Chapter 06.pdf

chapter

Perceiving the World

THEME:

Perception is an active

process; perceptual

impressions are not always

accurate representations

of events.

Key Questions

What are perceptual constancies, and

what is their role in perception?

How is perception altered by attention,

motives, values, and expectations?

What basic principles do we use to group

sensations into meaningful patterns?

Is extrasensory perception possible?

How reliable are eyewitness reports?

How is it possible to see depth and judge

distance?

What effect does learning have on per-

ception?

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Perceiving the World

189

Preview

Murder!

One of your authors was in a supermarket when an 8-

year-old girl suddenly came running around a corner. She

looked back and screamed, “Stop! Stop! You’re killing him!

You’re killing my father!” Here is your author’s account of

what followed:

collar. Obviously, the girl had misunderstood what was

happening to her father. As a psychologist, I still ﬁ nd it

fascinating that her words so dramatically inﬂ uenced

what I saw.

In the last chapter we discussed sensation, the process

of bringing information into the nervous system. This

chapter is about perception, or how we assemble

sensations into meaningful patterns. As we perceive

events, the brain actively selects, organizes, and integrates

sensory information to construct a “picture” or model of

the world. This process is so automatic that it can take

a drastic misperception like that of your author to call

attention to it.

Perception creates faces, melodies, works of art, illusions,

and on occasion, “murders” out of the raw material of

sensation. Let’s see how this takes place.

I retraced the girl’s path and saw two men on the ﬂ oor,

struggling violently. The guy on top had his victim by

the throat. There was blood everywhere. It was a murder

in progress! As a witness, I would have told a jury what

I just told you, were it not for an important fact: No

murder ever took place that day at the supermarket.

When the police arrived, they quickly discovered that

the “guy on the bottom” had passed out and hit his

head. That caused the cut (actually quite minor) that

explained the “blood everywhere.” The “guy on top”

saw the ﬁ rst man fall and was trying to loosen his

Perceptual Constancies—

Taming an Unruly World

times at various distances. We call this size constancy: The per-

ceived size of an object remains the same, even though the size of

its image on the retina changes.

To perceive your hand accurately, you had to draw on past ex-

perience. Some perceptions are so basic they seem to be native (in-

born). An example is the ability to see a line on a piece of paper.

Likewise, even newborn babies show some evidence of size con-

stancy (Slater, Mattock, & Brown, 1990). However, many of our

perceptions are empirical, or based on prior experience. For in-

stance, cars, houses, and people look like toys when seen from an

unfamiliar perspective, such as from the top of a skyscraper. This

suggests that although some size constancy is innate, it is also af-

fected by learning.

In shape constancy the shape of an object remains stable, even

though the shape of its retinal image changes. You can demonstrate

shape constancy by looking at this page from directly overhead

What would it be like to have your vision restored after a lifetime

of blindness? In Chapter 5 we met Bob Edens, who, after 50 years

of blindness, was overjoyed by the vividness of colors. But a ﬁ rst

look at the world can also be disappointing. Newly sighted per-

sons must learn to identify objects; to read clocks, numbers, and

letters; and to judge sizes and distances (Senden, 1960). Learning

to “see” can be quite frustrating. For instance, a cataract patient

named Mr. S. B. had been blind since birth. After an operation re-

stored his sight at age 52, Mr. S. B. struggled to use his vision. At

ﬁ rst, he could judge distance only in familiar situations (Gregory,

2000). One day he was found crawling out of a hospital window

to get a closer look at trafﬁ c on the street. It’s easy to understand

his curiosity, but he had to be restrained. His room was on the

fourth ﬂ oor!

Why would Mr. S. B. try to crawl out of a fourth-story window?

Couldn’t he at least tell distance from the size of the cars? No, you

must be familiar with objects to use their size to judge distance.

Try holding your left hand a few inches in front of your nose and

your right hand at arm’s length. Your right hand should appear to

be about half the size of your left hand. Still, you know your right

hand did not suddenly shrink, because you have seen it many

Perception The mental process of organizing sensations into

meaningful patterns.

Size constancy The perceived size of an object remains

constant, despite changes in its retinal image.

Shape constancy The perceived shape of an object is

unaffected by changes in its retinal image.

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CHAPTER 6

Visual perception involves ﬁ nding meaningful patterns in complex stimuli.

