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Sensation and
Pain: An Essential Sensation
lagued by a nagging headache, or smarting over a stubbed toe, you might curse your feeling of pain. A pain-free
existence might sound nice, but in fact pain is an important sensation. Consider the situation of Ashlyn Blocker,
age 12. She was born without the ability to sense pain. Ashlyn was a baby who did not cry, even when stinging eye
drops were put in her eyes, even when she had a terrible diaper rash.
Researchers have studied Ashlyn to understand not only the cause of her condition but also the nature of pain itself
(Staud & others, 2011). It turns out that Ashlyn has two genetic mutations that short-circuit the pain signals in her
brain. Without pain, she has missed some of life’s “warning signals,” suffering severe burns as well as two broken
ankles in her short life. She has had to remind herself that the sight of blood coming from a cut means that some-
thing is wrong. According to her mother, Ashlyn’s inability to experience pain has caused a lack of empathy for the
suffering of others: She cannot understand, for example, why a child would cry after falling off a swing (Chun, 2010).
The feeling of pain provides us with information about what is happening to us. This feeling, like all of our other
senses, connects us to the external world. We see a beloved friend’s face, feel a comforting hand on our shoulder, or
hear our name called from across a room. Our abilities to sense and to perceive are what allow us to reach out into
that world in the many ways we do every day.
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How We Sense and Perceive the World // 85
This chapter explores sensation and perception, the processes by which we engage
with the external world. We rst examine vision and then probe hearing, the skin
senses, taste, smell, and the kinesthetic and vestibular senses. Without the senses, we
would be isolated from the world around us; we would live in dark silence—and in a
tasteless, colorless, feelingless void.
Sensation and perception researchers represent a broad range of specialties, including
ophthalmology, the study of the eye’s structure, function, and diseases; audiology, the
science concerned with hearing; neurology, the scienti c study of the nervous system;
and many others. Understanding sensation and perception requires comprehending the
physical properties of the objects of our perception—light, sound, the texture of material
things, and so on. The psychological approach to these processes involves understanding
the physical structures and functions of the sense organs, as well as the brain’s conver-
sion of the information from these organs into experience.
The Processes and Purposes
of Sensation and Perception
Our world is alive with stimuli—all the objects and events that surround us. Sensation
and perception are the processes that allow us to detect and understand these various
stimuli. We do not actually experience these stimuli directly; rather, our senses allow us
to get information about aspects of our environment, and we then take that information
and form a perception of the world. Sensation is the process of receiving stimulus ener-
gies from the external environment and
transforming those energies into neural
energy. Physical energy such as light, sound,
and heat is detected by specialized receptor
cells in the sense organs—eyes, ears, skin,
nose, and tongue. When the receptor cells
register a stimulus, the energy is converted
into an electrochemical impulse or action
potential that relays information about the
stimulus through the nervous system to the
brain (Harris & Attwell, 2012). Recall from
Chapter 2 that an action potential is the brief
wave of electrical charge that sweeps down
the axon of a neuron for possible transmis-
sion to another neuron. When it reaches the
brain, the information travels to the appro-
priate area of the cerebral cortex (Swaminathan
& Freedman, 2012).
The brain gives meaning to sensation
through perception. Perception i s t h e p r o -
cess of organizing and interpreting sensory
The process of receiving
stimulus energies from the
external environment and
transforming those energies
into neural energy.
The process of organizing
and interpreting sensory
information so that it makes
How We Sense and Perceive the World
Through sensation we take in information from the world; through
perception we identify meaningful patterns in that information.
Thus sensation and perception work hand in hand when we enjoy a
hug and the sweet fragrance of a fl ower.
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86 // CHAPTER 3 // Sensation and Perception
information so that it makes sense. Receptor cells in our eyes record—that is, sense—
a sleek silver object in the sky, but they do not “see” a jet plane. So, sensation is
about the biological processing that occurs between our sensory systems and the
environment, while perception is our experience of those processes in action.
B O T T O M - U P A N D T O P - D O W N P R O C E S S I N G Psychologists
distinguish between bottom-up and top-down processing in sensation and per-
ception. In bottom-up processing, sensory receptors register information
about the external environment and send it up to the brain for interpretation.
Bottom-up processing means taking in information and trying to make sense
of it (McMains & Kastner, 2011). An example of bottom-up processing might
be the way you experience a song the rst time you hear it: You listen carefully
to get a “feel” for it. In contrast, top-down processing starts with cognitive process-
ing at the higher levels of the brain; in top-down processing we begin with some sense
of what is happening and apply that framework to information from the world (van Gaal
& Lamme, 2011). You can experience top-down processing by “listening” to your
favorite song in your head right now. As you “hear” the song in your mind’s ear,
you are engaged in perceptual experience produced by top-down processing.
Bottom-up and top-down processing work together in sensation and percep-
tion to allow us to function accurately and ef ciently (Meyer, 2011). By them-
selves our ears provide only incoming information about sound in the
environment. Only when we consider both what the ears hear (bottom-up
processing) and what the brain interprets (top-down processing) can we fully
understand how we perceive sounds in our world. In everyday life, the two
processes of sensation and perception are essentially inseparable. For this rea-
son, most psychologists refer to sensation and perception as a uni ed information-
processing system (Goldstein, 2010).
T H E P U R P O S E S O F S E N S A T I O N A N D P E R C E P T I O N W h y d o w e p e r -
ceive the world? From an evolutionary perspective, the purpose of sensation and percep-
tion is adaptation that improves a species’ chances for survival (Mader &
Windelspecht, 2012). An organism must be able to sense and respond quickly
and accurately to events in the immediate environment, such as the approach
of a predator, the presence of prey, and the appearance of a potential mate.
Not surprisingly, therefore, most animals—from gold sh to gorillas to
humans—have eyes and ears, as well as sensitivities to touch and chemicals
(smell and taste). Furthermore, a close comparison of sensory systems in animals
The operation in
sensation and
perception in
which sensory
receptors register
about the exter-
nal environment
and send it up
to the brain for
The operation
in sensation
and perception,
launched by cog-
nitive processing
at the brain’s
higher levels,
that allows the
organism to
sense what is
happening and
to apply that
framework to
information from
the world.
If youve ever begged
someone t o t r y your f avor i t e
food only to have the person give
you a s hr ug and meh, t hat s
the difference between sensation
and per cept i on. Bot h t ongues
had t he same exper i enc e.
But per cept i on i s
subj ect i ve.
Eat i n g s t r aw b er r i es ,
cher r i es, and r ed popsi cl es, l i t t l e
kids of t en get t he idea t hat
red ! sweet . . . t hen t hey
get a t as t e of r ed beet s ! I t s
al ways a f un moment when
bot t om- up exper i ence col l i des
wi t h t op- down expec t at i ons.
Do g s c a n s me l l b e t t e r
than humans. But that’s because
dogs need t o and humans don’ t .
Most predatory animals have
eyes at the front of the
face; most animals that are
prey have eyes on the side
of their head. Through these
adaptations, predators
perceive their prey accurately,
and prey gain a measure of
safety from their panoramic
view of their environment.
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How We Sense and Perceive the World // 87
reveals that each species is exquisitely adapted to the habitat in which it evolved
(Hoefnagels, 2012). Animals that are primarily predators generally have their eyes at the
front of their faces so that they can perceive their prey accurately. In contrast, animals
that are more likely to be someone else’s lunch have their eyes on either side of their
heads, giving them a wide view of their surroundings at all times.
Sensory Receptors and the Brain
All sensation begins with sensory receptors. Sensory receptors are specialized
cells that detect stimulus information and transmit it to sensory (afferent)
nerves and the brain. Sensory receptors are the openings through which the
brain and nervous system experience the world. Figure 3.1 shows the human
sensory receptors for vision, hearing, touch, smell, and taste.
Figure 3.2 depicts the ow of information from the environment to the brain.
Sensory receptors take in information from the environment, creating local electrical
currents. These currents are graded; that means they are sensitive to the intensity of
stimulation, such as the difference between a dim and a bright light. These receptors
trigger action potentials in sensory neurons, which carry that information to the central
nervous system. Because sensory neurons (like all neurons) follow the all-or-nothing
principle, described in Chapter 2, the intensity of the stimulus cannot be communicated
to the brain by changing the strength of the action potential. Instead, the receptor varies
the fr e quency of action potentials sent to the brain. So, if a stimulus is very intense, like
the bright sun on a hot day, the neuron will re more frequently (but with the same
strength) to let the brain know that the light is indeed very, very bright.
Other than frequency, the action potentials of all sensory nerves are alike. This same-
ness raises an intriguing question: How can an animal distinguish among sight, sound,
odor, taste, and touch? The answer is that sensory receptors are selective and have dif-
ferent neural pathways. They are specialized to absorb a particular type of energy—light
energy, sound vibrations, or chemical energy, for example—and convert it into an action
sensory receptors
Specialized cells
that detect stim-
ulus information
and transmit it to
sensory (afferent)
nerves and the
Type of
detection of
light, perceived
as sight
detection of
perceived as
detection of
perceived as
detection of
chemical stimuli,
perceived as
detection of
chemical stimuli,
perceived as
Touch TasteHearing Smell
Ears SkinEyes Nose Tongue
FIGURE 3.1 Human Senses:
Organs, Energy Stimuli, and
Sensory Receptors The receptor
cells for each sense are specialized
to receive particular types of energy
Ther e i s t hat wor d
agai n,
af f er ent
. Remember t hat
af f er ent
ar r i ves
at t he
br a i n.
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88 // CHAPTER 3 // Sensation and Perception
Cell membrane
Receptor Receptor
Energy Stim ulus
Sensory Receptor
Sensation and Perception
Sensation involves detecting and trans-
mitting information about different kinds
of energy. The sense organs and sensory
receptors fall into several main classes
based on the type of energy that is trans-
mitted. The functions of these classes
Photoreception: detection of light, per-
ceived as sight
Mechanoreception: d e t e c t i o n o f p r e s -
sure, vibration, and movement, per-
ceived as touch, hearing, and equilibrium
Chemoreception: detection of chemical
stimuli, perceived as smell and taste
Each of these processes belongs to a particular
class of receptors and brain processes. There are
rare cases, however, in which the senses can become
confused. The term synae s thesia d e s c r i b e s a n e x p e r i -
ence in which one sense (say, sight) induces an experience
in another sense (say, hearing) (Simner, 2012a, 2012b). An
individual might “see” music or “taste” a color, for example. One
woman was able to taste sounds, so that a piece of music might taste
like tuna sh (Beeli, Esslen, & Jancke, 2005). Neuroscientists are explor-
ing the neurological bases of synaesthesia, especially in the connections
between the various sensory regions of the cerebral cortex. For example, a recent
fMRI study identi ed the parietal cortex as the key auditory-visual location in the
brain for individuals with synaesthesia who experienced music as color (Neufeld &
others, 2012).
Phantom limb pain m i g h t b e a n o t h e r e x a m p l e o f c o n f u s e d s e n s e s . A s m a n y a s 9 5 p e r -
cent of individuals who have lost an arm or a leg report alarming and puzzling pain in the
amputated arm or leg. Although the limb that contains the sensory receptors is gone, the
areas of the brain and nervous system that received information from those receptors are
still there, causing confusion (Elbert, 2012; Foell & others, 2011). Amputee veterans of
combat in Iraq and Afghanistan have found some relief in an unexpected place: looking in
a mirror. In this treatment, individuals place a mirror in front of their existing limb and
move the limb around while watching the mirror. So, if a person’s left leg has been ampu-
tated, the mirror is placed so that the right leg is seen moving in the mirror where the left
leg would be if it had not been amputated. This procedure seems to trick the brain into
perceiving the missing limb as still there, allowing it to make sense of incoming sensation
(Flor & Diers, 2009). The success of this mirror therapy demonstrates how our senses
cooperate to produce experience—how the bottom-up processes (the incoming messages
from the missing limb) and the top-down processes (the brain’s efforts to make sense of
these) work together. A recent research review concluded that mirror therapy has mixed
results in treating phantom leg syndrome (Subedi & Grossberg, 2011).
I n t h e b r a i n , n e a r l y a l l s e n s o r y s i g n a l s p a s s t h r o u g h t h e t h a l a m u s , t h e b r a i n s r e l a y s t a -
tion, described in Chapter 2. From the thalamus, the signals go to the sensory areas of the
cerebral cortex, where they are modi ed and spread throughout a vast network of neurons.
FIGURE 3.2 Information Flow in
Senses The diagram shows a general ow of
sensory information from energy stimulus to
sensory receptor cell to sensory neuron to
sensation and perception.
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How We Sense and Perceive the World // 89
R e c a l l f r o m C h a p t e r 2 t h a t c e r t a i n a r e a s o f t h e c e r e b r a l c o r t e x a r e s p e c i a l i z e d t o
handle different sensory functions. Visual information is processed mainly in the
occipital lobes; hearing, in the temporal lobes; and pain, touch, and temperature,
in the parietal lobes. Keep in mind, however, that the interactions and pathways
of sensory information are complex, and the brain often must coordinate exten-
sive information and interpret it.
