Chapter 6: Perception
At any moment we are conscious of a very limited amount of all that we are capable of experiencing. One example of this selective attention is the cocktail party effect—attending to only one voice among many. Another example is inattentional blindness, which refers to our blocking of a brief visual interruption when focusing on other sights.
Visual and auditory illusions were fascinating scientists even as psychology emerged. Explaining illusions required an understanding of how we transform sensations into meaningful perceptions, so the study of perception became one of psychology’s first concerns. Conflict between visual and other sensory information is usually resolved with the mind’s accepting the visual data, a tendency known as visual capture.
From a top-down perspective, we see how we transform sensory information into meaningful perceptions when we are aided by knowledge and expectations.
The early Gestalt psychologists were impressed with the seemingly innate way we organize fragmentary sensory data into whole perceptions. Our minds structure the information that comes to us in several demonstrable ways:
To recognize an object, we must first perceive it (see it as a figure) as distinct from its surroundings (the ground). We must also organize the figure into a meaningful form. Several Gestalt principles—proximity, similarity, continuity, connectedness, and closure—describe this process.
Research on the visual cliff revealed that many species perceive the world in three dimensions at, or very soon after, birth. We transform two-dimensional retinal images into three-dimensional perceptions by using binocular cues, such as retinal disparity, and monocular cues, such as the relative sizes of objects.
Our brain computes motion as objects move across or toward the retina. Large objects appear to move more slowly than smaller objects. A quick succession of images, as in a motion picture or on a lighted sign, can also create an illusion of movement.
Having perceived an object as a coherent figure and having located it in space, how then do we recognize it—despite the varying images that it may cast on our retinas? Size, shape, and lightness constancies describe how objects appear to have unchanging characteristics regardless of their distance, shape, or motion. These constancies explain several of the well-known visual illusions. For example, familiarity with the size-distance relationships in a carpentered world of rectangular shapes makes people more susceptible to the Müller-Lyer illusion.
The most direct tests of the nature-nurture issue come from experiments that modify human perceptions.
Sensory Deprivation and Restored Vision
For many species, infancy is a critical period during which experience must activate the brain’s innate visual mechanisms. If cataract removal restores eyesight to adults who were blind from birth, they remain unable to perceive the world normally. Generally, they can distinguish figure from ground and can perceive colors, but they are unable to recognize shapes and forms. In controlled experiments, animals have been reared with severely restricted visual input. When their visual exposure is returned to normal, they, too, suffer enduring visual handicaps.
Human vision is remarkably adaptable. Given glasses that shift the world slightly to the left or right, or even turn it upside down, people manage to adapt their movements and, with practice, to move about with ease.
Clear evidence that perception is influenced by our experience—our learned assumptions and beliefs—as well as by sensory input comes from the many demonstrations of perceptual set and context effects. The schemas we have learned help us to interpret otherwise ambiguous stimu