Sensation and Perception - Cengage

Sensation and Perception

Cengage Learning

Cengage Learning

Chapter Contents

CHAPTER 1

WHY READ THIS BOOK?

THE PERCEPTUAL PROCESS The Stimulus Electricity Experience and Action Knowledge

DEMONSTRATION: Perceiving a Picture

HOW TO APPROACH THE STUDY OF PERCEPTION

Introduction to Perception

MEASURING PERCEPTION Description Recognition

METHOD: Recognition Detection METHOD: Determining the Absolute

Threshold METHOD: Determining the Difference

Threshold Magnitude Estimation

METHOD: Magnitude Estimation Search Other Methods of Measurement

SOMETHING TO CONSIDER:

Cengage Learning THRESHOLD MEASUREMENT

CAN BE INFLUENCED BY HOW A PERSON CHOOSES TO RESPOND

TEST YOURSELF 1.1

Think About It If You Want to Know More Key Terms Media Resources

VL VIRTUAL LAB

OPPOSITE PAGE Why are we able to perceive the forms, distances, colors, and lighting in this scene, even though it is a picture on a flat page? This is just one of the many questions about perception we will consider in this book.

Patrick Hyland Photography

VL The Virtual Lab icons direct you to specific animations and videos designed to help you visualize what you are reading about. The number beside each icon indicates the number of the clip you can access through your CD-ROM or your student website.

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Some Questions We Will Consider:

main problem first, and leave conscious experience until

Why should you read this book? (p. 4) How are your perceptions determined by processes that

you are unaware of? (p. 5) What is the difference between perceiving something

later.

The "science project" above is what this book is about. Our goal is to understand the human model, starting with the detectors--the eyes, ears, skin receptors, and receptors in the nose and mouth--and then moving on to the computer--

and recognizing it? (p. 8)

the brain. We want to understand how we sense things in

How can we measure perception? (p. 12)

the environment and interact with them. The paradox we face

in searching for this understanding is that although we still

I magine that you have been given the following hypothetical science project.

don't understand perception, perceiving is something that occurs almost effortlessly. In most situations, we simply open our eyes and see what is around us, or listen and hear sounds,

Science project:

without expending any particular effort.

Design a device that can locate, describe, and identify all

Because of the ease with which we perceive, many people

objects in the environment, including their distance

see perception as something that "just happens," and don't

from the device and their relationships to each other. In

see the feats achieved by our senses as complex or amaz-

addition, make the device capable of traveling from one

ing. "After all," the skeptic might say, "for vision, a picture

point to another, avoiding obstacles along the way.

of the environment is focused on the back of my eye, and

Extra credit: Make the device capable of having conscious experience, such as what people experience when they look out at a scene.

that picture provides all the information my brain needs to duplicate the environment in my consciousness." But the idea that perception is not complex is exactly what misled computer scientists in the 1950s and 1960s to propose that it would take only about a decade or so to create "perceiv-

Warning:

ing machines" that could negotiate the environment with

This project, should you decide to accept it, is extremely

humanlike ease. That prediction, made half a century ago,

difficult. It has not yet been solved by the best computer

has yet to come true, even though a computer defeated the

scientists, even though they have access to the world's

world chess champion in 1997. From a computer's point of

Cengage Learning most powerful computers.

Hint: Humans and animals have solved the problems above

view, perceiving a scene is more difficult than playing world championship chess.

In this chapter we will begin by introducing some basic principles to help us understand the complexities of

in a particularly elegant way. They use (1) two spheri-

perception. We will first consider a few practical reasons for

cal sensors called "eyes," which contain a light-sensitive

studying perception, then examine how perception occurs

chemical, to sense light; (2) two detectors on the sides

in a sequence of steps, and finally consider how to measure

of the head, which are fitted with tiny vibrating hairs

perception.

to sense pressure changes in the air; (3) small pressure

detectors of various shapes imbedded under the skin to

sense stimuli on the skin; and (4) two types of chemical detectors to detect gases that are inhaled and solids and

Why Read This Book?

liquids that are ingested.