An important part of perception is interpreting the sensory messages. Many
top-down factors determine this meaning, including signals from different parts of
the brain, prior learning, the person’s goals, and his or her degree of arousal. Moving in
the opposite direction, bottom-up signals from a sensory area may help other parts of
the brain maintain arousal, form an image of where the body is in space, or regulate
The principles we have surveyed so far apply to all of the senses. We’ve seen that the
senses are about detecting different energies and that all have specialized receptor cells
and areas of the brain that serve their functions. You have probably heard about a “sixth
sense”— extrasensory perception, or ESP. ESP means that a person can detect information
from the world without receiving concrete sensory input. Examples of ESP include
telepathy (the ability to read another person’s mind) and precognition (the ability to sense
future events). To read about how psychologists view these apparent phenomena, see
Challenge Your Thinking.
Any sensory system must be able to detect varying degrees of energy. This energy can
take the form of light, sound, chemical, or mechanical stimulation. How much of a
stimulus is necessary for you to see, hear, taste, smell, or feel something? What is the
lowest possible amount of stimulation that will still be detected?
A B S O L U T E T H R E S H O L D One way to think about the lowest limits of perception
is to assume that there is an absolute threshold, or minimum amount of stimulus energy
that a person can detect. When the energy of a stimulus falls below this absolute thresh-
old, we cannot detect its presence; when the energy of the stimulus rises above the
absolute threshold, we can detect the stimulus (Lim, Kyung, & Kwon, 2012). As an
example, nd a clock that ticks; put it on a table and walk far enough away that you no
longer hear it. Then gradually move toward the
clock. At some point, you will begin to hear it
ticking. Hold your position and notice that
occasionally the ticking fades, and you may
have to move forward to reach the threshold;
at other times, it may become loud, and you
can move backward.
I n t h i s e x p e r i m e n t , i f y o u m e a s u r e y o u r
absolute threshold several times, you likely
will record several different distances for
detecting the stimulus. For example, the rst
time you try it, you might hear the ticking at
25 feet from the clock. However, you probably
will not hear it every time at 25 feet. Maybe
you hear it only 38 percent of the time at this
distance, but you hear it 50 percent of the time
at 20 feet away and 65 percent of the time at
15 feet. People have different thresholds. Some
have better hearing than others, and some have
better vision. Figure 3.3 shows one person’s
absolute threshold
The minimum amount
of stimulus energy that a
person can detect.
In case you dont
remember: occipital ! t he back
of t he br ai n; t empor al ! on t he
sides; par i et al ! on t he t op.
(Youre welcome!)
Percentage of yes responses
30 25 20 15 510 0
Distance in feet from a ticking clock
FIGURE 3.3 Measuring Absolute Threshold Absolute threshold
is the minimum amount of energy we can detect. To measure absolute threshold,
psychologists have arbitrarily decided to use the criterion of detecting the stimulus
50 percent of the time. In this graph, the person’s absolute threshold for detecting
the ticking clock is at a distance of 20 feet.
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90 // CHAPTER 3 // Sensation and Perception
Vision A candle flame at 30 miles on a dark, clear night
Hearing A ticking clock at 20 feet under quiet conditions
Smell One drop of perfume diffused throughout three rooms
Taste A teaspoon of sugar in 2 gallons of water
Touch The wing of a fly falling on your neck from a distance of 1 centimeter
measured absolute threshold for detecting a clock’s ticking sound. Psychologists have
arbitrarily decided that absolute threshold is the point at which the individual detects the
stimulus 50 percent of the time—in this case, 20 feet away. Using the same clock,
another person might have a measured absolute threshold of 26 feet, and yet another,
18 feet. Figure 3.4 lists the approximate absolute thresholds of ve senses.
Challen ge
eople have experiences that seem to involve precognition—
for instance, “just knowing” that a friend is in trouble and
later nding out that he was in a car accident. Such experi-
ences can be fascinating and even spooky, but do they re ect the
existence of ESP? Or is simple coincidence at work?
There are many reasons to question the existence of ESP.
Think about precognition in the ways we have considered sensa-
tion and perception. Sensation involves detecting energy from
the environment. If ESP exists, consider: Which afferent neurons
send psychic messages from the future to the brain, and what
sort of energy conveys these messages? In truth, the notion that
human beings can sense future events challenges the basic
principle that cause precedes effect (and not the other way
around). Proof of the existence of precognition would require a
revision of not only psychology but also biology and physics
(Rouder & Morey, 2011).
Paraphrasing French mathematician Pierre-Simon Laplace, the
late American astrophysicist Carl Sagan famously observed,
“Extraordinary claims require extraordinary evidence.” The asser-
tion that ESP exists is an extraordinary claim. Is the evidence for
this claim equally extraordinary? For most psychologists, the an-
swer is a de nite no (French, 2010; Hyman, 2010; Wiseman &
Watt, 2006).
Distinguished social psychologist Daryl Bem (2011) rekindled
the ESP debate, publishing nine studies testing for the existence
of precognition. In eight of those studies, Bem claimed to show
that future events could in uence present behavior. For example,
in one study, participants were shown 48 common words for
3 seconds each. After seeing the words, they were asked to re-
call all of the words they could. After the memory test, the com-
puter randomly assigned the participants to study half of the
words. A staunch believer in ESP, Bem hypothesized that partici-
pants would do a better job of remembering the words that they
later were going to study. Results showed that participants re-
membered the words that they would later study about 2 percent
better than the words they would not study. Bem concluded that
the future act of studying had “in fact reach[ed] back in time and
facilitate[d]” word recall.
In another study, participants saw two pictures of curtains on
a computer screen. They were told that one curtain had a picture
behind it and the other did not. Their task was to select the cur-
tain with a picture. Some of the pictures contained erotic images;
others showed positive, negative, or neutral images. The com-
puter randomly placed the pictures behind the curtains after
participants had made their guesses. Bem predicted that partici-
pants would select the curtain that would eventually show the
erotic images at rates greater than chance—and the results did
reveal that they had selected that curtain 53.1 percent of the
time (higher than the chance rate of 50 percent).
Bems ndings and their publication in the most prestigious
journal in social psychology caused an uproar, inspiring articles
in the New York Times (Carey, 2011) and Science (Miller, 2011);
a urry of activity in the blogosphere; and an appearance by
Bem on the Colbert Report. Critics of Bem’s work pointed to
inconsistencies across the studies (LeBel & Peters, 2011;
Wagenmakers & others, 2011). For instance, in some studies
precognition was shown for erotic (but not negative) images, in
others for negative (but not erotic) images; in other studies
women (but not men) showed precognition; and in other cases
only extraverts did.
Can We Feel the Future?
FIGURE 3.4 Approximate Absolute Thresholds for Five Senses
These thresholds show the amazing power of our senses to detect even very slight
variations in the environment.
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How We Sense and Perceive the World // 91
The statistical tests Bem used are at the heart of the contro-
versy. To understand this issue, reconsider the questions posed
above: Do Bem’s results re ect ESP—or simple coincidence?
Psychologists typically use statistics to determine whether an
effect is real or a matter of chance (or coincidence). In the results
just described, participants were 3.1 percent more likely than
chance to select the curtain with the erotic image. Is this differ-
ence extraordinary evidence for an extraordinary claim? Before
answering, consider the following situation. Imagine that you are
offered $1 million if you can accurately assess whether a coin
isrigged (that is, biased to produce more heads than tails). If you
ip that coin 100 times and it comes up heads 53 times, should
you conclude the coin is biased? How sure are you of your conclu-
sion? How many times will you need to repeat your test to be
sure? The probability of Bem’s obtaining his result completely by
chance was 1 in 100. But is 1 in 100 convincing enough to sup-
port the existence of a phenomenon that challenges the very na-
ture of cause and effect? Many critics answered that question
with a resounding no (Hyman, 2010; Kruschke, 2011; LeBel &
Peters, 2011; Miller, 2011; Rouder & Morey, 2011; Wagenmakers
& others, 2011; Wetzels & others, 2011).
E. J. Wagenmakers and his colleagues (2011) analyzed the data
from each of Bems studies using a different statistical technique
and concluded that the results did not provide strong evidence for
ESP. Jeffrey Rouder and Richard Morey (2011) applied yet another
statistical tool to the entire package of studies and concluded that
although there might be some evidence in Bem’s data for the exis-
tence of precognition, it was too slight to overcome the appropriate
level of skepticism that scholars should have for ESP.
Clearly, although an important tool, statistics are not a direct
pathway to truth. Statistics are not a substitute for a sound ratio-
nale for predictions and conclusions. Raymond Hyman (2010), a
longtime critic of research on ESP, argues that in the absence of a
theory about why or how precognition exists, along with indepen-
dently repeatable methods, any
evidence for the phenomenon is
Statistically speaking, Bem’s nd-
ings were not terribly different from
the ndings in other published psy-
chological studies (Wetzels & others,
2011). Most researchers consider a
nding “real” if it is likely to occur
fewer than 5 times in 100 by
chance. Some have concluded
that the lesson from Bem’s work
is not so much that ESP exists as
that psychologists need to rethink
how they test their hypotheses
(LeBel & Peters, 2011; Wagenmakers
& others, 2011). Still, some scholars
continue to argue for the existence of
ESP and defend research like Bem’s (Bem, Utts, & Johnson,
2011; Dossey, 2011; Radin,
2006; Storm, Tressoldi, & Di
Risio, 2010).
The controversy over Bem’s
paper highlights a tension within
science between openness and
enthusiasm for ideas (no matter
how strange or counterintuitive
they might seem) and a deep
and intense skepticism. Recog-
nizing this dif cult tension be-
tween wonder and skepticism,
Carl Sagan concluded, “This is
how deep truths are winnowed
from deep nonsense.
What Do You Think?
Do you believe in the
phenomenon of ESP? Why
orwhy not?
What kind of evidence would
be necessary for you to
change your belief?
Should research on ESP be
held to a higher standard than
other research? Explain.
Under ideal circumstances, our senses have very low absolute thresholds, so we can
be remarkably good at detecting small amounts of stimulus energy. You might be sur-
prised to learn that the human eye can see a candle ame at 30 miles on a dark, clear
night. However, our environment seldom gives us ideal conditions with which to detect
stimuli. If the night were cloudy or the air smoky, for example, you would have to be
much closer to see the candle ame. In addition, other lights on the horizon—car or
house lights—would hinder your ability to detect the candle’s icker. Noise is the term
given to irrelevant and competing stimuli—not just sounds but any distracting stimuli for
our senses (Ikeda, Sekiguchi, & Hayashi, 2010).
D I F F E R E N C E T H R E S H O L D In addition to studying how much energy is required
for a stimulus to be detected, psychologists investigate the degree of difference that must
exist between two stimuli before the difference is detected. This is the difference threshold,
or just noticeable difference. An artist might detect the difference between two similar
shades of color. A fashion designer might notice a difference in the texture of two fabrics.
How different must the colors and textures be for someone to say, “These are different”?
Like the absolute threshold, the difference threshold is the smallest difference in stimu-
lation required to discriminate one stimulus from another 50 percent of the time.
Irrelevant and competing
stimuli—not only sounds but
also any distracting stimuli
for the senses.
difference threshold
The degree of difference
that must exist between two
stimuli before the difference
is detected.
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92 // CHAPTER 3 // Sensation and Perception
Difference thresholds increase as a stimulus becomes stronger. That means that at very
low levels of stimulation, small changes can be detected, but at very high levels, small
changes are less noticeable. When music is playing softly, you may notice when your
roommate increases the volume by even a small amount. If, however, he or she turns the
volume up an equal amount when the music is playing very loudly, you may not notice.
Weber’s law ( d i s c o v e r e d b y G e r m a n p h y s i o l o g i s t E . H . W e b e r m o r e t h a n 1 5 0 y e a r s a g o )
is the principle that two stimuli must differ by a constant proportion to be perceived
as different. For example, we add 1 candle to 20 candles and notice a difference in
the brightness of the candles; we add 1 candle to 120 candles and do not notice a
difference, but we would notice the difference if we added 6 candles to 120 candles.
Weber’s law generally holds true (Gao & Vasconcelos, 2009; Mohring, Libertus, &
Bertin, 2012).
S U B L I M I N A L P E R C E P T I O N Can sensations that occur below our absolute
threshold affect us without our being aware of them? Subliminal perception refers to
the detection of information below the level of conscious awareness. In 1957, James
Vicary, an advertising executive, announced that he was able to increase popcorn and
soft drink sales by secretly ashing the words “EAT POPCORN” and “DRINK COKE”
on a movie screen in a local theater (Weir, 1984). Vicary’s claims were a hoax, but
people have continued to wonder whether behavior can be in uenced by stimuli
that are presented so quickly that we cannot perceive them.
S t u d i e s h a v e s h o w n t h a t t h e b r a i n r e s p o n d s t o i n f o r m a t i o n t h a t i s p r e s e n t e d
below the conscious threshold, and such information can also in uence behav-
ior (Dupoux, de Gardelle, & Kouider, 2008; Radel, Sarrazin, & Pelletier, 2009).
