute thresholds and difference thresholds differ, and what effect, if any, do stimuli below the absolute threshold have on us?

At this moment, we are being struck by X-rays and radio waves, ultraviolet and infrared light, and sound waves of very high and very low frequencies. To all of these we are blind and deaf. Other animals with differing needs detect a world that lies beyond our experience. Migrating birds stay on course aided by an internal magnetic compass. Bats and dolphins locate prey using sonar, bouncing echoing sound off objects. Bees navigate on cloudy days by detecting invisible (to us) polarized light.

The shades on our own senses are open just a crack, allowing us a restricted awareness of this vast sea of energy. But for our needs, this is enough.

Absolute Thresholds

To some kinds of stimuli we are exquisitely sensitive. Standing atop a mountain on an utterly dark, clear night, most of us could see a candle flame atop another mountain 30 miles away. We could smell a single drop of perfume in a three-room apartment. We could feel the wing of a bee falling on our cheek (Galanter, 1962).

German scientist and philosopher Gustav Fechner (1801–1887) studied our awareness of these faint stimuli and called them our absolute thresholds—the minimum stimulation necessary to detect a particular light, sound, pressure, taste, or odor 50 percent of the time. To test your absolute threshold for sounds, a hearing specialist would expose each of your ears to varying sound levels (FIGURE 6.2). For each tone, the test would define where half the time you could detect the sound and half the time you could not. That 50-50 point would define your absolute threshold.

Detecting a weak stimulus, or signal (such as a hearing-test tone), depends not only on its strength but also on our psychological state—our experience, expectations, motivation, and alertness. Signal detection theory predicts when we will detect weak signals (measured as our ratio of “hits” to “false alarms”). Lonely people at speed-dating events often respond to potential dates unselectively—with a low threshold (McClure et al., 2010). Signal detection theorists seek to understand why people respond differently to the same stimuli, and why the same person’s reactions vary as circumstances change.

Stimuli you cannot consciously detect 50 percent of the time are subliminal—below your absolute threshold (see FIGURE 6.2). Under certain conditions, you can still be affected by stimuli so weak that you don’t consciously notice them. An unnoticed image or word can reach your visual cortex and briefly prime your response to a later question. In a typical experiment, the image or word is quickly flashed, then replaced by a masking stimulus that interrupts the brain’s processing before conscious perception (Herring et al., 2013; Van den Bussche et al., 2009). In one such experiment, researchers monitored brain activity as they primed people with either unperceived action words (such as go and start) or inaction words (such as still and stop). Without any conscious awareness, the inaction words automatically evoked brain activity associated with inhibiting behavior (Hepler & Albarracin, 2013).

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Another priming experiment illustrated the deep reality of sexual orientation. As people gazed at the center of a screen, a photo of a nude person was flashed on one side and a scrambled version of the photo on the other side (Jiang et al., 2006). Because the nude images were immediately masked by a colored checkerboard, viewers consciously perceived nothing but flashes of color and so were unable to state on which side the nude had appeared. To test whether this unseen image had unconsciously attracted their attention, the experimenters then flashed a geometric figure to one side or the other. This, too, was quickly followed by a masking stimulus. When asked to give the figure’s angle, straight men guessed more accurately when it appeared where a nude woman had been a moment earlier (FIGURE 6.3). Gay men (and straight women) guessed more accurately when the geometric figure replaced a nude man. As other experiments confirm, we can evaluate a stimulus even when we are not consciously aware of it—and even when we are unaware of our evaluation (Ferguson & Zayas, 2009).

How can we feel or respond to what we do not know and cannot describe? An imperceptibly brief stimulus often triggers a weak response that can be detected by brain scanning (Blankenburg et al., 2003; Haynes & Rees, 2005, 2006). The stimulus may reach consciousness only when it triggers synchronized activity in multiple brain areas (Dehaene, 2009, 2014). Such experiments reveal the dual-track mind at work: Much of our information processing occurs automatically, out of sight, off the radar screen of our conscious mind. Our conscious minds are top-level executives who delegate routine tasks to lower-level mental assistants.

