The ability to detect and interpret the events that are occurring around us allows us to respond to
these stimuli appropriately (Gibson & Pick, 2000).  In most cases the system is successful, but
as you can see from the above example, it is not perfect. In this chapter we will discuss the
strengths and limitations of these capacities, focusing on both sensation—awareness resulting
from the stimulation of a sense organ, and perception—the organization and interpretation of
sensations. Sensation and perception work seamlessly together to allow us to experience the
world through our eyes, ears, nose, tongue, and skin, but also to combine what we are currently
learning from the environment with what we already know about it to make judgments and to
choose appropriate behaviors.
The study of sensation and perception is exceedingly important for our everyday lives because
the knowledge generated by psychologists is used in so many ways to help so many people.
Psychologists work closely with mechanical and electrical engineers, with experts in defense and
military contractors, and with clinical, health, and sports psychologists to help them apply this
knowledge to their everyday practices. The research is used to help us understand and better
prepare people to cope with such diverse events as driving cars, flying planes, creating robots,
and managing pain (Fajen & Warren, 2003). 
We will begin the chapter with a focus on the six senses of seeing, hearing,smelling, touching, tasting, and monitoring the body’s positions (proprioception). We will see
that sensation is sometimes relatively direct, in the sense that the wide variety of stimuli around
us inform and guide our behaviors quickly and accurately, but nevertheless is always the result of
at least some interpretation. We do not directly experience stimuli, but rather we experience
those stimuli as they are created by our senses. Each sense accomplishes the basic process
of transduction—the conversion of stimuli detected by receptor cells to electrical impulses that
are then transported to the brain—in different, but related, ways.
After we have reviewed the basic processes of sensation, we will turn to the topic of perception,
focusing on how the brain’s processing of sensory experience can not only help us make quick
and accurate judgments, but also mislead us into making perceptual and judgmental errors, such
as those that allowed the Chaser group to breach security at the APEC meeting.
Sensory Thresholds: What Can We Experience?
Humans possess powerful sensory capacities that allow us to sense the kaleidoscope of sights,
sounds, smells, and tastes that surround us. Our eyes detect light energy and our ears pick up
sound waves. Our skin senses touch, pressure, hot, and cold. Our tongues react to the molecules
of the foods we eat, and our noses detect scents in the air. The human perceptual system is wired
for accuracy, and people are exceedingly good at making use of the wide variety of information
available to them (Stoffregen & Bardy, 2001). 
In many ways our senses are quite remarkable. The human eye can detect the equivalent of a
single candle flame burning 30 miles away and can distinguish among more than 300,000
different colors. The human ear can detect sounds as low as 20 hertz (vibrations per second) and
as high as 20,000 hertz, and it can hear the tick of a clock about 20 feet away in a quiet room.
We can taste a teaspoon of sugar dissolved in 2 gallons of water, and we are able to smell one
drop of perfume diffused in a three-room apartment. We can feel the wing of a bee on our cheek
dropped from 1 centimeter above (Galanter, 1962).
Although there is much that we do sense, there is even more that we do not. Dogs, bats, whales,
and some rodents all have much better hearing than we do, and many animals have a far richer
sense of smell. Birds are able to see the ultraviolet light that we cannot (see Figure 4.3
"Ultraviolet Light and Bird Vision") and can also sense the pull of the earth’s magnetic field.
Cats have an extremely sensitive and sophisticated sense of touch, and they are able to navigate
in complete darkness using their whiskers. The fact that different organisms have different
sensations is part of their evolutionary adaptation. Each species is adapted to sensing the things
that are most important to them, while being blissfully unaware of the things that don’t matter.
Psychophysics is the branch of psychology that studies the effects of physical stimuli on sensory
perceptions and mental states. The field of psychophysics was founded by the German
psychologist Gustav Fechner (1801–1887), who was the first to study the relationship between
the strength of a stimulus and a person’s ability to detect the stimulus.
The measurement techniques developed by Fechner and his colleagues are designed in part to
help determine the limits of human sensation. One important criterion is the ability to detect very
faint stimuli. The absolute threshold of a sensation is defined as the intensity of a stimulus that
allows an organism to just barely detect it.
