What happens when babies first open their eyes? Do they see the same sights, hear the same sounds, and smell the same smells as an adult does? Or is an infant's world, as William James once suggested, only a “blooming, buzzing confusion,” waiting to be organized by experience and learning? The truth lies somewhere between these two extremes.

In human beings, most basic sensory abilities and many perceptual skills are inborn or develop very early. Infants can distinguish salty from sweet and can discriminate among odors. They can distinguish a human voice from other sounds. They will startle to a loud noise and turn their heads toward its source, showing that they perceive sound as being localized in space. Many visual skills, too, are present at birth or develop shortly afterward. Human infants can discriminate sizes and colors very early, possibly even right away. They distinguish contrasts, shadows, and complex patterns after only a few weeks, and depth perception develops during the first few months.

Testing an infant's perception of depth requires considerable ingenuity. One classic procedure has been to place infants on a device called a visual cliff (Gibson & Walk, 1960). The “cliff” is a pane of glass covering a shallow surface and a deep one (see Figure6.18). Both surfaces are covered by a checkerboard pattern. The infant is placed on a board in the middle, and the child's mother tries to lure the baby across either the shallow side or the deep side. Babies only 6 months of age will crawl across the shallow side but will hesitate to crawl out over the “cliff,” suggesting that they have depth perception. Infants often spend a considerable amount of time at the edge of the “cliff,” exploring their options and investigating the situation (Adolph, Kretch, & LoBue, 2014). This would suggest that it's not fear that's keeping them from the deep side, but rather a curiosity to explorecoupled with a canny recognition that the circumstances are somehow not quite right for normal locomotion (Adolph, 2000; Adolph & Kretch, 2012).

Figure 6.18

A Cliff-Hanger

Although many perceptual abilities are inborn, experience also plays a vital role. If an infant misses out on certain experiences during a crucial window of time called a critical period, perception will be impaired. Innate abilities may not survive because cells in the nervous system deteriorate, change, or fail to form appropriate neural pathways.

One way to study critical periods is to see what happens when the usual perceptual experiences of early life fail to take place. To do this, researchers have studied animals whose sensory and perceptual systems are similar to our own, such as kittens. Like human infants, kittens are born with the visual ability to detect horizontal and vertical lines and other spatial orientations; at birth, kittens' brains are equipped with the same kinds of feature-detector cells that adult cats have. But if they are deprived of normal visual experience, these cells deteriorate or change and perception suffers (Crair, Gillespie, & Stryker, 1998; Hirsch & Spinelli, 1970). In one classic study, kittens were reared in darkness for 5 months after birth, but for several hours each day, they were put into a special cylinder that permitted them to see only vertical or horizontal lines and nothing else. Later, cats that were exposed only to vertical lines had trouble perceiving horizontal ones; they would bump into horizontal obstacles. Those exposed only to horizontal lines had trouble perceiving vertical ones; they would run to play with horizontal bars but not vertical ones (Blakemore & Cooper, 1970).

What about human beings? Because of the brain's impressive plasticity, some people who are unable to see or hear until middle childhood or even adulthood can regain enough perceptual ability to get along fine in daily life (Ostrovsky, Andalman, & Sinha, 2006; Šikl et al., 2013). However, their perception is unlikely to fully recover. When adults who have been blind from infancy have their vision restored, most of them do not see well. Areas in the brain normally devoted to vision may have taken on different functions when these people were blind. As a result, their depth perception may be poor, causing them to trip constantly (Ostrovsky et al., 2009). They cannot always make sense of what they see; to identify objects, they may have to touch or smell them. They may have trouble recognizing faces and emotional expressions. They may even lack size constancy and need to remind themselves that people walking away from them are not shrinking in size (Fine et al., 2003). Generally, the best recoveries occur when an infant's congenital blindness is corrected early, probably because a critical period for visual development occurs in infancy or early childhood.

Similar findings apply to hearing. When adults who were born deaf, or who lost their hearing before learning to speak, receive cochlear implants (devices that stimulate the auditory nerve and allow auditory signals to travel to the brain), they tend to find sounds confusing. They are unable to learn to speak normally, and sometimes they ask to have the implants removed. But cochlear implants are more successful in children and in adults who became deaf late in life (Rauschecker, 1999). Young children presumably have not yet passed through the critical period for processing sounds, and older adults have already had years of auditory experience.

In sum, our perceptual powers are both inborn and dependent on experience. Because neurological connections in infants' brains and sensory systems are not completely formed, their senses are far less acute than an adult's. It takes time and experience for their sensory abilities to fully develop. But an infant's world is clearly not the blooming, buzzing confusion that William James took it to be.

The fact that some perceptual processes appear to be innate does not mean that all people perceive the world in the same way. A camera doesn't care what it “sees.” A digital recorder doesn't ponder what it “hears.” But because we care about what we see, hear, taste, smell, and feel, psychological factors can influence what we perceive and how we perceive it. Here are a few of these factors:

1. Needs. When we need something, have an interest in it, or want it, we are especially likely to perceive it. That is why hungry people are faster than others at seeing words related to hunger when the words are flashed briefly on a screen (Radel & Clément-Guillotin, 2012; Wispé & Drambarean, 1953). People also tend to perceive objects that they want—a water bottle if they are thirsty, money they can win in a game, a personality test with favorable results—as being physically closer to them than objects they don't want or need. Some psychological scientists call these motivated misperceptions “wishful seeing” (Balcetis & Dunning, 2010).