If you look closely at this painting by the artist Yvaral, you will see that it is

entirely made up of small, featureless squares. An infant or newly sighted

person would see only a jumble of meaningless colors. But because the

squares form a familiar pattern, you should easily see Marilyn Monroe’s face.

(Or is that Madonna?)

Almost everyone’s family album has at least one photo like this. Extreme

viewing angles can make maintaining size constancy difﬁ cult, even for

familiar objects.

and then from an angle. Obviously, the page is rectangular, but

most of the images that reach your eyes are distorted. Yet although

the book’s image changes, your perception of its shape remains

constant. (For additional examples, see ● Figure 6.1.) On the high-

way, alcohol intoxication impairs size and shape constancy, adding

to the accident rate among drunk drivers (Farrimond, 1990).

Let’s say that you are outside in bright sunlight. Beside you, a

friend is wearing a gray skirt and a white blouse. Suddenly a cloud

shades the sun. It might seem that the blouse would grow dim-

mer, but it still appears to be bright white. This happens because

the blouse continues to reﬂ ect a larger proportion of light than

nearby objects. Brightness constancy refers to the fact that the

brightness of objects appears to stay the same

as lighting conditions change. However, this

holds true only if the blouse and other ob-

jects are all illuminated by the same amount

of light. You could make an area on your

friend’s gray skirt look whiter than the

shaded blouse by shining a bright spotlight

on the skirt.

To summarize, the energy patterns reach-

ing our senses are constantly changing, even

when they come from the same object. Size,

shape, and brightness constancy rescue us

from a confusing world in which objects

would seem to shrink and grow, change

shape as if made of rubber, and light up or

fade like neon lamps. Gaining these constan-

cies was only one of the hurdles Mr. S. B.

faced in learning to see. In the next section,

we will consider some others.

(a) (b)

● Figure 6.1 Shape constancy. (a) When a door is open, its image actually forms a trapezoid. Shape

constancy is indicated by the fact that it is still perceived as a rectangle. (b) With great effort you may be

able to see this design as a collection of ﬂ at shapes. However, if you maintain shape constancy, the distorted

squares strongly suggest the surface of a sphere. (From Spherescapes-1 by Scott Walter and Kevin McMahon, 1983.)

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191

Perceptual Organization—

Getting It All Together

1. Nearness. All other things being equal, stimuli that are near

one another tend to be grouped together (Kubovy & Hol-

combe, 1998). Thus, if three people stand near one another

and a fourth person stands 10 feet away, the adjacent three

will be seen as a group and the distant person as an outsider

(see ● Figure 6.3 a ).

2. Similarity. “Birds of a feather ﬂ ock together,” and stimuli that

are similar in size, shape, color, or form tend to be grouped to-

gether (see ● Figure 6.3 b ). Picture two bands marching side by

side. If their uniforms are different colors, the bands will be

seen as two separate groups, not as one large group.

3. Continuation, or continuity. Perceptions tend toward sim-

plicity and continuity. In ● Figure 6.3 c it is easier to visualize a

wavy line on a squared-off line than it is to see a complex row

of shapes.

4. Closure. Closure refers to the tendency to complete a ﬁ gure so

that it has a consistent overall form. Each of the drawings in

● Figure 6.3 d has one or more gaps, yet each is perceived as a

recognizable ﬁ gure. The “shapes” that appear in the two right

drawings in ● Figure 6.3 d are illusory ﬁ gures (implied shapes

that are not actually bounded by an edge or an outline). Even

young children see these shapes, despite knowing that they

are “not really there.” Illusory ﬁ gures reveal that our tendency

to form shapes—even with minimal cues—is powerful.

5. Contiguity. A principle that can’t be shown in ● Figure 6.3 is

contiguity, or nearness in time and space. Contiguity is often

responsible for the perception that one thing has caused an-

other (Michotte, 1963). A psychologist friend of ours demon-

strates this principle in class by knocking on his head with

one hand while knocking on a wooden table (out of sight)

with the other. The knocking sound is perfectly timed with

the movements of his visible hand. This leads to the irresist-

ible perception that his head is made of wood.