In one study, researchers randomly assigned participants to observe either
words related to being thirsty or control words of the same length being ashed
on a computer screen for 16 milliseconds while they performed an unrelated
task (Strahan, Spencer, & Zanna, 2002). All of the participants thought they
were participating in a taste test study, and all were thirsty. None of the partici-
pants reported seeing the ashed words, but when given a chance to drink a bever-
age afterward, those who had seen thirst-related words drank more. Research has also
supported the notion that people’s performance on learning tasks is affected by stimuli
that are too faint to be recognized at a conscious level (Cleeremans & Sarrazin, 2007).
Yo u w il l r e ad f urt her a bo ut t hes e e ffe cts i n C hap ter 6 ’s d isc uss io n o f p rim in g.
The notion that stimuli we do not consciously perceive can in uence our behavior
challenges the usefulness of the idea of thresholds (Rouder & Morey, 2009). If stimuli
that fall below the threshold can have an impact on us, you may be wondering, what do
thresholds really tell us? Further, you might have noticed that the de nition of absolute
threshold is not that absolute. It refers to the intensity of stimulation detected 50 percent
of the time . How can something absolute change from one trial to the next?
If, for example, you tried the ticking clock experiment described earlier, you
might have found yourself making judgment calls. Sometimes you felt very sure
you could hear the clock, but other times you were uncertain and probably took
a guess. Sometimes you guessed right, and other times you were mistaken.
Now, imagine that someone offered to pay you $50 for every correct answer
you gave—would the presence of that incentive change your judgments? Alter-
natively, what if you were charged $50 for every time you said you heard the
clock and it was not ticking? In fact, perception is often about making such
judgment calls.
A n a l t e r n a t i v e a p p r o a c h t o t h e q u e s t i o n o f w h e t h e r a s t i m u l u s i s d e t e c t e d
would emphasize that saying (or not saying) “Yes, I hear that ticking” is actually
a decision. T h i s a p p r o a c h i s c a l l e d s i g n a l d e t e c t i o n t h e o r y . Signal detection the-
ory f o c u s e s o n d e c i s i o n m a k i n g a b o u t s t i m u l i u n d e r c o n d i t i o n s o f u n c e r t a i n t y . I n
signal detection theory, detection of sensory stimuli depends on a variety of factors
besides the physical intensity of the stimulus and the sensory abilities of the observer
(Haase & Fisk, 2011; Olma & others, 2011). These factors include individual and
Weber’s law
The principle that
two stimuli must
differ by a con-
stant minimum
(rather than a
constant amount)
to be perceived
as different.
The detection
below the level
of conscious
signal detection
An approach
to perception
that focuses on
decision making
about stimuli
under conditions
of uncertainty.
Not e t hat
1:2 0 ! 6:120.
Thi s was an
ex per i ment . The par t i ci pant s
who saw t hi r st y wor d s wer e
the experimental group, and the
ot her par t i ci pant s wer e t he
cont r ol gr oup. Now, why wer e
they randomly assigned
to the conditions?
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How We Sense and Perceive the World // 93
contextual variations, such as fatigue, expecta-
tions, and the urgency of the moment. Figure
3.5 shows how signal detection works.
Perceiving Sensory
A s w e j u s t s a w , t h e p e r c e p t i o n o f s t i m u l i i s
in uenced by more than the characteristics of
the environmental stimuli themselves. Two
important factors in perceiving sensory stimuli
are attention and perceptual set.
A T T E N T I O N The world holds a lot of information to perceive. At this moment you
are perceiving the letters and words that make up this sentence. Now gaze around you
and x your eyes on something other than this book. Afterward, curl up the toes on your
right foot. In each of these circumstances, you engaged in selective attention, which
involves focusing on a speci c aspect of experience while ignoring others (Reed & Hicks,
2012). A familiar example of selective attention is the ability to focus on one voice
among many in a crowded airline terminal or noisy restaurant. Psychologists call this
common occurrence the cocktail party effect (Kuyper, 1972).
Not only is attention selective, but it also is shiftable. For example, you might be
paying close attention to your instructor’s lecture, but if the person next to you starts
texting a friend, you might look to see what is going on over there. The fact that we can
attend selectively to one stimulus and shift readily to another indicates that we must be
monitoring many things at once.
Certain features of stimuli cause people to attend to them. Novel stimuli (those that
are new, different, or unusual) often attract our attention. If a Ferrari convertible whizzes
by, you are more likely to notice it than you would a Ford. Size, color, and movement
also in uence our attention. Objects that are large, vividly colored, or moving are more
likely to grab our attention than objects that are small, dull-colored, or stationary.
S o m e t i m e s e v e n v e r y i n t e r e s t i n g s t i m u l i c a n b e m i s s e d i f o u r a t t e n t i o n i s o t h e r w i s e
occupied. Inattentional blindness refers to the failure to detect unexpected events when
attention is engaged by a task (Chabris & Simons, 2010). When we are working intently
on something, such as nding a seat in a packed movie theater,
we might not even see an unusual stimulus, such as a friend
waving to us in the crowd. Research conducted by Daniel
Simons and Christopher Chabris (1999) provides a remarkable
example of inattentional blindness. In that study, participants
were asked to watch a video of two teams playing basketball.
The participants were instructed to closely count the number
of passes thrown by each team. During the video, a small
woman dressed in a gorilla suit walked through the action,
clearly visible for 5 seconds. Surprisingly, over half of the par-
ticipants (who were apparently deeply engaged in the counting
task) never noticed the gorilla.
Inattentional blindness is more
likely to occur when a task is dif cult (Macdonald & Lavie,
2008) and when the distracting stimulus is very different from
stimuli that are relevant to the task at hand (White & Aimola
Davies, 2008; Wiemer, Gerdes, & Pauli, 2012).
Research on inattentional blindness suggests the dangers of
multitasking when one of the tasks is driving. Engaging in a
task such as talking on a cell phone or sending text messages
can so occupy attention that little is left over for the important
task of piloting a motor vehicle. Research revealed that
The act of focus-
ing on a specifi c
aspect of experi-
ence while ignor-
ing others.
Observer’s Response
Sig nal Present Hit (correct)
Yes, I see the signal.” No, I don’t see the
Miss (mistake)
Signal Absent False alarm (mistake)
Correct rejection
FIGURE 3.5 Four Outcomes in Signal Detection Signal
detection research helps to explain when and how perceptual judgments are
correct or mistaken.
“Now that I have your attention, dear . . .”
Used by permission of CartoonStock,
Inattentional Blindness
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94 // CHAPTER 3 // Sensation and Perception
individuals who text-message while they drive face 23 times the risk of a crash or
near-crash compared to non-distracted drivers (Blanco & others, 2009; Hanowski &
others, 2009). In this research, cameras continuously observed drivers for more than
6 million miles of driving. Texting drew the drivers’ eyes away from the road long
enough for the vehicle to travel the length of a football eld at 55 miles an hour.
C U L T U R E , A T T E N T I O N , A N D P E R C E P T I O N R e s e a r c h s h o w s t h a t c u l -
ture in uences which stimuli we attend to as we perceive the world. Individuals from
Western cultures are more likely to attend to objects in the foreground of scenes (or
focal objects ), while East Asians looking at the same scenes are more likely to notice
aspects of the context. For example, in one study (Masuda & Nisbett, 2001), Amer-
ican and Japanese participants were shown video clips of underwater scenes. When
asked to describe what they had seen, the Americans were more likely to talk about
the colorful sh swimming around, and Japanese participants were more likely to
talk about the locations of objects and aspects of the setting. Such differences have
led psychologists to conclude that Westerners take a more analytical orientation, while
Asians are more likely to see the big picture. Culture also in uences the kinds of
stimuli that are missed in inattentional blindness. Research on change blindness (the
tendency to miss changes that have occurred in a scene) shows that when objects in
the foreground change, Americans are more likely to notice, while Japanese are more
likely to notice when changes occur in the context (Masuda & Nisbett, 2006).
W h a t m i g h t e x p l a i n t h e s e c u l t u r a l d i f f e r e n c e s i n a t t e n t i o n ? O n e p o s s i b i l i t y m a y b e d i f f e r -
ences in the environments that individuals in these cultures typically encounter. In a series of
studies, Yuri Miyamoto and her colleagues (Miyamoto, Nisbett, & Masuda, 2006) found through
photographic comparisons that Japanese hotels and schools had more detail and were more
complex than American hotels and schools. Japanese individuals, then, may develop the ten-
dency to look at the whole picture because navigating their world requires such attention.
Interestingly, in another study, Miyamoto and her colleagues had American and Japanese par-
ticipants watch brief video clips of American or Japanese scenes (Miyamoto, Nisbett, & Masuda,
2006). They found that American and Japanese participants alike noticed changes in focal
objects in American scenes but noticed changes in the context in Japanese scenes. This research
suggests that while the mechanics of sensation are the same for human beings, the experience
of perception can be shaped by the physical environment in which each person lives.
P E R C E P T U A L S E T P l a c e y o u r h a n d o v e r t h e p l a y i n g c a r d s o n t h e r i g h t i n t h e
illustration and look at the playing cards on the left. As quickly as you can, count how
many aces of spades you see. Then place your hand over the cards on the left and count
the number of aces of spades among the cards on the right.
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How We Sense and Perceive the World // 95
Most people report that they see two or three aces of spades in the set of cards on the
left. However, if you look closely, you will see that there are ve. Two of the aces of
spades are black and three are red. When people look at the set of cards on the right,
they are more likely to count ve aces of spades. Why do we perceive the two sets of
cards differently? We expect the ace of spades to be black because it is always black in
a regular deck of cards. We do not expect red spades, so we skip right over the red ones:
Expectations in uence perceptions.
Psychologists refer to a predisposition or readiness to perceive something in a par-
ticular way as a perceptual set. Perceptual sets act as “psychological” lters in process-
ing information about the environment (Fei-Fei & others, 2007). Perceptual sets re ect
top-down in uences on perception. Interestingly, young children are more accurate at the
task involving the ace of spades than adults are. Why? Because they have not built up
the perceptual set that the ace of spades is black.
Sensory Adaptation
Turning out the lights in your bedroom at night, you stumble across the room to your
bed, blind to the objects around you. Gradually the objects reappear and become
clearer. The ability of the visual system to adjust to a darkened room is an example
of sensory adaptation —a change in the responsiveness of the sensory system based
on the average level of surrounding stimulation (Iglesias, 2012; Elliott & others,
You have experienced sensory adaptation countless times in your life. You adjust to
the water in an initially “freezing” swimming pool. You turn on your windshield wip-
ers while driving, and shortly you are unaware of their rhythmic sweeping back and
forth. When you rst enter a room, you might be bothered by the hum of the air con-
ditioner, but after a while you get used to it. All of these experiences represent sensory
In the example of adapting to the dark, when you turn out the lights, everything is
black. Conversely, when you step out into the bright sunshine after spending time in a
dark basement, light oods your eyes and everything appears light. These momentary
blips in sensation arise because adaptation takes time.
perceptual set
A predisposition or readi-
ness to perceive something
in a particular way.
A change in the
of the sensory
system based on
the average level
of surrounding
1. Every day, you see, hear, smell,
taste, and feel stimuli from the
outside world. Collecting data
about that world is the function
of , and interpreting the
data collected is the function
of .
A. the brain; the spinal cord
B. the spinal cord; the brain
C. sensation; perception
D. perception; sensation
2. The main classes into which the
sense organs and sensory receptors
fall include all of the following
A. chemoreception.
B. electroreception.
C. photoreception.
D. mechanoreception.
3. An architect is designing apartments
and wants them to be soundproof.
She asks a psychologist what the
smallest amount of sound is that
can be heard. Her question is most
related to
A. the absolute threshold.
B. the difference threshold.
C. Weber’s law.
D. the sensory receptors.
APPLY IT! 4. Trina, a first-year college
student, goes home at Thanksgiving break
after being away from home (for the first
time) for three months. She feels as if she
has changed a lot, but her parents still
treat her like a kid in high school. At
Thanksgiving dinner she confronts them,
bursting out, “Stop top-down processing
me!” Her parents think Trina has lost her
mind. Which of the following explains her
A. Trina feels that her parents are judging
her sophisticated college ways too
B. Trina probably ate too much turkey.
C. Trina feels that her parents have spent
too much time analyzing her behavior.
D. Trina believes that her parents are
letting their preconceived ideas
of who she is prevent them from see-
ing her as the person she has become.
Sensory Adaptation
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96 // CHAPTER 3 // Sensation and Perception
When Michael May of Davis, California, was 3 years old, an accident left him visually
impaired, with only the ability to perceive the difference between night and day. He
lived a rich, full life, marrying and having children, founding a successful company,
and becoming an expert skier. Twenty- ve years passed before doctors transplanted
stem cells into May’s right eye, a new procedure that gave him partial sight (Kurson,
2007). May can now see; his right eye is functional and allows him to detect color and
negotiate the world without the use of a cane or reliance on his seeing-eye dog. His
visual experience remains unusual, however: He sees the world as if it is an abstract
painting. He can catch a ball thrown to him by his sons, but he cannot recognize his
wife’s face. His brain has to work at interpreting the new information that his right
eye is providing. May’s experience highlights the intimate connection between the
brain and the sense organs in producing perception. Vision is a remarkable process that
involves the brain’s interpretation of the visual information sent from the eyes. We now
explore the physical foundations of the visual system and the processes involved in
the perception of visual stimuli.