In a typical psychophysics experiment, an individual is presented with a series of trials in which
a signal is sometimes presented and sometimes not, or in which two stimuli are presented that are
either the same or different. Imagine, for instance, that you were asked to take a hearing test. On
each of the trials your task is to indicate either “yes” if you heard a sound or “no” if you did not.
The signals are purposefully made to be very faint, making accurate judgments difficult.
The problem for you is that the very faint signals create uncertainty. Because our ears are
constantly sending background information to the brain, you will sometimes think that you heard
a sound when none was there, and you will sometimes fail to detect a sound that is there. Your
task is to determine whether the neural activity that you are experiencing is due to the
background noise alone or is a result of a signal within the noise.
My Story of Posttraumatic Stress Disorder
It is a continuous challenge living with post-traumatic stress disorder (PTSD), and I’ve suffered from it for
most of my life. I can look back now and gently laugh at all the people who thought I had the perfect life. I
was young, beautiful, and talented, but unbeknownst to them, I was terrorized by an undiagnosed
debilitating mental illness.
Having been properly diagnosed with PTSD at age 35, I know that there is not one aspect of my life that has
gone untouched by this mental illness. My PTSD was triggered by several traumas, most importantly a
sexual attack at knifepoint that left me thinking I would die. I would never be the same after that attack. For
me there was no safe place in the world, not even my home. I went to the police and filed a report. Rape
counselors came to see me while I was in the hospital, but I declined their help, convinced that I didn’t need it.
This would be the most damaging decision of my life.
For months after the attack, I couldn’t close my eyes without envisioning the face of my attacker. I suffered
horrific flashbacks and nightmares. For four years after the attack I was unable to sleep alone in my house. I
obsessively checked windows, doors, and locks. By age 17, I’d suffered my first panic attack. Soon I became
unable to leave my apartment for weeks at a time, ending my modeling career abruptly. This just became a
way of life. Years passed when I had few or no symptoms at all, and I led what I thought was a fairly normal
life, just thinking I had a “panic problem.”
Then another traumatic event retriggered the PTSD. It was as if the past had evaporated, and I was back in
the place of my attack, only now I had uncontrollable thoughts of someone entering my house and harming
my daughter. I saw violent images every time I closed my eyes. I lost all ability to concentrate or even
complete simple tasks. Normally social, I stopped trying to make friends or get involved in my community. I
often felt disoriented, forgetting where, or who, I was. I would panic on the freeway and became unable to
drive, again ending a career. I felt as if I had completely lost my mind. For a time, I managed to keep it
together on the outside, but then I became unable to leave my house again.
Around this time I was diagnosed with PTSD. I cannot express to you the enormous relief I felt when I
discovered my condition was real and treatable. I felt safe for the first time in 32 years. Taking medication
and undergoing behavioral therapy marked the turning point in my regaining control of my life. I’m
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rebuilding a satisfying career as an artist, and I am enjoying my life. The world is new to me and not limited
by the restrictive vision of anxiety. It amazes me to think back to what my life was like only a year ago, and
just how far I’ve come.
For me there is no cure, no final healing. But there are things I can do to ensure that I never have to suffer as
I did before being diagnosed with PTSD. I’m no longer at the mercy of my disorder, and I would not be here
today had I not had the proper diagnosis and treatment. The most important thing to know is that it’s never
too late to seek help. (Philips, 2010) 
The topic of this chapter is learning—the relatively permanent change in knowledge or behavior
that is the result of experience. Although you might think of learning in terms of what you need
to do before an upcoming exam, the knowledge that you take away from your classes, or new
skills that you acquire through practice, these changes represent only one component of learning.
In fact, learning is a broad topic that is used to explain not only how we acquire new knowledge
and behavior but also a wide variety of other psychological processes including the development
of both appropriate and inappropriate social behaviors, and even how a person may acquire a
debilitating psychological disorder such as PTSD.
