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and what we are doing, pain has a way of stealing the spotlight of our attention
(Eccleston & Crombez, 1999). But too much attention to pain can lead to depression,
so the person suffering from pain has to find a balance (Hulsebusch, Hasenbring, &
Rusu, 2016).
One popular method believed to reduce a sufferer’s pain experience during
medical procedures is distraction (Ryckeghem, Damme, Eccleston, & Crombez,
2018). In studies of pain tolerance, researchers have found that people can best
manage the effects of intense physical discomfort—and exhibit less activation in painresponsive areas of the brain—by focusing their attention on something else,
preferably something pleasant like a picture, music, film, odor, or even a virtual reality
environment (Inal & Kelleci, 2012; Malloy & Milling, 2010; Villemure & Bushnell,
2002). This is why children are asked to play a game when given a shot. In terms of
chronic pain, however, the findings across several studies demonstrate that
distraction has no significant impact on the pain experience or distress (Ryckeghem
et al., 2018). As we will learn in Chapter 4, hypnosis can also be used to combat
pain, in part through a refocusing of attention (Martin et al., 2018). Mind over
sensation.
Coordination
The five traditional senses and their subdivisions are vital adaptive mechanisms,
but by themselves they do not enable us to regulate sensory input through
movement. To bend, lean, stretch, climb, turn the head, maintain an upright posture,
and run from danger, we need to sense the parts of our bodies as well as our
orientation in space. The kinesthetic system monitors the positions of various body
parts in relation to each other. Just as vision comes to us through sensory receptors
in the eye, coordination of movement is provided by receptors in the joints, tendons,
and muscles. These receptors are linked to motor areas of the brain. Without this
system, an acrobat could not turn somersaults and cartwheels. Nor could gymnasts,
dancers, and athletes perform their feats of bodily magic. Nor, for that matter, could
we walk upright, deliver a firm handshake, aim food into our mouths, or touch our
noses with the tip of the index finger.
kinesthetic system. The structures distributed throughout the body that give us a
sense of position and movement of body parts.
From hanging upside down, to seamlessly unraveling as they descend, aerial silk
performers exhibit a remarkable sense of coordination and location in space thanks
to the kinesthetic and vestibular systems.
iStockphoto.com/sundrawalex
A related sensory mechanism is provided by the vestibular system, which
monitors head tilt and location in space. Situated in the inner ear, this system has two
parts: the semicircular canals, three fluid-filled tubes that are set at right angles to
one another; and two vestibular sacs, which are also filled with fluid. Whenever you
move about, the movement rotates and tilts your head, causing the fluid to slosh back
and forth, which pushes tiny hair cells. In turn, these hair cells send impulses to the
cerebellum, which signals from moment to moment whether you are sitting, lying
down, or standing on your head. The vestibular system provides us with the sense of
equilibrium, or balance. But sometimes this delicate sense is disrupted by an excess
of fluid, floating particles, an infection, or by certain types of motion. The result may
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be car sickness, sea sickness, or the dizzying aftereffects of twirling in circles
(Filippopulos et al., 2017; Howard, 1986; Thompson & Amedee, 2009).
vestibular system. The inner ear and brain structures that give us a sense of
equilibrium.
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Keeping the Signals Straight
In a world filled with lights and colors, voices and musical tones, smells and
tastes, and feelings of cold, warmth, pressure, pain, and other sensations, our
sensory abilities seem marvelously adaptive. How do we bring in so much information
without becoming overwhelmed? With neural impulses flooding the brain from
different receptors throughout the body, it is amazing that we do not get our signals
crossed. Why is it that we see light and hear sound rather than the other way
around?