The Visual Stimulus and the Eye
Our ability to detect visual stimuli depends on the sensitivity of our eyes to differences
in light.
L I G H T Light i s a f o r m o f e l e c t r o m a g n e t i c e n e r g y t h a t c a n b e d e s c r i b e d i n t e r m s o f
wavelengths. Light travels through space in waves. The wavelength o f l i g h t i s t h e d i s -
tance from the peak of one wave to the peak of the next. Wavelengths of visible light
range from about 400 to 700 nanometers (a nanometer is 1 billionth of a meter and is
abbreviated nm). The wavelength of light that is re ected from a stimulus determines
its hue or color.
Outside the range of visible light are longer radio and infrared radiation waves and
shorter ultraviolet and X rays (Figure 3.6). These other forms of electromagnetic energy
continually bombard us, but we do not see them.
We can also describe waves of light in terms of their height, or amplitude, which
determines the brightness of the stimulus. Finally, the purity o f t h e w a v e l e n g t h s w h e t h e r
they are all the same or a mix of waves—determines the perceived saturation, or rich-
ness, of a visual stimulus (Figure 3.7). The color tree shown in Figure 3.8 can help you
to understand saturation. White light is a combination of color wavelengths that is per-
ceived as colorless, like sunlight. Very pure colors have no white light in them. They are
located on the outside of the color tree. Notice how, the closer we get to the center of
the color tree, the more white light has been added to the single wavelength of a par-
ticular color. In other words, the deep colors at the edge fade into pastel colors toward
the center.
T H E S T R U C T U R E O F T H E E Y E The eye, like a camera, is constructed to get
the best possible picture of the world. An accurate picture is in focus, is not too dark or
too light, and has good contrast between the dark and light parts. Each of several struc-
tures in the eye plays an important role in this process.
If you look closely at your eyes in the mirror, you will notice three parts—the sclera,
iris, and pupil (Figure 3.9). The sclera is the white, outer part of the eye that helps to
maintain the shape of the eye and to protect it from injury. The iris is the colored part
of the eye, which might be light blue in one individual and dark brown in another. The
The Visual System
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The Visual System // 97
Longer Wavelengths
Low energy
Shorter Wavelengths
High energy
Radio Television
X rays
750 700 650 600 550 500 450
of movement
of movement
White light
FIGURE 3.6 The Electromagnetic Spectrum and Visible Light (Top) Visible light is only a narrow band in the electromagnetic
spectrum. Visible light wavelengths range from about 400 to 700 nanometers. X rays are much shorter, radio waves much longer. (Bottom) The two
graphs show how waves vary in length between successive peaks. Shorter wavelengths are higher in frequency, as re ected in blue colors; longer
wavelengths are lower in frequency, as re ected in red colors.
Light waves of smaller amplitude make
up dimmer light.
Light waves of greater amplitude make
up brighter light.
Smaller amplitude
Greater amplitude
FIGURE 3.7 Light Waves of
Varying Amplitude The top graph might
suggest a spotlight on a concert stage; the
bottom, a candlelit dinner.
FIGURE 3.8 A Color Tree Showing Color’s
Three Dimensions: Hue, Saturation, and
Brightness Hue is represented around the color tree—
saturation horizontally and brightness vertically.
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98 // CHAPTER 3 // Sensation and Perception
pupil, which appears black, is the opening in the center of the iris. The iris contains
muscles that control the size of the pupil, and hence regulates the amount of light that
enters the eye. To get a good picture of the world, the eye needs to be able to adjust
the amount of light that enters. In this sense, the pupil acts like the aperture of a
camera, opening to let in more light when it is needed and closing to let in less light
when there is too much.
Two structures bring the image into focus: the cornea, a clear membrane just in
front of the eye, and the lens, a t r a n s p a r e n t a n d s o m e w h a t exible, disklike structure
lled with a gelatin-like material. The function of both of these structures is to bend
the light falling on the surface of the eye just enough to focus it at the back. The
curved surface of the cornea does most of this bending, while the lens ne-tunes things.
When you are looking at faraway objects, the lens has a relatively at shape because
the light reaching the eye from faraway objects is parallel and the bending power of
the cornea is suf cient to keep things in focus. However, the light reaching the eye
from objects that are close is more scattered, so more bending of the light is required
to achieve focus.
Without this ability of the lens to change its curvature, the eye would have a
toughtime focusing on close objects such as reading material. As we get older, the
lens loses exibility and hence its ability to change from its normal attened shape
to the rounder shape needed to bring close objects into focus. That is why many
people with normal vision throughout their young adult lives require reading glasses
as they age.
The parts of the eye we have considered so far work together to give us the sharpest
picture of the world. This effort would be useless, however, without a vehicle for record-
ing the images the eyes take of the world—in essence, the lm of the camera. Photo-
graphic lm is made of a material that responds to light. At the back of the eye is the
eye’s “ lm, the multilayered retina, which is the light-sensitive surface that records
electromagnetic energy and converts it to neural impulses for processing in the brain.
The analogy between the retina and lm goes only so far, however. The retina is amaz-
ingly complex and elegantly designed. It is in fact the primary mechanism of sight. Even
after decades of intense study, the full marvel of this structure is far from understood
(Collin & others, 2012; Herberstein & Kemp, 2012).
T h e h u m a n r e t i n a h a s a p p r o x i m a t e l y 1 2 6 m i l l i o n r e c e p t o r c e l l s . T h e y t u r n t h e e l e c -
tromagnetic energy of light into a form of energy that the nervous system can process.
There are two kinds of visual receptor cells: rods and cones. Rods and cones differ
both in how they respond to light and in their patterns of distribution on the surface
The multilayered light-
sensitive surface in the eye
that records electromag-
netic energy and converts
it to neural impulses for
processing in the brain.
FIGURE 3.9 Parts of the Eye Note that the image of
the butter y on the retina is upside down. The brain allows us to
see the image right side up.
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The Visual System // 99
of the retina (A. Lewis & others, 2010; Okano & others, 2012). Rods
are the receptors in the retina that are sensitive to light, but they
are not very useful for color vision. Rods function well under
low illumination; they are hard at work at night. Humans have about
120 million rods. Cones a r e t h e r e c e p t o r s t h a t w e u s e f o r c o l o r p e r -
ception. Like rods, cones are light-sensitive. However, they require a
larger amount of light to respond than the rods do, so they operate
best in daylight or under high illumination. There are about 6 million
cone cells in human eyes. Figure 3.10 shows what rods and cones
look like.
The most important part of the retina is the fovea, a tiny area in the
center of the retina at which vision is at its best (see Figure 3.9). The
fovea contains only cones and is vital to many visual tasks. Rods are
found almost everywhere on the retina except in the fovea.
Figure 3.11 shows how the rods and cones at the back of the retina
convert light into electrochemical impulses. The signal is transmitted
to the bipolar cells and then moves on to another layer of specialized
cells called ganglion cells (tom Dieck & Brandstatter, 2006). The axons
of the ganglion cells make up the optic nerve, which carries the visual
information to the brain for further processing. Figure 3.12 summarizes
the characteristics of rods and cones.
One place on the retina contains neither rods nor cones. This area,
the blind spot, is the area on the retina where the optic nerve leaves
the eye on its way to the brain (see Figure 3.11). We cannot see any-
thing that reaches only this part of the retina. To prove to yourself that you have a
blind spot, look at Figure 3.13. Once you have seen the yellow pepper disappear,
you have probably noticed it took a while to succeed at this task. Now shut
one eye and look around. You see a perfectly continuous picture of the world
around you; there is no blind spot. This is a great example of top-down pro-
cessing and a demonstration of the constructive aspect of perception. Your
brain lls in the gap for you (the one that ought to be left by your blind spot)
with some pretty good guesses about what must be in that spot, like a creative
artist painting in the blind spot.
The receptor
cells in the retina
that are sensitive
to light but not
very useful for
color vision.
The receptor
cells in the retina
that allow for
color perception.
optic nerve
The structure at
the back of the
eye, made up of
axons of the gan-
glion cells, that
carries visual in-
formation to the
brain for further
FIGURE 3.10 Rods and Cones In real
life, rods and cones look somewhat like stumps
and corncobs. To get a sense of how well the cones
in the fovea work, try reading out of the corner of
your eye. It is dif cult because the fovea doesn’t
get to do the reading for you. Keep in mind that the
visual information in the retina that is closest to
the nose crosses over, and the visual information
on the outer side of the retina stays on that side of
the brain.
Rod and cone
Blind spot
FIGURE 3.11 Direction of Light
in the Retina After light passes through the
cornea, pupil, and lens, it falls on the retina.
Three layers of specialized cells in the retina
convert the image into a neural signal that can
be transmitted to the brain. First, light triggers
a reaction in the rods and cones at the back of
the retina, transducing light energy into electro-
chemical neural impulses. The neural impulses
activate the bipolar cells, which in turn activate
the ganglion cells. Then light information is
transmitted to the optic nerve, which conveys it
to the brain. The arrows indicate the sequence in
which light information moves in the retina.
If you want to see a
ver y f aint s t ar , you s houl d gaze
sl ight l y away f r om i t , t o allow
your r ods t o do t hei r wor k.
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100 // CHAPTER 3 // Sensation and Perception
Visual Processing in the Brain
The eyes are just the beginning of visual perception. The next step occurs when neural
impulses generated in the retina are dispatched to the brain for analysis and integration
(Teismann & others, 2012).
The optic nerve leaves the eye, carrying information about light toward the brain.
Light travels in a straight line; therefore, stimuli in the left visual eld are registered
in the right half of the retina in both eyes, and stimuli in the right visual eld are
registered in the left half of the retina in both eyes (Figure 3.14). In the brain, at a
point called the optic chiasm, the optic nerve bers divide, and approximately half of
the nerve bers cross over the midline of the brain. As a result, the visual infor-
mation originating in the right halves of the two retinas is transmitted to the
right side of the occipital lobe in the cerebral cortex, and the visual informa-
tion coming from the left halves of the retinas is transmitted to the left side
of the occipital lobe. These crossings mean that what we see in the left side
of our visual eld is registered in the right side of the brain, and what we see in the
right visual eld is registered in the left side of the brain (see Figure 3.14). Then this
information is processed and combined into a recognizable object or scene in the
visual cortex.
T h e visual cortex, l o c a t e d i n t h e o c c i p i t a l l o b e a t t h e b a c k
of the brain, is the part of the cerebral cortex involved in vision. Most visual information
travels to the primary visual cortex, where it is processed, before moving to other visual
areas for further analysis (Schira & others, 2010).
A n i m p o r t a n t a s p e c t o f v i s u a l i n f o r m a t i o n p r o c e s s i n g i s t h e s p e c i a l i z a t i o n o f n e u -
rons. Like the cells in the retina, many cells in the primary visual cortex are highly
specialized (Shushruth & others, 2012). Feature detectors are neurons in the brain’s
visual system that respond to particular features of a stimulus. David Hubel and Torsten
Wiesel (1963) won a Nobel Prize for their research on feature detectors. By recording
the activity of a single neuron in a cat while it looked at patterns that varied in size,
shape, color, and movement, the researchers found that the visual cortex has neurons
that are individually sensitive to different types of lines and angles. One neuron might
visual cortex
Located in the
occipital lobe,
the part of the
cerebral cortex
involved in
feature detectors
Neurons in the brain’s visual
system that respond to par-
ticular features of a stimulus.
Wh a t s o n t h e n o s e s i d e
of t he r et i na cr osses over .
Type of vision
to light
Black and white
Dimly lit
Thin and long
Not on fovea
Well lit
Short and fat
On fovea and
of fovea
Characteristics of Rods and Cones Rods
and cones differ in shape, location, and function.
The Eye’s Blind Spot There is a normal
blind spot in your eye, a small area where the optic nerve leads to the
brain. To nd your blind spot, hold this book at arm’s length, cover your
left eye, and stare at the red pepper on the left with your right eye. Move
the book slowly toward you until the yellow pepper disappears. To nd
the blind spot in your left eye, cover your right eye, stare at the yellow
pepper, and adjust the book until the red pepper disappears.
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The Visual System // 101
Left visual field Right visual field
Processing at retina
Optic nerve
Optic chiasm
Visual cortex
in occipital lobe
Processing area
within the thalamus
FIGURE 3.14 Visual Pathways to and Through the
Brain Light from each side of the visual eld falls on the opposite
side of each eye’s retina. Visual information then travels along the optic
nerve to the optic chiasm, where most of the visual information crosses
over to the other side of the brain. From there visual information goes
to the occipital lobe at the rear of the brain. All these crossings mean
that what we see in the left side of our visual eld (here, the shorter,
dark-haired woman) is registered in the right side of our brain, and
what we see in the right visual eld (the taller, blonde woman) is
registered in the left side of our brain.
show a sudden burst of activity when stimulated by
lines of a particular angle; another neuron might re
only when moving stimuli appear; yet another neuron
might be stimulated when the object in the visual
eld has a combination of certain angles, sizes, and
Hubel and Wiesel also noted that when deprived of
certain types of visual stimulation early on, kittens
lost the ability to perceive these patterns. This nding
suggested that there might be a critical period in visual
development and that the brain requires stimulation in
its efforts to delegate its resources to different percep-
tual tasks. The brain “learns” to perceive through
experience. This explains Michael May’s unusual
experience, described at the beginning of our exami-
nation of the visual system. Once deprived of stimula-
tion, the brain will redistribute its resources to other
P A R A L L E L P R O C E S S I N G Sensory information
travels quickly through the brain because of parallel
processing, t h e s i m u l t a n e o u s d i s t r i b u t i o n o f i n f o r m a -
tion across different neural pathways (Joubert & others,
2008). A sensory system designed to process informa-
tion about sensory qualities serially or consecutively
(such as processing rst the shapes of images, then their
colors, then their movements, and nally their loca-
tions) would be too slow to keep us current with a rap-
idly changing world. To function, we need to “see” all
of these characteristics at once, which is parallel pro-
cessing. There is some evidence suggesting that parallel
processing also occurs for sensations of touch and hear-
ing (Recanzone & Sutter, 2008).