Learning is perhaps the most important human capacity. Learning allows us to create effective
lives by being able to respond to changes. We learn to avoid touching hot stoves, to find our way
home from school, and to remember which people have helped us in the past and which people
have been unkind. Without the ability to learn from our experiences, our lives would be
remarkably dangerous and inefficient. The principles of learning can also be used to explain a
wide variety of social interactions, including social dilemmas in which people make important,
and often selfish, decisions about how to behave by calculating the costs and benefits of different
The study of learning is closely associated with the behaviorist school of psychology, in which it
was seen as an alternative scientific perspective to the failure of introspection. The behaviorists,
including John B. Watson and B. F. Skinner, focused their research entirely on behavior, to the
exclusion of any kinds of mental processes. For behaviorists, the fundamental aspect of learning
is the process ofconditioning—the ability to connect stimuli (the changes that occur in the
environment) with responses (behaviors or other actions).
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But conditioning is just one type of learning. We will also consider other types, including
learning through insight, as well as observational learning (also known as modeling). In each
case we will see not only what psychologists have learned about the topics but also the important
influence that learning has on many aspects of our everyday lives. And we will see that in some
cases learning can be maladaptive—for instance, when a person like P. K. Philips continually
experiences disruptive memories and emotional responses to a negative event.
She Was Certain, but She Was Wrong
In 1984 Jennifer Thompson was a 22-year-old college student in North Carolina. One night a man broke into her
apartment, put a knife to her throat, and raped her. According to her own account, Ms. Thompson studied her rapist
throughout the incident with great determination to memorize his face. She said:
I studied every single detail on the rapist’s face. I looked at his hairline; I looked for scars, for tattoos, for
anything that would help me identify him. When and if I survived.
Ms. Thompson went to the police that same day to create a sketch of her attacker, relying on what she believed was
her detailed memory. Several days later, the police constructed a photographic lineup. Thompson identified Ronald
Cotton as the rapist, and she later testified against him at trial. She was positive it was him, with no doubt in her
I was sure. I knew it. I had picked the right guy, and he was going to go to jail. If there was the possibility of
a death sentence, I wanted him to die. I wanted to flip the switch.
As positive as she was, it turned out that Jennifer Thompson was wrong. But it was not until after Mr. Cotton had
served 11 years in prison for a crime he did not commit that conclusive DNA evidence indicated that Bobby Poole was
the actual rapist, and Cotton was released from jail. Jennifer Thompson’s memory had failed her, resulting in a
substantial injustice. It took definitive DNA testing to shake her confidence, but she now knows that despite her
confidence in her identification, it was wrong. Consumed by guilt, Thompson sought out Cotton when he was released
from prison, and they have since become friends (Innocence Project, n.d.; Thompson, 2000). 
Picking Cotton: A Memoir of Injustice and Redemption
Although Jennifer Thompson was positive that it was Ronald Cotton who had raped her, her memory was inaccurate.
Conclusive DNA testing later proved that he was not the attacker. Watch this book trailer about the story.Jennifer Thompson is not the only person to have been fooled by her memory of events. Over the past 10 years,
almost 400 people have been released from prison when DNA evidence confirmed that they could not have
committed the crime for which they had been convicted. And in more than three-quarters of these cases, the cause of
the innocent people being falsely convicted was erroneous eyewitness testimony (Wells, Memon, & Penrod, 2006). 
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Watch this video for Lesley Stahl’s 60 Minutes segment on this case.
The two subjects of this chapter are memory, defined as the ability to store and retrieve
information over time, and cognition, defined as the processes of acquiring and using knowledge.
It is useful to consider memory and cognition in the same chapter because they work together to
help us interpret and understand our environments.
Memory and cognition represent the two major interests of cognitive psychologists. The
cognitive approach became the most important school of psychology during the 1960s, and the
field of psychology has remained in large part cognitive since that time. The cognitive school
was influenced in large part by the development of the electronic computer, and although the
differences between computers and the human mind are vast, cognitive psychologists have used
the computer as a model for understanding the workings of the mind.
Differences between Brains and Computers
In computers, information can be accessed only if one knows the exact location of the memory. In the brain,
information can be accessed through spreading activation from closely related concepts.
The brain operates primarily in parallel, meaning that it is multitasking on many different actions at the same
time. Although this is changing as new computers are developed, most computers are primarily serial—they
finish one task before they start another.