6. Common region. As you can see in ● Figure 6.3 e, stimuli that

are found within a common region or area tend to be seen as a

group (Palmer, 1992). On the basis of similarity and nearness,

the stars in ● Figure 6.3 e should be one group and the dots an-

other. However, the colored backgrounds deﬁ ne regions that

create three groups of objects (four stars, two stars plus two

dots, and four dots). Perhaps the principle of common region

explains why we tend to mentally group together people from

a particular country, state, province, or geographic region.

We have seen that Mr. S. B. had to learn to understand his visual

sensations. He was soon able to tell time from a large clock and

read block letters he had known only from touch. At a zoo, he

recognized an elephant from descriptions he had heard. However,

handwriting meant nothing to him for more than a year after he

regained sight, and many objects were meaningless until he

touched them. Thus, although Mr. S. B. had visual sensations, his

ability to perceive remained limited.

How are sensations organized into meaningful perceptions? The

simplest organization involves grouping some sensations into an

object, or ﬁ gure, that stands out on a plainer background. Figure-

ground organization is probably inborn, because it is the ﬁ rst

perceptual ability to appear after cataract patients regain sight. In

normal ﬁ gure-ground perception, only one ﬁ gure is seen. In re-

versible ﬁ gures, however, ﬁ gure and ground can be switched. In

● Figure 6.2 it is equally possible to see either a wineglass ﬁ gure

on a dark background or two face proﬁ les on a light background.

As you shift from one pattern to the other, you should get a clear

sense of what ﬁ gure-ground organization means.

Gestalt Principles

How do we separate a ﬁ gure from its background? The Gestalt

psychologists (see Chapter 1) studied this question in detail.

They concluded that even if you were seeing for the ﬁ rst time,

several factors would bring some order to your perceptions ( ● Fig-

ure 6.3).

To learn about how the principles that guide perceptual orga-

nization can be applied to practical problems, see “Designing for

Human Use.”

Brightness constancy The apparent (or relative) brightness of

objects remains the same as long as they are illuminated by the

same amount of light.

Figure-ground organization Part of a stimulus appears

to stand out as an object (ﬁ gure) against a less prominent

background (ground).

● Figure 6.2 A reversible Figure-ground design. Do you see two faces in

proﬁ le or a wineglass?

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CHAPTER 6

● Figure 6.3 How we organize

perceptions.

(a) Principle of nearness

Notice how differently a group of six

objects can be perceptually organized,

depending on their spacing.

(b) Principle of similarity

In these examples, organization

depends on similarity of color.

Similarity and nearness can

be combined to produce a new

organization.

This?

plus

This?

(d) Principle of closure

(e) Principle of common region

Clearly, the Gestalt principles shape our day-to-day percep-

tions, but so do learning and past experience. Take a moment and

look for the camouﬂ aged animal pictured in ● Figure 6.5. (Cam-

ouﬂ age patterns break up ﬁ gure-ground organization). If you had

never seen similar animals before, could you have located this

one? Mr. S. B. would have been at a total loss to ﬁ nd meaning in

such a picture.

In a way, we are all detectives, seeking patterns in what we

see. In this sense, a meaningful pattern represents a perceptual

hypothesis, or initial guess about how to organize sensations.

Have you ever seen a “friend” in the distance, only to have the

person turn into a stranger as you drew closer? Preexisting ideas

and expectations actively guide our interpretation of sensations

(Most et al., 2005).

The active nature of perception is perhaps most apparent for

ambiguous stimuli (patterns allowing more than one interpreta-

tion). If you look at a cloud, you may discover dozens of ways to

organize its contours into fanciful shapes and scenes. Even clearly

deﬁ ned stimuli may permit more than one interpretation. Look

at Necker’s cube in ● Figure 6.6 if you doubt that perception is an

active process. Visualize the top cube as a wire box. If you stare at

the cube, its organization will change. Sometimes it will seem to

project upward, like the lower left cube; other times it will project

downward. The difference lies in how your brain interprets the

same information. In short, we actively construct meaningful per-

ceptions; we do not passively record the events and stimuli

around us (Coren, Ward, & Enns, 2004).

In some instances, a stimulus may offer such conﬂ icting infor-

mation that perceptual organization becomes impossible. For ex-

ample, the tendency to make a three-dimensional object out of a

drawing is frustrated by the “three-pronged widget” ( ● Figure

6.7), an impossible ﬁ gure. Such patterns cannot be organized into

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