B I N D I N G Some neurons respond to color, others
to shape, and still others to movement; but note that
all of these neurons are involved in responding to a
given stimulus—for instance, a toddler running
toward you. How does the brain know that these
physical features, communicated by different neu-
rons, all belong to the same object of perception? The
answer is, binding.
One of the most exciting topics in visual perception
today, binding is the bringing together and integration
of what is processed by different neural pathways or
cells (Hong & Shevell, 2009; Tacca, 2012). Through
binding, you can integrate information about the tod-
dler’s body shape, smile, and movement into a complete
image in the cerebral cortex. How binding occurs is a
puzzle that fascinates neuroscientists (McMahon &
Olson, 2009).
Researchers have found that all the neurons throughout pathways that are activated
by a visual object pulse together at the same frequency (Engel & Singer, 2001). Within
the vast network of cells in the cerebral cortex, this set of neurons appears to bind
together all the features of the objects into a uni ed perception.
The simultaneous
distribution of in-
formation across
different neural
In the sense of
vision, the bring-
ing together and
integration of
what is processed
by different neural
pathways or cells.
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102 // CHAPTER 3 // Sensation and Perception
Color Vision
Imagine how dull the world would be without color. Art museums are lled with paint-
ings that are remarkable for their use of color, and owers would lose much of their
beauty if we could not see their rich hues. The process of color perception starts in the
retina, the eyes’ lm. Interestingly, theories about how the retina processes color were
developed long before methods existed to study the anatomical and neurophysiological
bases of color perception. Instead, psychologists made some extraordinarily accurate
guesses about how color vision occurs in the retina by observing how people see. The
two main theories proposed were the trichromatic theory and opponent-process theory.
Both turned out to be correct.
T h e trichromatic theory, proposed by Thomas Young in 1802 and extended by
Hermann von Helmholtz in 1852, states that color perception is produced by three types
of cone receptors in the retina that are particularly sensitive to different but overlapping
ranges of wavelengths. The theory is based on experiments showing that a person with
normal vision can match any color in the spectrum by combining three other wave-
lengths. Young and Helmholtz reasoned that if the combination of any three wavelengths
of different intensities is indistinguishable from any single pure wavelength, the visual
system must base its perception of color on the relative responses of three receptor
systems—cones sensitive to red, blue, and green.
T h e s t u d y o f d e f e c t i v e c o l o r v i s i o n , o r color blindness ( F i g u r e 3 . 1 5 ) , p r o v i d e s f u r t h e r
support for the trichromatic theory. Complete color blindness is rare; most color-blind
people, the vast majority of whom are men, can see some colors but not others. The nature
of color blindness depends on which of the three kinds of cones is inoperative (Machado,
Oliveira, & Fernandes, 2009). The three cone systems are green, red, and blue. In the
most common form of color blindness, the green cone system malfunctions in some way,
rendering green indistinguishable from certain combinations of blue and red.
I n 1 8 7 8 , t h e G e r m a n p h y s i o l o g i s t E w a l d H e r i n g o b s e r v e d t h a t s o m e c o l o r s c a n n o t
exist together, whereas others can. For example, it is easy to imagine a greenish blue but
nearly impossible to imagine a reddish green. Hering also noticed that trichromatic the-
ory could not adequately explain afterimages, sensations that remain after a stimulus is
removed (Figure 3.16 gives you a chance to experience an afterimage). Color afterimages
are common and involve particular pairs of colors. If you look at red long enough,
eventually a green afterimage will appear. If you look at yellow long enough, eventually
a blue afterimage will appear.
H e r i n g s o b s e r v a t i o n s l e d h i m t o p r o p o s e t h a t t h e r e w e r e n o t t h r e e t y p e s o f c o l o r
receptor cones (as proposed by trichromatic theory) but four, organized into complemen-
tary pairs: red-green and blue-yellow. Hering’s view, opponent-process theory, states
that cells in the visual system respond to red-green and blue-yellow colors; a given cell
trichromatic theory
Theory stating that color
perception is produced by
three types of cone recep-
tors in the retina that are
particularly sensitive to
different but overlapping
ranges of wavelengths.
opponent-process theory
Theory stating that cells in
the visual system respond to
complementary pairs of red-
green and blue-yellow col-
ors; a given cell might be
excited by red and inhibited
by green, whereas another
cell might be excited by yel-
low and inhibited by blue.
FIGURE 3.15 Examples of Stimuli
Used to Test for Color Blindness People
with normal vision see the number 16 in the left
circle and the number 8 in the right circle. People
with red-green color blindness may see just the 16,
just the 8, or neither. A complete color-blindness
assessment involves the use of 15 stimuli.
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The Visual System // 103
might be excited by red and inhibited by green, whereas another cell might be excited
by yellow and inhibited by blue. Researchers have found that opponent-process theory
does indeed explain afterimages (Jameson & Hurvich, 1989). If you stare at red, for
instance, your red-green system seems to “tire,and when you look away, it rebounds
and gives you a green afterimage.
If the trichromatic theory of color perception is valid and we do in fact have three
kinds of cone receptors like those predicted by Young and Helmholtz, then how can the
opponent-process theory also be accurate? The answer is that the red, blue, and green
cones in the retina are connected to retinal ganglion cells in such a way that the three-
color code is immediately translated into the opponent-process code (Figure 3.17). For
example, a green cone might inhibit and a red cone might excite a particular ganglion
cell. Thus, both the trichromatic and opponent-process theories are correct—the eye and
the brain use both methods to code colors.
Perceiving Shape, Depth,
Motion, and Constancy
P e r c e i v i n g v i s u a l s t i m u l i m e a n s o r g a n i z i n g a n d i n t e r p r e t i n g t h e f r a g m e n t s o f i n f o r m a -
tion that the eye sends to the visual cortex. Information about the dimensions of what
we are seeing is critical to this process. Among these dimensions are shape, depth,
motion, and constancy.
S H A P E Think about the visible world and its shapes—buildings against the sky,
boats on the horizon, the letters on this page. We see these shapes because they are
FIGURE 3.16 Negative Afterimage
Complementary Colors If you gaze steadily
at the dot in the colored panel on the left for a few
moments, then shift your gaze to the gray box on
the right, you will see the original hues’ comple-
mentary colors. The blue appears as yellow, the red
as green, the green as red, and the yellow as blue.
This pairing of colors has to do with the fact that
color receptors in the eye are apparently sensitive
as pairs: When one color is turned off (when you
stop staring at the panel), the other color in the
receptor is brie y turned on. The afterimage effect
is especially noticeable with bright colors.
Ganglion Cells
To optic nerve
and brain
FIGURE 3.17 Trichromatic and Opponent-
Process Theories: Transmission of Color
Information in the Retina Cones responsive
to green, blue, or red light form a trichromatic receptor
system in the retina. As information is transmitted to
the retina’s ganglion cells, opponent-process cells are
activated. As shown here, a retinal ganglion cell is
inhibited by a green cone (") and excited by a red
cone (#), producing red-green color information.
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104 // CHAPTER 3 // Sensation and Perception
marked off from the rest of what we see by
contour, a l o c a t i o n a t w h i c h a s u d d e n c h a n g e
of brightness occurs (Cavina-Pratesi & others,
2010). Now think about the letters on this
page. As you look at the page, you see letters,
which are shapes or gures, in a eld or back-
ground—the white page. The gure-ground
relationship is the principle by which we
organize the perceptual eld into stimuli that
stand out ( gure ) a n d t h o s e t h a t a r e l e f t o v e r
( background, o r ground ) . G e n e r a l l y t h i s p r i n -
ciple works well for us, but some gure-
ground relationships are highly ambiguous,
and it may be dif cult to tell what is gure
and what is ground. Figure 3.18 shows a well-
known ambiguous gure-ground relationship.
As you look at this illustration, your percep-
tion is likely to shift from seeing two faces to
seeing a single goblet.
The gure-ground relationship is a gestalt
principle (Figure 3.19 shows others). G e stalt is
German for “con guration” or “form, and
gestalt psychology is a school of thought that
probes how people naturally organize their
perceptions according to certain patterns. One of gestalt psychology’s main principles is
that the whole is different from the sum of its parts. For example, when you watch a
movie, the motion you see in the lm cannot be found in the lm itself; if you examine
the lm, you see only separate frames. When you watch the lm, the frames move past
a light source at a rate of many per second, and you perceive a whole that is very dif-
ferent from the separate frames that are the lm’s parts. Similarly, thousands of tiny
pixels make up an image (whole) on a computer screen.
D E P T H P E R C E P T I O N Images appear on our retinas in two-dimensional form,
yet remarkably we see a three-dimensional world. Depth perception is the ability to
perceive objects three-dimensionally. Look around you. You do not see your surround-
ings as at. You see some objects farther away, some closer. Some objects overlap
each other. The scene and objects that you are looking at have depth. How do you
see depth? To perceive a world of depth, we use two kinds of information, or cues—
binocular and monocular.
Because we have two eyes, we get two views of the world, one from each eye.
Binocular cues are depth cues that depend on the combination of the images in the
left and right eyes and on the way the two eyes work together. The pictures are slightly
different because the eyes are in slightly different positions. Try holding your hand
The principle by
which we orga-
nize the percep-
tual fi eld into
stimuli that stand
out (fi gure) and
those that are left
over (ground).
A school of
thought inter-
ested in how
people naturally
organize their
perceptions ac-
cording to cer-
tain patterns.
depth perception
The ability
to perceive
objects three-
binocular cues
Depth cues that depend
on the combination of the
images in the left and right
eyes and on the way the
two eyes work together.
(a) (b) (c)
FIGURE 3.19 Gestalt Principles of Closure, Proximity, and Similarity (a) Closure: When we see disconnected or incomplete gures, we ll
in the spaces and see them as complete gures. (b) Proximity: When we see objects that are near each other, they tend to be seen as a unit. You are likely to
perceive the grouping as four columns of four squares, not one set of 16 squares. (c) Similarity: When we see objects that are similar to each other, they tend to
be seen as a unit. Here, you are likely to see vertical columns of circles and squares in the left box but horizontal rows of circles and squares in the right box.
FIGURE 3.18 Reversible Figure-Ground Pattern Do
you see the silhouette of a goblet or a pair of faces in pro le? Use
this gure to think again about bottom-up and top-down processes.
> What processes did you use the rst moment you looked at the
picture—top-down or bottom-up? > Now try to see the opposite
image (if you saw a goblet, look for the faces; if you saw the faces,
look for the goblet). Is this top-down or bottom-up processing?
> Ask some friends what they see rst in this image. What do they
report? > What do you think accounts for the differences?
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The Visual System // 105
about 10 inches from your face. Alternately close and open your left and right
eyes so that only one eye is open at a time. The image of your hand will
appear to jump back and forth, because the image is in a slightly different
place on the left and right retinas. The disparity, or difference, between
the images in the two eyes is the binocular cue the brain uses to deter-
mine the depth, or distance, of an object. The combination of the two
images in the brain, and the disparity between them in the eyes, give us
information about the three-dimensionality of the world (Preston, Kourtzi, &
Welchman, 2009).
Convergence is another binocular cue to depth and distance. When we use our two
eyes to look at something, they are focused on the same object. If the object is near us,
our eyes converge, or move together, almost crossing. If the object is farther away, we
can focus on it without pulling our eyes together. The muscle movements involved in
convergence provide information about how far away or how deep something is.
I n a d d i t i o n t o u s i n g b i n o c u l a r c u e s t o g e t a n i d e a o f o b j e c t s d e p t h , w e r e l y o n a
number of monocular cues, o r d e p t h c u e s , a v a i l a b l e f r o m t h e i m a g e i n o n e e y e , e i t h e r
right or left. Monocular cues are powerful, and under normal circumstances they can
provide a compelling impression of depth. Try closing one eye—your perception of the
world still retains many of its three-dimensional qualities. Examples of monocular
1. Familiar size: This cue to the depth and distance of objects is based on what we
have learned from experience about the standard sizes of objects. We know how
large oranges tend to be, so we can tell something about how far away an orange is
likely to be by the size of its image on the retina.
2. Height in the eld of view: All other things being equal, objects positioned higher
in a picture are seen as farther away.