In computers, short-term (random-access) memory is a subset of long-term (read-only) memory. In the brain,
the processes of short-term memory and long-term memory are distinct.
In the brain, there is no difference between hardware (the mechanical aspects of the computer) and software (the
programs that run on the hardware).
In the brain, synapses, which operate using an electrochemical process, are much slower but also vastly more
complex and useful than the transistors used by computers.
Computers differentiate memory (e.g., the hard drive) from processing (the central processing unit), but in brains
there is no such distinction. In the brain (but not in computers) existing memory is used to interpret and store
incoming information, and retrieving information from memory changes the memory itself.
The brain is self-organizing and self-repairing, but computers are not. If a person suffers a stroke, neural
plasticity will help him or her recover. If we drop our laptop and it breaks, it cannot fix itself.
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The brain is significantly bigger than any current computer. The brain is estimated to have
25,000,000,000,000,000 (25 million billion) interactions among axons, dendrites, neurons, and
neurotransmitters, and that doesn’t include the approximately 1 trillion glial cells that may also be important for
information processing and memory.
Although cognitive psychology began in earnest at about the same time that the electronic computer was first being
developed, and although cognitive psychologists have frequently used the computer as a model for understanding
how the brain operates, research in cognitive neuroscience has revealed many important differences between brains
and computers. The neuroscientist Chris Chatham (2007)  provided the list of differences between brains and
computers shown here. You might want to check out the website and the responses to it
We will begin the chapter with the study of memory. Our memories allow us to do relatively
simple things, such as remembering where we parked our car or the name of the current
president of the United States, but also allow us to form complex memories, such as how to ride
a bicycle or to write a computer program. Moreover, our memories define us as individuals—
they are our experiences, our relationships, our successes, and our failures. Without our
memories, we would not have a life.
At least for some things, our memory is very good (Bahrick, 2000).  Once we learn a face, we
can recognize that face many years later. We know the lyrics of many songs by heart, and we can
give definitions for tens of thousands of words. Mitchell (2006)  contacted participants 17
years after they had been briefly exposed to some line drawings in a lab and found that they still
could identify the images significantly better than participants who had never seen them.
For some people, memory is truly amazing. Consider, for instance, the case of Kim Peek, who
was the inspiration for the Academy Award–winning film Rain Man (Figure 8.1 "Kim
Peek" andNote 8.5 "Video Clip: Kim Peek"). Although Peek’s IQ was only 87, significantly
below the average of about 100, it is estimated that he memorized more than 10,000 books in his
lifetime (Wisconsin Medical Society, n.d.; “Kim Peek,” 2004).  The Russian psychologist A.
R. Luria (2004)  has described the abilities of a man known as “S,” who seems to have
unlimited memory. S remembers strings of hundreds of random letters for years at a time, and
seems in fact to never forget anything.
In this chapter we will see how psychologists use behavioral responses (such as memory tests
and reaction times) to draw inferences about what and how people remember. And we will see
that although we have very good memory for some things, our memories are far from perfect
(Schacter, 1996).  The errors that we make are due to the fact that our memories are not simply
recording devices that input, store, and retrieve the world around us. Rather, we actively process
and interpret information as we remember and recollect it, and these cognitive processes
influence what we remember and how we remember it. Because memories are constructed, not
recorded, when we remember events we don’t reproduce exact replicas of those events (Bartlett,
In the last section of the chapter we will focus primarily on cognition, with a particular
consideration for cases in which cognitive processes lead us to distort our judgments or
misremember information. We will see that our prior knowledge can influence our memory.
People who read the words “dream,sheets, rest, snore, blanket, tired, and bed” and then are
asked to remember the words often think that they saw the word sleep even though that word
was not in the list (Roediger & McDermott, 1995).  And we will see that in other cases we are
influenced by the ease with which we can retrieve information from memory or by the
information that we are exposed to after we first learn something.
Although much research in the area of memory and cognition is basic in orientation, the work
also has profound influence on our everyday experiences. Our cognitive processes influence the
accuracy and inaccuracy of our memories and our judgments, and they lead us to be vulnerable
to the types of errors that eyewitnesses such as Jennifer Thompson may make. Understanding
these potential errors is the first step in learning to avoid them.