There are, however, interesting exceptions. Exhibiting a very rare condition known
as synesthesia (“joining the senses”), some individuals report that they experience
sensory “crossovers”—that bright lights are loud, that the sound of a jazz trumpet is
hot, that colors can be felt through touch, or that they can “enjoy the sweet smell of
purple” or “taste the sound of raindrops” (Stein & Meredith, 1993). As described in
Richard Cytowic’s (1999) The Man Who Tasted Shapes, a number of fascinating
cases have been reported over the years. Recent studies indicate that the condition
is found in only 1 out of 2,000 people—and that most are women (Cytowic, 2002;
Harrison, 2001).
synesthesia. A rare condition in which stimulation in one sensory modality
triggers sensations in another sensory modality.
Can the self-reports of those claiming to have synesthesia be trusted? Although
there is reason to be skeptical, recent studies provide intriguing evidence. In one
study, nine women with word-color synesthesia and nine control subjects were asked
to report on the color sensations triggered by 130 letters, words, and phrases. When
retested a year later, without warning, the synesthetic women reported the identical
sensations 92 percent of the time—compared to only 38 percent in the control group.
In a second study, six synesthetic women and six control subjects listened to words
while blindfolded. PET scans revealed that this auditory stimulation activated the
language areas of the brain in both groups. Among the synesthetic women, it also
activated certain areas of the visual cortex (Paulesu et al., 1995). Other studies, too,
have provided independent evidence of this rare condition in adults (Martino & Marks,
2001; Smilek, Dixon, Cudahy, & Merikle, 2002) and children (Simner & Bain, 2013;
Simner, Harrold, Creed, Monro, & Foulkes, 2009). In fact, some argue that we are all
born with synesthesia (Maurer & Maurer, 1988; Spector & Maurer, 2011), but the
evidence is debatable (Deroy & Spence, 2013).
Although there are exceptions, our sensory systems generally do not cross. The
reason is that different receptors are sensitive only to certain types of energy and
stimulate only certain nerve pathways to the brain. Rods respond to light, not to
sound, and they transmit impulses through the optic nerve, not the auditory nerve.
There may be “normal” exceptions—as when pressing on a closed eyelid stimulates
the optic nerve and causes you to “see” a flash of light—but each sensory system
operates independent of the others (Gardner & Martin, 2000).
Two other aspects of sensation enable us to respond to volumes of information
without confusion. First, all of our sensory systems are designed to detect novelty,
contrast, and change—not sameness. After constant exposure to a stimulus,
sensation fades. This decline in sensitivity is known as sensory adaptation. We saw
earlier that the eyes gradually adapt to bright light and darkness. The same is true of
the other senses. After a while, you simply get used to the new contact lenses in your
eyes, the new watchband on your wrist, the noise level at work, or the coldness of
winter. To those sensitive to smells that often pervade the hallways of apartment
buildings (or dormitories), it is comforting to know that people also adjust to chronic
odors (Dalton & Wysocki, 1996; Pellegrino, Sinding, Wijk, & Hummel, 2017; Yoder et
al., 2014). By adapting to repeated stimulation, you are free to detect important
changes in the environment.
sensory adaptation. A decline in sensitivity to a stimulus as a result of constant
exposure.
A second adaptive mechanism is selective attention. People can choose to focus
on some sensory input and block out the rest. This selective attention enables us to
pick out a face or a voice in a crowded room or to find distractions from pain and
discomfort. Parents thus can hear their baby cry over the sounds of a TV, traders on
the floor of the stock exchange can hear orders to buy and sell amid all the noise,
and commuters in a city can spot yellow cabs in the street through all the commotion
of rush-hour traffic. People are not passive sensation-recording devices. We have a
way of “zooming in” on sensations—such as pain—that are personally important.
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LEARNING CHECK
Common Senses
Listed below are the five traditional senses, plus two additional sensory systems
covered in this chapter. Choose among them for answers to the questions below.
(Answers: 1. smell; 2. touch; 3. hearing; 4. vision; 5. kinesthetic system; 6. taste;
7. vestibular system.)
PERCEPTION
LEARNING OBJECTIVES
Summarize the steps we take to perceive our world as adults.