3. Linear perspective and relative size: Objects that are farther away take up less
space on the retina. So, things that appear smaller are perceived to be farther away.
As Figure 3.20 shows, as an object recedes into the distance, parallel lines in the
scene appear to converge.
A binocular cue to depth
and distance in which the
muscle movements in an
individual’s two eyes pro-
vide information about
howdeep and/or far away
something is.
monocular cues
Powerful depth
cues available
from the image
in one eye, either
the right or the
FIGURE 3.20 An
Artist’s Use of the
Monocular Cue of Linear
Perspective Famous
landscape artist J. M. W. Turner
used linear perspective to give
the perception of depth in Rain,
Steam, and Speed (1844).
3-D glasses use
di spar i t y as wel l t hey pr esent
di f f er ent i mages t o each eye.
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106 // CHAPTER 3 // Sensation and Perception
4. Overlap: We perceive an object that partially conceals or overlaps
another object as closer.
5. Shading: This cue involves changes in perception due to the posi-
tion of the light and the position of the viewer. Consider an egg
under a desk lamp. If you walk around the desk, you will see
different shading patterns on the egg.
6. Texture gradient: Texture becomes denser and ner the farther
away it is from the viewer (Figure 3.21).
Depth perception is a remarkably complex adaptation. Individuals
with only one functioning eye cannot see depth in the way that those
with two eyes can. Other disorders of the eye can also lead to a lack
of depth perception. Oliver Sacks (2006) described the case of Susan
Barry, who had been born with crossed eyes. The operation to correct
her eyes left her cosmetically normal, but she was unable to perceive
depth throughout her life. As an adult, she became determined to see
depth. With a doctor’s aid, she found special glasses and performed
eye muscle exercises to improve her chances of perceiving in three dimensions. It was
a dif cult and long process, but one day she noticed things starting to stick out” at
her—as you might when watching a lm in 3-D. Although Barry had successfully
adapted to life in a at visual world, she had come to realize that relying on mon-
ocular cues was not the same as experiencing the rich visual world of binocular vision.
She described owers as suddenly appearing “in ated. She noted how “ordinary
things looked extraordinaryas she saw the leaves of a tree, an empty chair, and her
of ce door projecting out from the background. For the rst time, she had a sense of
being inside the world she was viewing.
M O T I O N P E R C E P T I O N Motion perception plays an important role in the lives
of many species (Boeddeker & Hemmi, 2010). Indeed, for some animals, motion
perception is critical for survival. Both predators and their prey depend on being
able to detect motion quickly (Borst, Haag, & Reiff, 2010). Frogs and some other
simple vertebrates may not even see an object unless it is moving. For example, if
a dead y is dangled motionlessly in front of a frog, the frog cannot sense its
winged meal. The bug-detecting cells in the frogs retinas are wired only to sense
Whereas the retinas of frogs can detect movement, the retinas of humans and other
primates cannot. According to one neuroscientist, “The dumber the animal, the ‘smarter’
the retina” (Baylor, 2001). In humans the brain takes over the job of analyzing motion
through highly specialized pathways (Lee & Lee, 2012).
How do humans perceive movement? First, we have neurons that are specialized to
detect motion. Second, feedback from our body tells us whether we are moving or
whether someone or some object is moving; for example, you move your eye muscles
as you watch a ball coming toward you. Third, the environment we see is rich in cues
that give us information about movement (Badler & Heinen, 2006). For example, when
we run, our surroundings appear to be moving.
Psychologists are interested in both real movement and apparent movement, which
occurs when we perceive a stationary object as moving. You can experience apparent
movement at IMAX movie theaters. In watching a lm of a climb of Mount Everest,
you may nd yourself feeling breathless as your visual eld oods with startling images.
In theaters without seats, viewers of these lms are often warned to hold the handrail
because perceived movement is so realistic that they might fall.
P E R C E P T U A L C O N S T A N C Y Retinal images change constantly. Yet even though
the stimuli that fall on our retinas change as we move closer to or farther away from
The perception
that a stationary
object is moving.
FIGURE 3.21 Texture Gradient The
gradients of texture create an impression of depth
on a at surface.
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The Visual System // 107
objects, or as we look at objects from different orientations
and in light or dark settings, our perception of them remains
stable. Perceptual constancy is the recognition that objects
are constant and unchanging even though sensory input
about them is changing.
W e e x p e r i e n c e t h r e e t y p e s o f p e r c e p t u a l c o n s t a n c y
size constancy, shape constancy, and color constancy—as
Size constancy is the recognition that an object remains
the same size even though the retinal image of the object
changes (Figure 3.22). Experience is important to size
perception: No matter how far away you are from your
car, you know how large it is.
Shape constancy is the recognition that an object
retains the same shape even though its orientation to
you changes. Look around. You probably see objects
of various shapes—chairs and tables, for example.
If you walk around the room, you will view these
objects from different sides and angles. Even though
the retinal image of the object changes as you walk,
you still perceive the objects as having the same shape
(Figure 3.23).
Color constancy is the recognition that an object
retains the same color even though different amounts
of light fall on it. For example, if you are reaching
for a green Granny Smith apple, it looks green to
youwhether you are having it for lunch, in the bright
noon sun, or as an evening snack in the pale pink of
Perceptual constancy tells us about the crucial role of interpretation in perception: We
interpret sensation. That is, we perceive objects as having particular characteristics
regardless of the retinal image detected by our eyes. Images may ow across the retina,
but experiences are made sensible through perception. The many cues we use to visually
perceive the real world can lead to optical illusions when they are taken out of that real-
world context, as you can experience for yourself in Figure 3.24.
As you look over these illusions, consider that culture can in uence the extent to
which people experience these illusions. In cultures where two-dimensional images, such
as drawings on a piece of paper, are not typically used, geometrical illusions are less
likely to lead to errors (Segall, Campbell, & Herskovits, 1966).
The recognition
that objects are
constant and
unchanging even
though sensory
input about them
is changing.
FIGURE 3.22 Size Constancy Even though our
retinal images of the hot air balloons vary, we still realize the
balloons are approximately the same size. This illustrates the
principle of size constancy.
FIGURE 3.23 Shape Constancy The various projected images
from an opening door are quite different, yet you perceive a rectangular door.
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108 // CHAPTER 3 // Sensation and Perception
Pattern Recognition
Although the diagram contains no actual
triangles, your brain “sees” two overlapping
triangles. The explanation is that the
notched circles and angled lines merely
suggest gaps in which complete objects
should be. The brain fills in the missing
Induction Illusion
The yellow patches are identical, but they
look different and seem to take on the charac-
teristics of their surroundings when they
appear against different-color backgrounds.
Blinking Effect Illusion
Stare at the white circles and notice the
intermittent blinking effect. Your eyes make
the static figure seem dynamic, attempting to
fill in the white circle intersections with the
black of the background.
Ponzo Illusion
The top line looks much longer than the
bottom, but they are the same length.
Rotational Illusion
The two rings appear to rotate in different
directions when we approach or move away
from this figure while fixing our eyes on the
FIGURE 3.24 Perceptual Illusions These illusions show how adaptive perceptual cues can lead to errors when taken out of context. They are
de nitely fun, but keep in mind that these illusions are based on processes that are quite adaptive in real life. Remember, not everyone sees these illusions. In
cultures where exposure to two-dimensional representations is not common, individuals are less fooled by geometric illusions.
1. When we refer to the hue of a light
wave, we are referring to what we
perceive as
A. intensity.
B. radiation.
C. brightness.
D. color.
2. To read this question, you are looking
at it. After the light passes into
youreyes, the incoming light waves
are recorded by receptor cells
l ocatedin the
A. retina.
B. cornea.
C. blind spot.
D. optic chiasm.
3. If you are in a well-lighted room, your
rods are being used and
cones are being used .
A. infrequently; frequently
B. infrequently; infrequently
C. frequently; infrequently
D. frequently; frequently
APPLY IT! 4. Sondra was driving in the
country one afternoon. There was not much
traffic on the long, straight road, though
Sondra noticed a man walking along the
roadside some distance away. Suddenly, as
she approached the person, he drifted to-
ward the middle of the road, and Sondra,
with screeching brakes, was shocked to re-
alize she had nearly hit a child. Fortunately,
the child was not harmed. It had become
clear to Sondra that what had seemed like
aman some distance away was actually a
child who was much closer than she real-
ized. What explains this situation?
A. Sondra’s occipital lobe must be damaged.
B. Because objects that are smaller on the
retina are typically farther away, Sondra
was fooled by relative size.
C. Because objects in the mirror are closer
than they appear, Sondra was not able to
detect the just-noticeable difference.
D. Because objects that are smaller on the
retina are typically closer than they
appear, Sondra was fooled by shape
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The Auditory System // 109
Just as light provides us with information about the environment, so does sound. Sounds
tell us about the presence of a person behind us, the approach of an oncoming car, the
force of the wind, and the mischief of a 2-year-old. Perhaps most important, sounds allow
us to communicate through language and song.
The Nature of Sound and How We
Experience It
At a reworks display, you may feel the loud boom of the explosion in your chest.At
a concert, you might have sensed that the air around you was vibrating. Bass instruments
are especially effective at creating mechanical pulsations, even causing the oor to
vibrate. When the bass is played loudly, we can sense air molecules being pushed forward
in waves from the speaker. How does sound generate these sensations?
Sound waves are vibrations in the air that are processed by the auditory (hearing)
system. Remember that light waves are much like the waves in the ocean moving toward
the beach. Sound waves are similar. Sound waves also vary in length. Wavelength deter-
mines the sound wave’s frequency, that is, the number of cycles (full wavelengths) that
pass through a point in a given time interval. Pitch is the perceptual interpretation of the
frequency of a sound. We perceive high-frequency sounds as having a high pitch, and
low-frequency sounds as having a low pitch. A soprano voice sounds high-pitched; a
bass voice has a low pitch. As with the wavelengths of light, human sensitivity is limited
to a range of sound frequencies. It is common knowledge that dogs, for example, can
hear higher frequencies than humans can.
Sound waves vary not only in frequency but also, like light waves, in amplitude (see
Figure 3.7). A sound wave’s amplitude, measured in decibels (dB), is the amount of
pressure the sound wave produces relative to a standard. The typical standard, 0 decibels,
is the weakest sound the human ear can detect. Loudness is the perception of the sound
wave’s amplitude. In general, the higher the amplitude of the sound wave, or the higher
the decibel level, the louder we perceive the sound to be. Thus, in terms of amplitude,
the air is pressing more forcibly against you and your ears during loud sounds and more
gently during quiet sounds.
So far we have been describing a single sound wave with just one
frequency. A single sound wave is similar to the single wavelength of
pure colored light, discussed in the context of color matching. Most
sounds, including those of speech and music, are complex sounds,
those in which numerous frequencies of sound blend together. Timbre
is the tone saturation, or the perceptual quality, of a sound. Timbre is
responsible for the perceptual difference between a trumpet and a
trombone playing the same note and for the quality differences we
hear in human voices. Figure 3.25 illustrates the physical differences
in sound waves that produce the various qualities of sounds.
Structures and Functions
of the Ear
W h a t h a p p e n s t o s o u n d w a v e s o n c e t h e y r e a c h y o u r e a r ? H o w d o
various structures of the ear transform sound waves into signals that the
brain will recognize as sound? Functionally the ear is analogous to the
The Auditory System
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110 // CHAPTER 3 // Sensation and Perception
eye. The ear serves the purpose of transmitting a high- delity version of sounds in the
world to the brain for analysis and interpretation. Just as an image needs to be in focus
and suf ciently bright for the brain to interpret it, a sound needs to be transmitted in a
way that preserves information about its location, its frequency (which helps us distin-
guish the voice of a child from that of an adult), and its timbre (which allows us to
identify the voice of a friend on the telephone). The ear is divided into three parts: outer
ear, middle ear, and inner ear (Figure 3.26).
O U T E R E A R The outer ear consists of the pinna and the external auditory canal.
The funnel-shaped pinna (plural, pinnae ) is the outer, visible part of the ear. (Elephants
have very large pinnae.) The pinna collects sounds and channels them into the interior
of the ear. The pinnae of many animals, such as cats, are movable and serve a more
important role in sound localization than do the pinnae of humans. Cats turn their ears
in the direction of a faint and interesting sound.
outer ear
The outermost part of the
ear, consisting of the pinna
and the external auditory
Physical Difference
in Sound Waves and
the Qualities of Sound
They Produce Here we
can see how the input of
sound stimuli requires our
ears and brain to attend to
varying characteristics of the
rich sensory information that
is sound.
Physical Dimension Perceptual Dimension
Amplitude (intensity) Loudness
Frequency Pitch
Loud Soft
Low High
Complex sounds
Form of Sound Waves
(Form of sound wave from a clarinet)
Outer ear
Auditory canal
Middle ear
Inner ear
FIGURE 3.26 The
Outer, Middle, and
Inner Ear On entering the
outer ear, sound waves travel
through the auditory canal,
where they generate vibrations
in the eardrum. These vibra-
tions are transferred via the
hammer, anvil, and stirrup to
the uid- lled cochlea in the
inner ear. There the mechanical
vibrations are converted into
an electrochemical signal that
the brain will recognize as
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The Auditory System // 111
The hammer , anv i l ,
and s t i r r up ar e al s o cal l ed t he
oss i cl es.