So can we be controlled by subliminal messages? For more on that question, see Thinking Critically About: Subliminal Persuasion.

Difference Thresholds

To function effectively, we need absolute thresholds low enough to allow us to detect important sights, sounds, textures, tastes, and smells. We also need to detect small differences among stimuli. A musician must detect minute discrepancies when tuning an instrument. Parents must detect the sound of their own child’s voice amid other children’s voices. I [DM] noticed while living two years in Scotland that sheep baas all sound alike to my ears. But not to those of ewes, who, after shearing, will streak directly to the baa of their lamb amid the chorus of other distressed lambs.

The difference threshold (or the just noticeable difference [jnd]) is the minimum difference a person can detect between any two stimuli half the time. That difference threshold increases with the size of the stimulus. If we listen to our music at 40 decibels, we might detect an added 5 decibels. But if we increase the volume to 110 decibels, we probably won’t detect a 5-decibel change. In the late 1800s, Ernst Weber described this with a principle so simple and so widely applicable that we still refer to it as Weber’s law. This law states that for an average person to perceive a difference, two stimuli must differ by a constant minimum percentage (not a constant amount). The exact percentage varies, depending on the stimulus. Two lights, for example, must differ in intensity by 8 percent. Two objects must differ in weight by 2 percent. And two tones must differ in frequency by only 0.3 percent (Teghtsoonian, 1971).

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6-5 What is the function of sensory adaptation?

Entering your neighbors’ living room, you smell a musty odor. You wonder how they endure it, but within minutes you no longer notice it. Sensory adaptation has come to your rescue. When we are constantly exposed to an unchanging stimulus, we typically become less aware of it because our nerve cells fire less frequently. (To experience sensory adaptation, move your watch up your wrist an inch. You will feel it—but only for a few moments.)

Why, then, if we stare at an object without flinching, does it not vanish from sight? Because, unnoticed by us, our eyes are always moving (FIGURE 6.4). This continual flitting from one spot to another ensures that stimulation on the eyes’ receptors continually changes.

What if we actually could stop our eyes from moving? Would sights seem to vanish, as odors do? To find out, psychologists have devised ingenious instruments that maintain a constant image on the eye’s inner surface. Imagine that we have fitted a volunteer, Mary, with one of these instruments—a miniature projector mounted on a contact lens (FIGURE 6.5a). When Mary’s eye moves, the image from the projector moves as well. So everywhere that Mary looks, the scene is sure to go.

If we project images through this instrument, what will Mary see? At first, she will see the complete image. But within a few seconds, as her sensory system begins to fatigue, things get weird. Bit by bit, the image vanishes, only to reappear and then disappear—often in fragments (FIGURE 6.5b).

Although sensory adaptation reduces our sensitivity to constant stimulation, it offers an important benefit: freedom to focus on informative changes in our environment without being distracted by background chatter. Stinky or heavily perfumed people don’t notice their odor because, like you and me, they adapt to what’s constant and detect only change. Our sensory receptors are alert to novelty; bore them with repetition and they free our attention for more important things. The point to remember: We perceive the world not exactly as it is, but as it is useful for us to perceive it.

Our sensitivity to changing stimulation helps explain television’s attention-grabbing power. Cuts, edits, zooms, pans, sudden noises—all demand attention. The phenomenon is irresistible even to TV researchers. One noted that during conversations, “I cannot for the life of me stop from periodically glancing over to the screen” (Tannenbaum, 2002).

Sensory adaptation even influences how we perceive emotions. By creating a 50-50 morphed blend of an angry face and a scared face, researchers showed that our visual system adapts to a static facial expression by becoming less responsive to it (Butler et al., 2008; FIGURE 6.6). The effect is created by our brain, not our retinas. We know this because the illusion also works when we view either side image with one eye, and the center image with the other eye.

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Sensory adaptation and sensory thresholds are important ingredients in our perceptions of the world around us. Much of what we perceive comes not just from what’s “out there,” but also from what’s behind our eyes and between our ears.

6-6 How do our expectations, contexts, motivation, and emotions influence our perceptions?