The most obvious answer to the question "Why read this

Additional note:

book?" is that it is required reading for a course you are

Designing the detectors is just the first step in design-

taking. Thus, it is probably an important thing to do if

ing the system. An information processing system is

you want to get a good grade. But beyond that, there are

also needed. In the case of the human, this information

a number of other reasons for reading this book. For one

processing system is a "computer" called the brain, with

thing, the material will provide you with information that

100 billion active units and interconnections so com-

may be helpful in other courses and perhaps even your fu-

plex that they have still not been completely deciphered.

ture career. If you plan to go to graduate school to become

Although the detectors are an important part of the

a researcher or teacher in perception or a related area, this

project, the design of the computer is crucial, because

book will provide you with a solid background to build on.

the information that is picked up by the detectors needs

In fact, a number of the research studies you will read about

to be analyzed. Note that operation of the human sys-

were carried out by researchers who were introduced to the

tem is still not completely understood and that the best

field of perception by earlier editions of this book.

scientific minds in the world have made little progress

The material in this book is also relevant to future stud-

with the extra credit part of the problem. Focus on the

ies in medicine or related fields, since much of our discussion

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CHAPTER 1 Introduction to Perception

is about how the body operates. A few medical applications that depend on knowledge of perception are devices to restore perception to people who have lost vision or hearing, and treatments for pain. Other applications include robotic vehicles that can find their way through unfamiliar environments, speech recognition systems that can understand what someone is saying, and highway signs that are visible to drivers under a variety of conditions.

But reasons to study perception extend beyond the possibility of useful applications. Because perception is something you experience constantly, knowing about how it works is interesting in its own right. To appreciate why, consider what you are experiencing right now. If you touch the page of this book, or look out at what's around you, you might get the feeling that you are perceiving exactly what is "out there" in the environment. After all, touching this page puts you in direct contact with it, and it seems likely that what you are seeing is what is actually there. But one of the things you will learn as you study perception is that everything you see, hear, taste, feel, or smell is created by the mechanisms of your senses. This means that what you are perceiving is determined not only by what is "out there," but also by the properties of your senses. This concept has fascinated philosophers, researchers, and students for hundreds of years, and is even more meaningful now because of recent advances in our understanding of the mechanisms responsible for our perceptions.

Another reason to study perception is that it can help

Cengage you become more aware of the nature of your own percep-

tual experiences. Many of the everyday experiences that you take for granted--such as listening to someone talking, tasting food, or looking at a painting in a museum--can be appreciated at a deeper level by considering questions such as "Why does an unfamiliar language sound as if it is one continuous stream of sound, without breaks between words?" "Why do I lose my sense of taste when I have a cold?" and "How do artists create an impression of depth in a picture?" This book will not only answer these questions but will answer other questions that you may not have thought of, such as "Why don't I see colors at dusk?" and "How come the scene around me doesn't appear to move as I walk through it?" Thus, even if you aren't planning to become a physician or a robotic vehicle designer, you will come away from reading this book with a heightened appreciation of both the complexity and the beauty of the mechanisms responsible for your perceptual experiences, and perhaps even with an enhanced awareness of the world around you.

In one of those strange coincidences that occasionally happen, I received an e-mail from a student (not one of my own, but from another university) at exactly the same time that I was writing this section of the book. In her e-mail, "Jenny" made a number of comments about the book, but the one that struck me as being particularly relevant to the question "Why read this book?" is the following: "By reading your book, I got to know the fascinating processes that

take place every second in my brain, that are doing things I don't even think about." Your reasons for reading this book may turn out to be totally different from Jenny's, but hopefully you will find out some things that will be useful, or fascinating, or both.

The Perceptual Process

One of the messages of this book is that perception does not just happen, but is the end result of complex "behind the scenes" processes, many of which are not available to your awareness. An everyday example of the idea of behind-thescenes processes is provided by what's happening as you watch a play in the theater. While your attention is focused on the drama created by the characters in the play, another drama is occurring backstage. An actress is rushing to complete her costume change, an actor is pacing back and forth to calm his nerves just before he goes on, the stage manager is checking to be sure the next scene change is ready to go, and the lighting director is getting ready to make the next lighting change.

Just as the audience sees only a small part of what is happening during a play, your perception of the world around you is only a small part of what is happening as you perceive. One way to illustrate the behind-the-scenes processes involved in perception is by describing a sequence of steps, which we will call the perceptual process.