These ar e t he t iniest
bones i n t he human body.
Cross section of cochlea
Basilar membrane
Basilar membrane
lined with hair cells
Auditory nerve
Tectorial membrane
Oval window
Hair cell
FIGURE 3.27 The Cochlea The cochlea is a spiral structure consisting of uid- lled canals. When the stirrup vibrates against the oval window, the uid
in the canals vibrates. Vibrations along portions of the basilar membrane correspond to different sound frequencies. The vibrations exert pressure on the hair cells
(between the basilar and tectorial membranes); the hair cells in turn push against the tectorial membrane, and this pressure bends the hairs. This triggers an action
potential in the auditory nerve.
M I D D L E E A R A f t e r p a s s i n g t h e p i n n a , s o u n d w a v e s m o v e t h r o u g h t h e a u d i t o r y c a n a l
to the middle ear. The middle ear c h a n n e l s t h e s o u n d t h r o u g h t h e e a r d r u m , h a m m e r , a n v i l ,
and stirrup to the inner ear. The eardrum, o r tympanic membrane, s e p a r a t e s t h e o u t e r e a r
from the middle ear and vibrates in response to sound. It is the rst structure that sound
touches in the middle ear. The hammer, anvil, a n d stirrup a r e a n i n t r i c a t e l y c o n n e c t e d
chain of very small bones. When they vibrate, they transmit sound waves to the uid- lled
inner ear (Stenfelt, 2006). The muscles that operate these tiny bones take the vibration of
the eardrum and transmit it to the oval window, t h e o p e n i n g o f t h e i n n e r e a r .
If you are a swimmer, you know that sound travels far more easily in air
than in water. Sound waves entering the ear travel in air until they reach the
inner ear. At the border between the middle and the inner ear—which, as we
will see below, is a border between air and uid—sound meets the same kind
of resistance as do shouts directed at an underwater swimmer when the shouts
hit the surface of the water. To compensate, the muscles of the middle ear can
maneuver the hammer, anvil, and stirrup to amplify the sound waves. Importantly, these
muscles, if necessary, can also work to decrease the intensity of sound waves, to protect
the inner ear.
I N N E R E A R The function of the inner ear, which includes the oval window, cochlea,
and basilar membrane, is to convert sound waves into neural impulses and send them on
to the brain (Gregan, Nelson, & Oxenham, 2011). The stirrup is connected to the mem-
branous oval window, which transmits sound waves to the cochlea. The cochlea is a
tubular, uid- lled structure that is coiled up like a snail (Figure 3.27). The basilar
middle ear
The part of the ear that
channels sound through the
eardrum, hammer, anvil, and
stirrup to the inner ear.
inner ear
The part of the ear that
includes the oval window,
cochlea, and basilar mem-
brane and whose function is
to convert sound waves into
neural impulses and send
them to the brain.
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112 // CHAPTER 3 // Sensation and Perception
membrane lines the inner wall of the cochlea and runs its entire length. It is narrow and
rigid at the base of the cochlea but widens and becomes more exible at the top. The
variation in width and exibility allows different areas of the basilar membrane to vibrate
more intensely when exposed to different sound frequencies (Wojtczak & Oxenham,
2009). For example, the high-pitched tinkle of a little bell stimulates the narrow region
of the basilar membrane at the base of the cochlea, whereas the low-pitched tones of a
tugboat whistle stimulate the wide end.
In humans and other mammals, hair cells line the basilar membrane (see Figure 3.27).
These hair cells are the sensory receptors of the ear. They are called hair cells because
of the tufts of ne bristles, or cilia, that sprout from the top of them. The movement of
the hair cells against the tectorial membrane, a jellylike ap above them, generates result-
ing impulses that the brain interprets as sound (Nowatny & Gummer, 2011). Hair cells
are so delicate that exposure to loud noise can destroy them, leading to deafness or dif-
culties in hearing. Once lost, hair cells cannot regenerate.
C o c h l e a r i m p l a n t s a r e d e v i c e s t h a t w e r e s p e c i cally developed to replace damaged
hair cells. A cochlear implant a small electronic device that is surgically implanted
in the ear and head—allows deaf or profoundly hard-of-hearing individuals to detect
sound (Sparreboom, Snik, & Mylanus, 2012). The implant works by using electronic
impulses to directly stimulate whatever working auditory nerves the recipient has in
his or her cochlea (Zhou, Xu, & P ngst, 2012). In the United States, approximately
41,500 adults and 25,500 children have had cochlear implants (U.S. Food and Drug
Administration, 2009).
Theories of Hearing
One of the auditory system’s mysteries is how the inner ear registers the frequency of
sound. Two theories aim to explain this mystery: place theory and frequency theory.
Place theory states that each frequency produces vibrations at a particular place on
the basilar membrane. Georg von Békésy (1960) studied the effects of vibration applied
at the oval window on the basilar membrane of human cadavers. Through a micro-
scope, he saw that this stimulation produced a traveling wave on the basilar
membrane. A traveling wave is like the ripples that appear in a pond when
you throw in a stone. However, because the cochlea is a long tube, the ripples
can travel in only one direction, from the oval window at one end of the
cochlea to the far tip of the cochlea. High-frequency vibrations create traveling
waves that maximally displace, or move, the area of the basilar membrane next to the
oval window; low-frequency vibrations maximally displace areas of the membrane closer
to the tip of the cochlea.
Place theory adequately explains high-frequency sounds but not low-frequency
sounds. A high-frequency sound, like the screech of a referees whistle or the piercing
high note of an opera diva, stimulates a precise area on the basilar membrane, just
as the theory suggests. However, a low-frequency sound, like the tone of a tuba or
the croak of a bullfrog, causes a large part of the basilar membrane to be displaced,
making it hard to identify an exact location that is associated with hearing this kind
of sound. Looking only at the movement of the basilar membrane, you would get
the impression that humans are probably not very good at hearing low-frequency
sounds, and yet we are. Therefore, some other factors must be at play in low- frequency
Frequency theory gets at these other in uences by stating that the perception of a
sound’s frequency depends on how often the auditory nerve res. Higher-frequency
sounds cause the auditory nerve to re more often than do lower-frequency sounds. One
limitation of frequency theory, however, is that a single neuron has a maximum ring
rate of about 1,000 times per second. Therefore, frequency theory does not apply to tones
with frequencies that would require a neuron to re more rapidly.
place theory
Theory on how
the inner ear
registers the fre-
quency of sound,
stating that each
frequency pro-
duces vibrations
at a particular
place on the bas-
ilar membrane.
frequency theory
Theory on how the inner
earregisters the frequency
of sound, stating that the
perception of a sound’s
frequency depends on
howoften the auditory
nerve fi res.
k ésy won a Nobel
Pr i z e i n 1 9 6 1 f o r h i s r e s e a r c h
on t he ba si l ar membr ane .
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The Auditory System // 113
Phew! T wo t heor i es
and a pr i nci pl e j us t t o expl ai n
how we hear .
T o d e a l w i t h t h i s l i m i t a t i o n o f f r e q u e n c y t h e o r y , r e s e a r c h e r s d e v e l o p e d t h e volley
principle, which states that a cluster of nerve cells can re neural impulses in rapid
succession, producing a volley of impulses. Individual neurons cannot re faster than
1,000 times per second, but if the neurons team up and alternate their neural ring, they
can attain a combined frequency above that rate. To get a sense for how the volley prin-
ciple works, imagine a troop of soldiers who are all armed with guns that can re only
one round at a time and that take time to reload. If all the soldiers re at the same time,
the frequency of ring is limited and cannot go any faster than it takes to reload those
guns. If, however, the soldiers are coordinated as a group and re at different times,
some of them can re while others are reloading, leading to a greater frequency of
ring. Frequency theory better explains the perception of sounds below 1,000 times
per second, whereas a combination of frequency theory and place theory is needed
to account for sounds above 1,000 times per second.
Auditory Processing in the Brain
As we considered in the discussion of the visual system, once our receptors pick up
energy from the environment, that energy must be transmitted to the brain for processing
and interpretation. We saw that in the retina, the responses of the rod and cone receptors
feed into ganglion cells and leave the eye via the optic nerve. In the auditory system,
information about sound moves from the hair cells of the inner ear to the auditory nerve,
which carries neural impulses to the brain’s auditory areas. Remember that it is the
movement of the hair cells that transforms the physical stimulation of sound waves into
the action potential of neural impulses.
Auditory information moves up the auditory pathway via electrochemical transmission
in a more complex manner than does visual information in the visual pathway. Many
synapses occur in the ascending auditory pathway, with most bers crossing over the
midline between the hemispheres of the cerebral cortex, although some proceed directly
to the hemisphere on the same side as the ear of reception (Lewald & Getzmann, 2011).
This means that most of the auditory information from the left ear goes to the right side
of the brain, but some also goes to the left side of the brain. The auditory nerve extends
from the cochlea to the brain stem, with some bers crossing over the midline. The
cortical destination of most of these bers is the temporal lobes of the brain (beneath
the temples of the head). As in the case of visual information, researchers have found
that features are extracted from auditory information and transmitted along parallel
pathways in the brain (Recanzone & Sutter, 2008).
Localizing Sound
When we hear a re engine’s siren or a dog’s bark, how do we know where
the sound is coming from? The basilar membrane gives us information about
the frequency, pitch, and complexity of a sound, but it does not tell us where
a sound is located.
Earlier in the chapter we saw that because our two eyes see slightly dif-
ferent images, we can determine how near or far away an object is. Similarly,
having two ears helps us to localize a sound because each receives somewhat
different stimuli from the sound source. A sound coming from the left has
to travel different distances to the two ears, so if a barking dog is to your
left, your left ear receives the sound sooner than your right ear. Also, your
left ear will receive a slightly more intense sound than your right ear in this
case. The sound reaching one ear is more intense than the sound reaching
the other ear for two reasons: (1) It has traveled less distance, and (2) the
other ear is in what is called the sound shadow of the listener’s head, which
volley principle
Modifi cation of frequency
theory stating that a cluster
of nerve cells can fi re neural
impulses in rapid succes-
sion, producing a volley of
auditory nerve
The nerve structure that
receives information about
sound from the hair cells of
the inner ear and carries
these neural impulses to the
brain’s auditory areas.
Imagine hearing impossible sounds,
nonexistent words, or three voices
where only two exist. Welcome to
the world of auditory illusions. In
Figure 3.24 you tried out some
visual illusions. Did you know that
there are auditory illusions as
well? Search the web for “auditory
illusions” and try some out. They
can be truly amazing and baffl ing!
Keep in mind, just as you did when
looking at the optical illusions, that
these illusions emerge as a function
of capacities that work very well in
the “real world.”
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114 // CHAPTER 3 // Sensation and Perception
provides a barrier that reduces the sound’s intensity (Figure 3.28). Blind individuals use
the sound shadow to orient themselves.
T h u s , d i f f e r e n c e s i n b o t h t h e timing o f t h e s o u n d a n d t h e intensity o f t h e s o u n d h e l p
you to localize a sound (Salminen & others, 2010). You often have dif culty localizing a
sound that is coming from a source directly in front of you because it reaches both ears
simultaneously. The same is true for sounds directly above your head or directly behind
you, because the disparities that provide information about location are not present.
FIGURE 3.28 The Sound
Shadow The sound shadow is caused
by the listener’s head, which forms a
barrier that reduces the sound’s intensity.
Here the sound is to the person’s left,
so the sound shadow will reduce the
intensity of the sound that reaches the
right ear.
Left ear Right ear
Sound shadow
1. Your mother’s and sister’s voices have
the same pitch and loudness, but you
can tell them apart on the telephone.
This is due to the perceptual quality, or
, of their voices.
A. timbre
B. wavelength
C. frequency
D. amplitude
2. The major function of the hammer, an-
vil, and stirrup of the middle ear is
A. to soften the tone of incoming stim-
uli for appropriate processing.
B. to stir cochlear fl uid so that bone
conduction hearing can occur.
C. to amplify vibrations and pass them
on to the inner ear.
D. to clean the external auditory canal
of any potential wax buildup.
3. The bones of the middle ear are set into
motion by vibrations of the
A. cochlea.
B. eardrum.
C. saccule.
D. basilar membrane.
APPLY IT! 4. Conservative radio person-
ality Rush Limbaugh experienced sudden
hearing loss in 2001, after which he received
a cochlear implant. He has described his
ability to listen to music as dependent on
what he heard before becoming deaf. Ifhe
had heard a song prior to becoming deaf, he
could hear it, but if it was a new song, he
could not make sense of it. Which of the fol-
lowing explains Limbaugh’s experience?
A. He is no longer able to listen to music
from a top-down perspective.
B. He is able to engage in top-down
listening, but not bottom-up listening.
C. He is likely to have experienced damage
to the temporal lobes.
D. He is not able to experience any
auditory sensation.
Beyond vision and hearing, the body has other sensory systems. These include the skin
senses and the chemical senses (smell and taste), as well as the kinesthetic and vestibu-
lar senses (systems that allow us to stay upright and to coordinate our movements).
