LOQ LearningObjectiveQuestion

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 senses detect the same information yours would, and they transmit that information to her brain. And her perception—the processes by which her brain organizes and interprets the sensory input—is almost normal. Thus, she may recognize people from their hair, walk, voice, or peculiar build, just not from their face. Her experience is much like the struggle any human would have trying to recognize a specific penguin.

Under normal circumstances, your sensory and perceptual processes work together to help you decipher the world around you.

As your brain absorbs the information in FIGURE 5.1, bottom-up processing enables your sensory systems to detect the lines, angles, and colors that form the flower and leaves. Using top-down processing, you interpret what your senses detect.

But how do you do it? How do you create meaning from the blizzard of sensory stimuli bombarding your body 24 hours a day? Meanwhile, in a silent, cushioned, inner world, your 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 pathways in our brain? 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 look at some processes that cut across all our sensory systems.

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

The process of converting one form of energy into another form that your brain can use is 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, we’ll consider these three steps—receiving, transforming, and delivering the information to the brain.

First, though, let’s explore some strengths and weaknesses in our ability to detect and interpret stimuli in the vast sea of energy around 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 beyond our human experience. Migrating birds stay on course aided by an internal magnetic compass. Bats and dolphins locate prey using sonar, bouncing sounds off objects. Bees navigate on cloudy days by detecting aspects of sunlight we cannot see.

Our senses open the shades just a crack, giving us only a tiny glimpse of the energy around us. But for our needs, this is enough.

Absolute Thresholds

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

Our awareness of these faint stimuli illustrates our absolute thresholds—the minimum stimulation needed 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 send tones, at varying levels, into each of your ears and record whether you could hear each tone. The test results would show the point where, for any sound frequency, half the time you could detect the sound and half the time you could not. That 50-50 point would define your absolute threshold.

Stimuli you cannot detect 50 percent of the time are subliminal—below your absolute threshold (FIGURE 5.2). To consider their effects, see Thinking Critically About: Subliminal Sensation and 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. Many of life’s important decisions also depend on our ability to detect small differences among stimuli. A musician must detect tiny differences when tuning an instrument. Parents must detect the sound of their own child’s voice amid other children’s voices. Even after I [DM] had lived two years in Scotland, all lambs’ baas sounded alike to my ears. But not to their mother’s, as I observed. After shearing, each ewe would streak directly to the baa of her 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 two stimuli half the time. That detectable difference 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 then detect an additional 5-decibel change.

In the late 1800s, Ernst Weber noted something so simple and so useful that we still refer to it as Weber’s law. It 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).

Sitting down on the bus, you are overwhelmed by your seatmate’s heavy perfume. You wonder how she can stand it, but within minutes you no longer notice. Sensory adaptation has come to your rescue. When constantly exposed to an unchanging stimulus, we become less aware of it because our nerve cells fire less frequently. (To experience sensory adaptation, roll up your sleeve. 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. This continual flitting from one spot to another ensures that stimulation on the eyes’ receptors is always changing.

What if we actually could stop our eyes from moving? Would sights seem to vanish, as odors do? To find out, psychologists have designed clever instruments that maintain a constant image on the eye’s inner surface. Imagine that we have fitted a volunteer, Mary, with such an instrument—a miniature projector mounted on a contact lens (FIGURE 5.3a). 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 tire, things will get weird. Bit by bit, the image will vanish, only to reappear and then disappear—often in fragments (FIGURE 5.3b).

Although sensory adaptation reduces our sensitivity, it offers an important benefit. It frees us to focus on informative changes in our environment without being distracted by background chatter. The attention-grabbing power of changing stimulation helps explain why television’s cuts, edits, zooms, pans, and sudden noises are so hard to ignore. One TV researcher marveled at television’s ability to distract, even during interesting conversations: “I cannot for the life of me stop from periodically glancing over to the screen” (Tannenbaum, 2002).

Sensory adaptation influences our perceptions of emotions, too. By creating a 50-50 morphed blend of an angry and a scared face, researchers showed that our visual system adapts to an unchanging facial expression by becoming less responsive to that expression (Butler et al., 2008) (FIGURE 5.4). The effect is created by our brain, not by our retinas. How do 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.

The point to remember: Our sensory receptors are alert to novelty; bore them with repetition and they free our attention for more important things. We will see this principle again and again: We perceive the world not exactly as it is, but as it is useful for us to perceive it.

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. In 1972, a British newspaper published “the most amazing pictures ever taken”—of a lake “monster” in Scotland’s Loch Ness. If this information creates in you the same expectations it did in most of the paper’s readers, you, too, will see a monster in a similar photo in FIGURE 5.5. But when a skeptical researcher approached the photos with different expectations, he saw a curved tree limb—as had others the day the photo was shot (Campbell, 1986). What a difference a new perceptual set makes.

To believe is also to hear. Consider the kindly airline pilot who, on a takeoff run, looked over at his unhappy 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).

Our expectations can also influence our taste perceptions. In one 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). Another experiment invited bar patrons to sample free beer—regular beer and a well-known brand-name beer (Lee et al., 2006). The brand-name beer samples had secretly been doctored with a few drops of vinegar. Which beer did tasters prefer? The brand-name beer—except when they were told in advance that they would be drinking vinegar-laced beer. Then they expected, and usually experienced, a worse taste. In both cases, people’s past experiences (tastes they had enjoyed or not enjoyed) led them to form concepts, or schemas, that they then used to interpret new stimuli.

Perceptual set influences how we interpret stimuli. But our immediate context, and the motivation and emotion we bring to a situation, also affect our interpretations.

CONTEXT EFFECTS Social psychologist Lee Ross invited us to recall our own perceptions in different contexts: “Ever notice that when you’re driving you hate pedestrians, the way they saunter through the crosswalk, almost daring you to hit them, but when you’re walking you hate drivers?” (Jaffe, 2004).

Some other examples of the power of context:

MOTIVATION Motives give us energy as we work toward a goal. Like context, they can bias our interpretations of neutral stimuli. Consider these findings.

EMOTION Other clever experiments have demonstrated that emotions can shove our perceptions in one direction or another.

The moral of all these examples? 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. Through top-down processing, our experiences, assumptions, and expectations—and even our context, motivation, and emotions—can shape and color our views of reality.

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The processes we’ve discussed so far are features shared by all our sensory systems. Let’s turn now to the ways those systems are unique. We’ll start with our most prized and complex sense, vision.