Learning The perceptual process, shown in Figure 1.1, is a sequence of processes that work together to determine our experience of and reaction to stimuli in the environment. We will consider each step in the process individually, but first let's consider the boxes in Figure 1.1, which divide the process into four categories: Stimulus, Electricity, Experience and Action, and Knowledge. Stimulus refers to what is out there in the environment, what we actually pay attention to, and what stimulates our receptors. Electricity refers to the electrical signals that are created by the receptors and transmitted to the brain. Experience and Action refers to our goal--to perceive, recognize, and react to the stimuli. Knowledge refers to knowledge we bring to the perceptual situation. This box is located above the other three boxes because it can have its effect at many different points in the process. We will consider each box in detail, beginning with the stimulus.

The Stimulus

The stimulus exists both "out there," in the environment, and within the person's body.

Environmental Stimuli and Attended Stimuli These two aspects of the stimulus are in the environment. The environmental stimulus is all of the things in our environment that we can potentially perceive. Consider,

The Perceptual Process

5

Knowledge

7 Perception

8 Recognition

9 Action

Experience and

action

6 Processing

1 Environmental stimulus

5 Transmission

Electricity

4 Transduction

Stimulus

2 Attended stimulus

3 Stimulus on the receptors

Figure 1.1 The perceptual

process. The steps in this process are arranged in a circle to emphasize that the process is dynamic and continually changing. See text for descriptions of each step in the process.

for example, the potential stimuli that are presented to Ellen, who is taking a walk in the woods. As she walks along

interesting pattern on the tree trunk at first, but suddenly realizes that what she at first took to be a patch of moss is

the trail she is confronted with a large number of stimuli actually a moth (Figure 1.2b). When Ellen focuses on this

(Figure 1.2a)--trees, the path on which she is walking, rus- moth, making it the center of her attention, it becomes the

Cengage Learning tling noises made by a small animal scampering through the

leaves. Because there is far too much happening for Ellen to take in everything at once, she scans the scene, looking from

attended stimulus. The attended stimulus changes from moment to moment, as Ellen shifts her attention from place to place.

one place to another at things that catch her interest.

When Ellen's attention is captured by a particularly dis- The Stimulus on the Receptors When Ellen

tinctive looking tree off to the right, she doesn't notice the focuses her attention on the moth, she looks directly at it,

1. Environmental stimulus

2. Attended stimulus

3. Stimulus on the receptors

Image of moth

(a) The woods

(b) Moth on tree

Retina (c) Image on Ellen's retina

Figure 1.2 (a) We take

the woods as the starting point for our description of the perceptual process. Everything in the woods is the environmental stimulus. (b) Ellen focuses on the moth, which becomes the attended stimulus. (c ) An image of the moth is formed on Ellen's retina.

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CHAPTER 1 Introduction to Perception

and this creates an image of the moth and its immediate surroundings on the receptors of her retina, a 0.4-mm-thick network of light-sensitive receptors and other neurons that line the back of the eye (Figure 1.2c). (We will describe the retina and neurons in more detail in Chapters 2 and 3.) This step is important because the stimulus--the moth--is transformed into another form--an image on Ellen's retina.

Because the moth has been transformed into an image, we can describe this image as a representation of the moth. It's not the actual moth, but it stands for the moth. The next steps in the perceptual process carry this idea of representation a step further, when the image is transformed into electricity.

Electricity

One of the central principles of perception is that everything we perceive is based on electrical signals in our nervous system. These electrical signals are created in the receptors, which transform energy from the environment (such as the light on Ellen's retina) into electrical signals in the nervous system--a process called transduction.

Transduction Transduction is the transformation of one form of energy into another form of energy. For example, when you touch the "withdrawal" button on an ATM machine, the pressure exerted by your finger is transduced into electrical energy, which causes a device that uses me-

Cengage chanical energy to push your money out of the machine.

Transduction occurs in the nervous system when energy in the environment--such as light energy, mechanical pressure, or chemical energy--is transformed into electrical energy. In our example, the pattern of light created on Ellen's

retina by the moth is transformed into electrical signals in thousands of her visual receptors (Figure 1.3a).

