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Philosophers have debated whether our perceptual abilities should be credited to our nature or our nurture. To what extent do we learn to perceive? German philosopher Immanuel Kant (1724–1804) maintained that knowledge comes from our inborn ways of organizing sensory experiences. Indeed, we come equipped to process sensory information. But British philosopher John Locke (1632–1704) argued that through our experiences we also learn to perceive the world. Indeed, we learn to link an object’s distance with its size. So, just how important is experience? How radically does it shape our perceptual interpretations?

17-9 What does research on restored vision, sensory restriction, and perceptual adaptation reveal about the effects of experience on perception?

RESTORED VISION AND SENSORY RESTRICTION Writing to John Locke, William Molyneux wondered whether “a man born blind, and now adult, taught by his touch to distinguish between a cube and a sphere” could, if made to see, visually distinguish the two. Locke’s answer was No, because the man would never have learned to see the difference.

Molyneux’s hypothetical case has since been put to the test with a few dozen adults who, though blind from birth, later gained sight (Gregory, 1978; Huber et al., 2015; von Senden, 1932). Most were born with cataracts—clouded lenses that allowed them to see only diffused light, rather as you might see a foggy image through a Ping-Pong ball sliced in half. After cataract surgery, the patients could distinguish figure from ground and could sense colors—suggesting that these aspects of perception are innate. But much as Locke supposed, they often could not visually recognize objects that were familiar by touch.

Seeking to gain more control than is provided by clinical cases, researchers have outfitted infant kittens and monkeys with goggles through which they could see only diffuse, unpatterned light (Wiesel, 1982). After infancy, when the goggles were removed, these animals exhibited perceptual limitations much like those of humans born with cataracts. They could distinguish color and brightness, but not the form of a circle from that of a square. Their eyes had not degenerated; their retinas still relayed signals to their visual cortex. But lacking stimulation, the cortical cells had not developed normal connections. Thus, the animals remained functionally blind to shape. Experience guides, sustains, and maintains the brain’s neural organization that enables our perceptions.

In both humans and animals, similar sensory restrictions later in life do no permanent harm. When researchers cover the eye of an adult animal for several months, its vision will be unaffected after the eye patch is removed. When surgeons remove cataracts that develop during late adulthood, most people are thrilled at the return to normal vision.

The effect of sensory restriction on infant cats, monkeys, and humans suggests that for normal sensory and perceptual development there is a critical period—an optimal period when exposure to certain stimuli or experiences is required. Surgery on blind children in India reveals that children blind from birth can benefit from removal of cataracts. The younger they are, the more they will benefit, but their visual acuity (sharpness) may never be normal (Sinha, 2013). Early nurture sculpts what nature has endowed. In less dramatic ways, it continues to do so throughout our lives. Our visual experience matters.

PERCEPTUAL ADAPTATION Given a new pair of glasses, we may feel slightly disoriented, even dizzy. Within a day or two, we adjust. Our perceptual adaptation to changed visual input makes the world seem normal again. But imagine a far more dramatic new pair of glasses—one that shifts the apparent location of objects 40 degrees to the left. When you first put them on and toss a ball to a friend, it sails off to the left. Walking forward to shake hands with someone, you veer to the left.

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Could you adapt to this distorted world? Not if you were a baby chicken. When fitted with such lenses, baby chicks continue to peck where food grains seem to be (Hess, 1956; Rossi, 1968). But we humans adapt to distorting lenses quickly. Within a few minutes your throws would again be accurate, your stride on target. Remove the lenses and you would experience an aftereffect: At first your throws would err in the opposite direction, sailing off to the right; but again, within minutes you would readapt.

Indeed, given an even more radical pair of glasses—one that literally turns the world upside down—you could still adapt. Psychologist George Stratton (1896) experienced this. He invented, and for eight days wore, optical headgear that flipped left to right and up to down, making him the first person to experience a right-side-up retinal image while standing upright. The ground was up, the sky was down.

At first, when Stratton wanted to walk, he found himself searching for his feet, which were now “up.” Eating was nearly impossible. He became nauseated and depressed. But he persisted, and by the eighth day he could comfortably reach for an object in the right direction and walk without bumping into things. When Stratton finally removed the headgear, he readapted quickly.

In later experiments, people wearing the optical gear have even been able to ride a motorcycle, ski the Alps, and fly an airplane (Dolezal, 1982; Kohler, 1962). The world around them still seemed above their heads or on the wrong side. But by actively moving about in these topsy-turvy worlds, they adapted to the context and learned to coordinate their movements.