To see is to believe. As we less fully appreciate, to believe is to see. Through experience, we come to expect certain results. Those expectations may give us a perceptual set—a set of mental tendencies and assumptions that affects, top-down, what we hear, taste, feel, and see.

Consider: Is the center image in FIGURE 6.7 below an old or young woman? What we see in such a drawing can be influenced by first looking at either of the two unambiguous versions (Boring, 1930).

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Everyday examples of perceptual set abound. In 1972, a British newspaper published unretouched photographs of a “monster” in Scotland’s Loch Ness—“the most amazing pictures ever taken,” stated the paper. If this information creates in you the same expectations it did in most of the paper’s readers, you, too, will see the monster in a similar photo in FIGURE 6.8. But when a skeptical researcher approached the original photos with different expectations, he saw a curved tree limb—as had others the day that photo was shot (Campbell, 1986). What a difference a new perceptual set makes.

Perceptual set can also affect what we hear. Consider the kindly airline pilot who, on a takeoff run, looked over at his sad co-pilot and said, “Cheer up.” Expecting to hear the usual “Gear up,” the co-pilot promptly raised the wheels—before they left the ground (Reason & Mycielska, 1982).

Perceptual set similarly affects taste. One experiment invited bar patrons to sample free beer (Lee et al., 2006). The tasters preferred the brand-name beer, even when researchers secretly added a few drops of vinegar to it—unless they had been told they were drinking vinegar-laced beer. Then they expected, and usually experienced, a worse taste. In another experiment, preschool children, by a 6-to-1 margin, thought french fries tasted better when served in a McDonald’s bag rather than a plain white bag (Robinson et al., 2007).

What determines our perceptual set? Through experience we form concepts, or schemas, that organize and interpret unfamiliar information. Our preexisting schemas for monsters and tree branches influence how we apply top-down processing to interpret ambiguous sensations.

In everyday life, stereotypes about gender (another instance of perceptual set) can color perception. Without the obvious cues of pink or blue, people will struggle over whether to call the new baby “he” or “she.” But told an infant is “David,” people (especially children) have perceived “him” as bigger and stronger than if the same infant was called “Diana” (Stern & Karraker, 1989). Some differences, it seems, exist merely in the eyes of their beholders.

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A given stimulus may trigger radically different perceptions, partly because of our differing perceptual set (FIGURE 6.9), but also because of the immediate context. Some examples:

Perceptions are also influenced, top-down, by our motivation and emotions.

Hearing sad rather than happy music can predispose people to perceive a sad meaning in spoken homophonic words—mourning rather than morning, die rather than dye, pain rather than pane (Halberstadt et al., 1996). After listening to irritating (and anger-cuing) music, people perceive a harmful action such as robbery as more serious (Seidel & Prinz, 2013). Dennis Proffitt (2006a, b; Schnall et al., 2008) and others have demonstrated the power of emotions with other clever experiments showing that

Motives also matter. Desired objects, such as a water bottle when thirsty, seem closer (Balcetis & Dunning, 2010). This perceptual bias energizes our going for it. Our motives also direct our perception of ambiguous stimuli.

Emotions and motives color our social perceptions, too. People more often perceive solitary confinement, sleep deprivation, and cold temperatures as “torture” when experiencing a small dose of such themselves (Nordgren et al., 2011). Spouses who feel loved and appreciated perceive less threat in stressful marital events—“He’s just having a bad day” (Murray et al., 2003). The moral of these stories: To believe is, indeed, to see.

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Learning Objectives

Test Yourself by taking a moment to answer each of these Learning Objective Questions (repeated here from within the chapter). Research suggests that trying to answer these questions on your own will improve your long-term memory of the concepts (McDaniel et al., 2009).

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Terms and Concepts to Remember

Test yourself on these terms.

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Experience the Testing Effect

Test yourself repeatedly throughout your studies. This will not only help you figure out what you know and don’t know; the testing itself will help you learn and remember the information more effectively thanks to the testing effect.

Question 6.1

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Question 6.7

Use to create your personalized study plan, which will direct you to the resources that will help you most in .