2. Beliefs. What we hold to be true about the world can affect our interpretation of ambiguous sensory signals. Images that remind people of Jesus or Mary have been reported on walls, dishes, tortillas, and sandwiches; the Arabic script for “Allah” has been reported on fish scales, chicken eggs, and beans. Such images cause great excitement among those who believe that divine messages can be found on everyday objects. However, mundane events inevitably prove to be the explanation. An image of Jesus on a garage door in California attracted crowds until it turned out to be caused by two streetlights that merged the shadows of a bush and a “For Sale” sign in the yard.

   

3. Emotions. Emotions can also influence our interpretation of sensory information, as when a small child afraid of the dark sees a ghost instead of a robe hanging on the bedroom door. Pain, as we noted, is particularly intensified by negative emotions such as anxiety and sadness. Conversely, soldiers who are seriously wounded often deny being in much pain, even though they are alert and are not in shock. Their relief at being alive may offset the worry and fear that would otherwise make their pain worse (although the body's own pain-fighting mechanisms may also be involved). Interestingly, when people perceive their pain as resulting from another person's malicious intent (e.g., they think the other person intentionally stepped on their toe), they feel the hurt more than they would if they thought it was simply due to a clumsy accident (Gray & Wegner, 2008).

4. Expectations. Previous experiences affect how we perceive the world. The tendency to perceive things in a certain way based on your expectations is called a perceptual set. Such sets can come in handy, helping us fill in words in sentences when we haven't really heard every one. But perceptual sets can also cause misperceptions, as you can see in the illustration here.

   

   Expectations can even reduce our reactions to stimuli that would otherwise be unpleasant, such as the universally obnoxious sound of fingernails scratching a chalkboard (human ears have a physiological sensitivity to sounds in that frequency range). In one study, people rated the unpleasantness of such sounds while they were wired up to devices that measured physiological markers of stress, including heart rate and sweating. Some people were told what the sounds really were; others were told that the sounds were from musical compositions. Those who thought the sounds were “music” found them less unpleasant than people who knew that the sounds were fingernails scraping, even though both groups showed similar stress responses (Reuter & Oehler, 2011).

As we saw earlier in our discussion of selective attention, even when people are oblivious to speech sounds, they are processing and recognizing those sounds at some level. Much of our perception occurs without our conscious awareness, and the things we see, hear, touch, or even smell without conscious awareness may nonetheless influence our behavior.

Behavior can be affected even by stimuli that are so weak or brief that they are below a person's absolute threshold for detecting them—that is, subliminal. People sometimes correctly sense a change in a scene (say, in the color or location of an object) even though the change took place too quickly to be consciously recognized and identified (Rensink, 2004). And when people are subliminally exposed to a face, they will later tend to prefer that face over one they did not “see” in this way (Bornstein, Leone, & Galley, 1987).

Thus, people often know more than they know they know. However, even in the laboratory, where researchers have considerable control, subliminal perception can be difficult to demonstrate and replicate. The strongest evidence for its existence comes from studies using simple stimuli (faces or single words such as bread) rather than complex stimuli such as sentences (“Eat whole-wheat bread, not white bread, if you know what's good for you”).

If subliminal exposure to stimuli can affect judgments and preferences in the laboratory, you may be wondering whether it can be used to manipulate people's attitudes and behavior in ordinary life. In the 1950s an advertising executive claimed to have increased popcorn and Coke sales at a theater by secretly flashing the words EAT POPCORN and DRINK COKE on the movie screen. The claim turned out to be a hoax, devised to save the man's struggling advertising company.

Ever since, scientists have been skeptical, but that has not deterred people who market subliminal recordings that promise to help you lose weight, stop smoking, relieve stress, or boost your motivation, all without any effort on your part. Ah, if only those claims were true! But they are not. In study after study, placebo recordings, which do not contain the messages that participants think they do, have been just as “effective” as those containing the supposed subliminal messages (Eich & Hyman, 1992; Merikle & Skanes, 1992; Moore, 1992, 1995). In one typical experiment, people listened to recordings labeled “memory” or “self-esteem,” but some heard recordings that were incorrectly labeled. About half showed improvement in the area specified by the label whether or not it was correct; the improvement was due to expectations alone (Greenwald et al., 1991).

However, previous efforts at subliminal persuasion may have left out an important ingredient: the person's motivation. A team of researchers used subliminal messages—the words thirst and dry—to make participants feel thirsty and incline them to drink. Later, when given a chance to drink, these people did in fact drink more than those in a control group did, though only if they had been moderately thirsty to begin with (Strahan, Spencer, & Zanna, 2002).

Does this mean that advertisers can seduce us into buying soft drinks or voting for political candidates by slipping subliminal slogans and images into what we watch and hear? Given the many studies that have found no evidence of subliminal persuasion in real life and the subtlety of the effects that occur in the laboratory (e.g., you have to be somewhat thirsty already to be influenced to drink more), we think there's little cause for worry about subliminal manipulation.