RESULTADOS

CONSTANCIES

When a closed door suddenly swings open, it casts a different image on our retina, but we still perceive it as a door. Our perceptual hypothesis remains the same. Were it not for perceptual con-

stancies, which allow us to recognize familiar stimuli

under varying conditions, we would have to liter- ally rediscover what something is each time it ap- peared under different conditions. Thus, you can recognize a tune even if it is played in a different octave, as long as the relations among its notes are maintained. You can detect the flavor of a particu- lar spice even when it occurs in foods having very different tastes.

In vision, several constancies are important. Shape constancy allows us to recognize people and other objects from many different angles, as in the

Focus 23

What factors account for shape, brightness, and size constancy in vision?

(a) (b)

FIGURE 5.37

Reversible perceptions. Here are

two examples of how the same stimulus can give rise to different perceptions. (a) After viewing this comic strip panel by Gustave Verbeek, turn it upside down for a different spin on the story. (b) Stare at this Necker cube for a while; the front of the cube will suddenly become the back, and it will appear as if you’re viewing the cube from a different angle.

FIGURE 5.38

Who’s bigger? Size constancy based on distance cues causes us to perceive the person in the background

as being of normal size. When the same stimulus is seen in the absence of the distance cues, size constancy breaks down. The two men no longer look similar in size, nor do the photographic images of the man in the blue shirt.

What Do You Think?

WHY DOES THAT RISING MOON LOOK SO BIG?

Just before bedding down for the night on a back- packing trip, a friend of ours poked his head outside of his tent and gasped to his wife, “Look at the moon! Just look at that moon!” Indeed, a bright-red full moon had just come over the horizon, and it was so enor- mous that it dwarfed the mammoth peaks surrounding them. The couple gazed at it in wonder for a few min- utes and then retired into their tent. Later that night, they looked outside again and were puzzled to see a rather small, ordinary full moon approaching the zenith.

You too may have exclaimed over the size of a rising moon, only to notice later that the moon, well above the horizon, seemed to have shrunk. What can explain this phenomenon? Think about it, then see page 168.

I N R E V I E W

 Perception involves both bottom-up processing, in which individual stimulus fragments are combined into a perception, and top-down processing, in which existing knowledge and perceptual schemas are applied to interpret stimuli.  Attention is an active process in which we focus

on certain stimuli while blocking out others. We cannot attend completely to more than one thing at a time, but we are capable of rapid attentional shifts. Inattentional blindness refers to a failure to perceive certain stimuli when attending to other stimulus elements. Attentional processes are affected by the nature of the stimulus and by personal factors such as motives and interests. The perceptual system appears to be especially vigilant to stimuli that denote threat or danger.

 Gestalt psychologists identified a number of prin- ciples of perceptual organization, including figure- ground relations and the laws of similarity, proximity, closure, and continuity. Gregory sug- gested that perception is essentially a hypothesis about what a stimulus is, based on previous expe- rience and the nature of the stimulus.

 Perceptual sets involve a readiness to perceive stimuli in certain ways, based on our expecta- tions, assumptions, motivations, and current emo- tional state.

 Perceptual constancies allow us to recognize fa- miliar stimuli under changing conditions. In the visual realm, there are three constancies: shape, brightness, and size.

case of the swinging door. Perhaps you have had the experience of sitting up front and off to one side of the screen in a crowded movie theater. At first the picture probably looked distorted, but after a while your visual system corrected for the distortion and objects on the screen looked nor- mal again.

Because of brightness constancy, the relative brightness of objects remains the same under different conditions of illumination, such as full sunlight and shade. Brightness constancy occurs because the ratio of light intensity between an object and its surroundings is usually constant. The actual brightness of the light that illuminates an object does not matter, as long as the same light intensity illuminates both the object and its surroundings.

When we take off in an airplane, we know that the cars on the highway below are not shrinking and becoming the size of ants. Size constancy is the perception that the size of ob- jects remains relatively constant even though images on our retina change in size with varia- tions in distance. Thus, a man who is judged to be 6 feet tall when standing 5 feet away is not perceived to be 3 feet tall at a distance of 10 feet, even though the size of his image on the retina is reduced to half its original size (Figure 5.38).

PERCEPTION OF DEPTH,