The Skin Senses
You know when a friend has a fever by putting your hand to her head; you know how
to nd your way to the light switch in a darkened room by groping along the wall; and
you know whether a pair of shoes is too tight by the way the shoes touch different parts
Other Senses
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Other Senses // 115
of your feet when you walk. Many of us think of our skin as a canvas rather than a
sense. We color it with cosmetics, dyes, and tattoos. In fact, the skin is our largest sen-
sory system, draped over the body with receptors for touch, temperature, and pain. These
three kinds of receptors form the cutaneous senses .
T O U C H Touch is one of the senses that we most often take for granted, yet our
ability to respond to touch is astounding. What do we detect when we feel touch”?
What kind of energy does our sense of touch pick up from our external environment?
In vision we detect light energy. In hearing we detect the vibrations of air or sound
waves pressing against our eardrums. In touch we detect mechanical energy, or
pressure against the skin. The lifting of a single hair causes pressure on the skin
around the hair shaft. This tiny bit of mechanical pressure at the base of the hair is
suf cient for us to feel the touch of a pencil point. More commonly we detect the
mechanical energy of the pressure of a car seat against our buttocks or of a pencil in
our hand. Is this energy so different from the kind of energy we detect in vision or
hearing? Sometimes the only difference is one of intensity—the sound of a rock
band playing softly is an auditory stimulus, but at the high volumes that make
a concert hall reverberate, this auditory stimulus is also felt as mechanical
energy pressing against our skin.
How does information about touch travel from the skin through the nervous
system? Sensory bers arising from receptors in the skin enter the spinal cord.
From there the information travels to the brain stem, where most bers from
each side of the body cross over to the opposite side of the brain. Next the
information about touch moves on to the thalamus, which serves as a relay station.
The thalamus then projects the map of the body’s surface onto the somatosensory areas
of the parietal lobes in the cerebral cortex (Hirata & Castro-Alamancos, 2010).
Just as the visual system is more sensitive to images on the fovea than to images in
the peripheral retina, our sensitivity to touch is not equally good across all areas of the
skin. Human toolmakers need excellent touch discrimination in their hands, but they
require much less touch discrimination in other parts of the body, such as the torso and
legs. The brain devotes more space to analyzing touch signals coming from the hands
than from the legs.
T E M P E R A T U R E We not only can feel the warmth of a comforting hand on our
hand, we also can feel the warmth or coolness of a room. We must be able to detect
temperature in order to maintain our body temperature. Thermoreceptors, sensory nerve
endings under the skin, respond to temperature changes at or near the skin and provide
input to keep the body’s temperature at 98.6 degrees Fahrenheit. There are two types of
thermoreceptors: warm and cold. Warm thermoreceptors respond to the warming of the
skin, and cold thermoreceptors respond to the cooling of the skin. When warm and cold
receptors that are close to each other in the skin are stimulated simultaneously, we expe-
rience the sensation of hotness. Figure 3.29 illustrates this “hot” experience. To read
about fascinating research on the social signi cance of the feeling of temperature and
other tactile experiences, see the Intersection.
P A I N Pain is the sensation that warns us of damage to our bodies. When contact
with the skin takes the form of a sharp pinch, our sensation of mechanical pressure
changes from touch to pain. When a pot handle is so hot that it burns our hand, our
sensation of temperature becomes one of pain. Intense stimulation of any one of the
senses can produce pain—too much light, very loud sounds, or too many habanero pep-
pers, for example. Our ability to sense pain is vital for our survival as a species. It
functions as a quick-acting messenger that tells the brain’s motor systems that they must
act fast to minimize or eliminate damage.
Pain receptors are dispersed widely throughout the body—in the skin, in the sheath
tissue surrounding muscles, in internal organs, and in the membranes around bone.
Sensory nerve
endings under
the skin that
respond to
changes in tem-
perature at or
near the skin and
provide input to
keep the body’s
temperature at
98.6 degrees
The sensation
that warns an in-
dividual of dam-
age to the body.
A “Hot” Experience
When two pipes, one con-
taining cold water and the
other warm water, are braided
together, a person touching
the pipes feels a sensation
of “hot.The perceived heat
coming from the pipes is so
intense that the individual
cannot touch them for longer
than a couple of seconds.
Your abi l i t y t o f i nd
a ni ckel i n your pocket wi t hout
looking is a t ruly amazing feat
of t ouch. Not even t he
mo s t s o p h i s t i c a t e d
robot can do it .
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116 // CHAPTER 3 // Sensation and Perception
e might not notice the
softness of our pillow at
night or the hardness of
a chair we have sat in
thousands of times. Research by so-
cial cognitive psychologist John Bargh
and his colleagues (Ackerman, Nocera,
& Bargh, 2010; Bargh & Shalev,
2012) shows, however, that such tac-
tile experiences can have surprising
consequences for social perception
and behavior. They have learned, for
example, that what we feel with our
hands can change the way we feel
about other people.
Words used for the tactile sensa-
tions, such as heavy, rough, and hard,
have metaphorical meanings that ap-
ply to the social world. Consider the
results of a series of studies examin-
ing how touching various objects in u-
ences social perception. Participants
judged a job candidate as more im-
portant if the candidate’s résumé was
presented on a heavy (rather than
light) clipboard, evaluated interper-
sonal interactions as more dif cult af-
ter solving a puzzle with rough (rather than smooth) pieces, and
were more rigid in a negotiation after handling hard (rather than
soft) objects (Ackerman, Nocera, & Bargh, 2010). The metaphors
embodied by these literal tactile experiences apparently in u-
enced how participants felt, guratively.
Temperature serves as an especially powerful example of the
metaphoric meaning of the sense of touch. Warm and cold can
refer not only to temperature but to social feelings as well. A
“warm” person is kind and gentle, while a “cold” person is harsh
and rejecting. When we are loved by others, we might feel “all
warm inside,and when we are lonely
or rejected, we might feel “left out in
the cold.” How does the physical
experience of temperature relate to
these social experiences? In one
study, participants held either a hot
cup of coffee or a cup of iced coffee
and then rated another person. Par-
ticipants who held the hot cup of Joe
rated the individual as warmer than
those who held the iced coffee
(Williams & Bargh, 2008). In a similar
study, participants who held a hot
pack (versus a cold pack) were more
likely to behave in an interpersonally
warm way, choosing a gift for a friend
rather than for themselves (Williams
& Bargh, 2008). In yet another study,
individuals who handled a hot pack
were more trusting in an investment
game than those who handled a cold
pack (Kang & others, 2010). The
strong connection between physical
warmth and interpersonal warmth
has prompted Bargh to suggest that
we might substitute one kind of
warmth for another. If we feel lonely,
we might warm ourselves up with long hot bath (Bargh & Shalev,
These clever studies are reminders of the intricate links
between the body and the mind. Our mental experiences are
deeply embedded in a physical body.
The sense of touch conveys informa-
tion about the world, and that infor-
mation can color our perceptions
and behaviors without our even be-
ing aware ofit.
Social Psychology and Perception:
The Social Glow of Feeling Warm
How have all the things
you have felt (literally) today
influenced how you are
feeling (figuratively) today?
Although all pain receptors are anatomically similar, they differ in the type of physical
stimuli to which they most readily react, with some responding to pressure, others to
heat, and others to both. Many pain receptors are chemically sensitive and respond to a
range of pain-producing substances.
Pain receptors have a much higher threshold for ring than receptors for temperature
and touch (Bloom, Nelson, & Lazerson, 2001). Pain receptors react mainly to physical
stimuli that distort them or to chemical stimuli that irritate them into action. In amed
joints or sore, torn muscles produce prostaglandins, fatty acids that stimulate the receptors
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Other Senses // 117
and cause the experience of pain. Drugs such as aspirin likely reduce
the feeling of pain by reducing prostaglandin production.
Two different neural pathways transmit pain messages to the brain:
a fast pathway and a slow pathway (Bloom, Nelson, & Lazerson,
2001). In the fast pathway, bers connect directly with the thalamus
and then to the motor and sensory areas of the cerebral cortex. This
pathway transmits information about sharp, localized pain, as when you
cut your skin. The fast pathway may serve as a warning system, provid-
ing immediate information about an injury—it takes less than a second
for the information in this pathway to reach the cerebral cortex. In the
slow pathway, pain information travels through the limbic system, a
detour that delays the arrival of information at the cerebral cortex by
seconds. The unpleasant, nagging pain that characterizes the slow path-
way may function to remind the brain that an injury has occurred and
that we need to restrict normal activity and monitor the pain (Gao &
Ji, 2010).
Many neuroscientists believe that the brain actually generates the
experience of pain. There is evidence that turning pain signals on and
off is a chemical process that probably involves endorphins. R e c a l l
from Chapter 2 that endorphins are neurotransmitters that function as
natural opiates in producing pleasure and pain (Mirilas & others, 2010). Endorphins are
believed to be released mainly in the synapses of the slow pathway.
Perception of pain is complex and often varies from one person to the next (H. S.
Smith, 2010). Some people rarely feel pain; others seem to be in great pain if they
experience a minor bump or bruise. To some degree, these individual variations may be
physiological. A person who experiences considerable pain even with a minor injury may
have a neurotransmitter system that is de cient in endorphin production. However, per-
ception of pain goes beyond physiology. Although it is true that all sensations are affected
by factors such as motivation, expectation, and other related decision factors, the percep-
tion of pain is especially susceptible to these factors (Watson & others, 2006).
R e s e a r c h e r s h a v e r e p o r t e d t h a t w o m e n e x p e r i e n c e m o r e c l i n i c a l p a i n a n d s u f f e r m o r e
pain-related distress than men (Jarrett, 2011). However, a recent research review of stud-
ies from 1998 to 2008 of laboratory-induced pain in nonclinical participants found no sex
differences in perception of pain intensity and unpleasantness and no sex differences in
many types of pain (Racine & others, 2012a, 2012b). An important factor to consider in
the realm of gender differences in pain is the role of cultural expectations. Research has
shown that men are particularly likely to show high pain tolerance when the experimenter
is a woman, and especially if she is an attractive woman (Levine & De Simone, 1991).
Cultural and ethnic contexts in uence the degree to which an individual experiences
or reports pain (Jarrett, 2011). Compared to Americans, Japanese consider expressing
physical pain to be inappropriate (Hobara, 2005). One study of dental pain found that
although Japanese were more sensitive to stimulation, such as a needle prick to the gums,
they reported less pain than a comparison group of Belgians (Komiyama, Kawara, & De
Laat, 2007). Further, pain can mean different things to different people. For example,
one pain researcher described a ritual performed in India in which a chosen person
travels from town to town delivering blessings to the children and the crops while sus-
pended from metal hooks embedded in his back (Melzack, 1973). The individual appar-
ently reports no sensation of pain and appears to be in ecstasy.
The Chemical Senses
The information processed through our senses comes in many diverse forms: electromag-
netic energy in vision, sound waves in hearing, and mechanical pressure and temperature
in the skin senses. The two senses we now consider, smell and taste, are responsible for
Used by permission of
Inability to Feel Pain
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118 // CHAPTER 3 // Sensation and Perception
processing chemicals in our environment. Through smell, we detect airborne chemicals,
and through taste we detect chemicals that have been dissolved in saliva. Smell and
taste are frequently stimulated simultaneously. We notice the strong links between
the two senses when a nasty cold with lots of nasal congestion takes the pleasure
out of eating. Our favorite foods become “tasteless” without their characteris-
tic smells. Despite this link, taste and smell are two distinct systems.
T A S T E Think of your favorite food. Why do you like it? Imagine that
food without its avor. The thought of giving up a favorite taste, such as
chocolate, can be depressing. Indeed, eating food we love is a major source
of pleasure.
How does taste happen? To get at this question, try this. Take a drink of
milk and allow it to coat your tongue. Then go to a mirror, stick out your tongue,
and look carefully at its surface. You should be able to see rounded bumps above
the surface. Those bumps, called papillae, contain taste buds, the receptors for taste.
Your tongue houses about 10,000 taste buds, which are replaced about every two weeks.
As people age, however, this replacement process is not quite as ef cient, and an older
individual may have just 5,000 working taste buds at any given moment. As with all of
the other sensory systems we have studied, the information picked up by these taste
receptors is transmitted to the brain for analysis and, when necessary, for a response
(spitting something out, for example).
The taste bers leading from a taste bud to the brain often respond strongly to a
range of chemicals spanning multiple taste elements, such as salty and sour. The brain
processes these somewhat ambiguous incoming signals and integrates them into a
perception of taste (Iannilli & others, 2012). So although people often categorize
taste sensations along the four dimensions of sweet, bitter, salty, and sour, our
tasting ability goes far beyond these. Recently, researchers and chefs have been
exploring a taste called umami (Maruyama & others, 2006). Umami is the
Japanese word for “delicious” or “yummy. The taste of umami, one that
Asian cooks have long recognized, is the avor of L-glutamate. What is that
taste? Umami is a savory avor that is present in many seafoods as well as
soy sauce, parmesan and mozzarella cheese, anchovies, mushrooms, and hearty
meat broths.
Culture certainly in uences the experience of taste. Any American who has
watched the Japanese version of the TV series Iron Chef quickly notices