Transmission After the moth's image has been transformed into electrical signals in Ellen's receptors, these signals activate other neurons, which in turn activate more neurons (Figure 1.3b). Eventually these signals travel out of the eye and are transmitted to the brain. The transmission step is crucial because if signals don't reach the brain, there is no perception.

Processing As electrical signals are transmitted through Ellen's retina and then to the brain, they undergo neural processing, which involves interactions between neurons (Figure 1.3c). What do these interactions between neurons accomplish? To answer this question, we will compare how signals are transmitted in the nervous system to how signals are transmitted by your cell phone.

Let's first consider the phone. When a person says "hello" into a cell phone (right phone in Figure 1.4a), this voice signal is changed into electrical signals, which are sent out from the cell phone. This electrical signal, which represents the sound "hello," is relayed by a tower to the receiving cell phone (on the left), which transforms the signal into the sound "hello." An important property of cell phone transmission is that the signal that is received is the same as the signal that was sent.

The nervous system works in a similar way. The image

Learning of the moth is changed into electrical signals in the receptors, which eventually are sent out the back of the eye (Figure 1.4b). This signal, which represents the moth, is relayed through a series of neurons to the brain, which transforms this signal into a perception of the moth. Thus, with a cell

4. Transduction

5. Transmission

6. Processing

Light in

Electricity out

(a) Electricity created

(b) One neuron activates another (c) Interactions between neurons

Figure 1.3 (a) Transduction

occurs when the receptors create electrical energy in response to light. (b) Transmission occurs as one neuron activates the next one. (c) This electrical energy is processed through networks of neurons.

The Perceptual Process

7

Signal received (same as sent)

"Hello"

Signal sent "Hello"

Copy of stimulus (a)

Transmission

Stimulus

Signal in the brain (different than sent)

Perception (b)

Processing

Signal sent Stimulus

Figure 1.4 Comparison of signal

transmission by cell phones and the nervous system. (a) The sending cell phone on the right sends an electrical signal that stands for "hello." The signal that reaches the receiving cell phone on the left is the same as the signal sent. (b) The nervous system sends electrical signals that stand for the moth. The nervous system processes these electrical signals, so the signal responsible for perceiving the moth is different from the original signal sent from the eye.

Cengage Learning phone, electrical signals that represent a stimulus ("hello") mission, and processing is transformed into things we are

are transmitted to a receiver (another cell phone), and in the aware of--perceiving, recognizing, and acting on objects in

nervous system, electrical signals representing a stimulus the environment.

(the moth) are also transmitted to a receiver (the brain).

There are, however, differences between information Perception Perception is conscious sensory experi-

transmission in cell phones and in the nervous system. With ence. It occurs when the electrical signals that represent the

cell phones, the signal received is the same as the signal moth are transformed by Ellen's brain into her experience of

sent. The goal for cell phones is to transmit an exact copy of seeing the moth (Figure 1.5a). In the past, some accounts of

the original signal. However, in the nervous system, the sig- the perceptual process have stopped at this stage. After all,

nal that reaches the brain is transformed so that, although once Ellen sees the moth, hasn't she perceived it? The answer

it represents the original stimulus, it is usually very different to this question is yes, she has perceived it, but other things

from the original signal.

have happened as well--she has recognized the form as a

The transformation that occurs between the receptors "moth" and not a "butterfly," and she has taken action based

and the brain is achieved by neural processing, which hap- on her perception by walking closer to the tree to get a bet-

pens as the signals that originate in the receptors travel ter look at the moth. These two additional steps--recognition

through a maze of interconnected pathways between the re- and action--are behaviors that are important outcomes of

ceptors and the brain and within the brain. In the nervous the perceptual process.

system, the original electrical representation of the stimulus

that is created by the receptors is transformed by processing Recognition Recognition is our ability to place an

into a new representation of the stimulus in the brain. In object in a category, such as "moth," that gives it meaning

Chapter 2 we will describe how this transformation occurs.

(Figure 1.5b). Although we might be tempted to group per-

ception and recognition together, researchers have shown

Experience and Action

that they are separate processes. For example, consider the case of Dr. P., a patient described by neurologist Oliver

We have now reached the third box of the perceptual pro- Sacks (1985) in the title story of his book The Man Who Mis-

cess, where the "backstage activity" of transduction, trans- took His Wife for a Hat.

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CHAPTER 1 Introduction to Perception

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