List the ways that perception is an active mental process.
Discuss how we manage to identify objects despite apparent changes in their
size, shape, and other features.
Describe how we perceive depth in three-dimensional space.
Examine whether our perceptual skills are inborn or learned from experience.
Critique why humans fall prey to perceptual illusions.
Our sensory systems convert physical energy from a multitude of sources into
neural signals that are transmitted to the brain. But we do not see inverted retinal
images, hear the bending and swaying of hair cells in the cochlea, or smell the
absorption of odorant molecules in the nose. These and other sensations must be
further processed to make sense. Perception is not a mere “copying” process, and
the brain does more than just serve as a sensory screenshot. As perceivers, we must
select, organize, and interpret input from the world in ways that are adaptive. Putting
the sensory pieces together is a “constructive” mental process (Brown, 2017; Gibson,
2015; Palmer, 1999).
To illustrate this point, consider the image in Figure 3.18. Some people might see
an elegant young woman looking over her right shoulder. But as the caption reveals,
the picture is a reversible figure, and it could also be seen as an elderly woman with
a very prominent nose and chin. Do you see both images? Look away for a few
seconds, and then look back. Can you see the elderly woman and the young
woman? The lines and shading have not changed, but if you look at the young
woman’s chin as a nose, the choker necklace as a mouth, and the ear as an eye, you
might be able to switch your brain into perceiving the elderly woman.
reversible figure. A drawing that one can perceive in different ways by reversing
figure and ground.
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Figure 3.18 A Reversible Figure Source: This image is available from the United
States Library of Congress's Prints and Photographs division under the digital ID
ds.00175.
Maybe not. The point is, visual input can often be processed in different ways, as
illustrated in Figure 3.19. The sensation may be the same, but the perception can
vary from one person and moment to the next.
Description
Figure 3.19 Perception and Interpretation
When Virgil’s eyesight was restored at the age of 50, he was able to detect lights,
shadows, colors, shapes, and textures, but he could not separate one figure from
another or identify common objects just by looking at them. In busy settings such as a
supermarket, he was so overwhelmed by sensory information that “everything ran
together.” In this section, we examine the ways in which the brain organizes and
interprets sensory input. As much of the research does, we will focus on visual
perception.
Perceptual Organization
In 1912, Max Wertheimer discovered that people perceive two stationary lights
flashing in rapid succession as a single light moving back and forth. This illusion of
apparent motion explains why we see flashing neon signs as a continuous stream
rather than as a series of separate lights. At the time, this illusion also paved the way
for Gestalt psychology—a school of thought arising in Germany that was founded
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on the premise that the whole (perception) is different from the sum of its parts
(sensations). The word gestalt is German for “pattern” or “whole,” and Gestalt
psychologists believed that humans have an inborn tendency to construct meaningful
perceptions from fragments of sensory input. A classic example is the way we listen
to music. A melody has a form that is different from the individual notes that make it
up. So, if the melody is transposed to another key, even if that means changing every
note, listeners would still recognize the music because its form would be the same.
The perception of music is based on a gestalt, not on a particular set of notes (Koffka,
1935; Kohler, 1947; Schindler, Herdener, & Bartels, 2013). The same is true of the
way people view and extract meaning from works of art and complex visual scenes
(Livingstone, 2002; Wagemans et al., 2012).
Gestalt psychology. A school of thought rooted in the idea that the whole
(perception) is different from the sum of its parts (sensation).
Figure and Ground
The first gestalt principle of perceptual organization is that people automatically
focus on some objects in the perceptual field to the exclusion of others. What we
focus on is called the figure. Everything else fades into the ground. A teacher
standing in front of a blackboard, the printed black words on a page or screen, the
lights on the car ahead of us on a dark highway, a scream in the night, and the lead
singer’s voice in a rock band—all are common figures and grounds. Gestalt
psychologists were quick to point out that these perceptions are in the eyes (or ears)
of the beholder—but also that we are prone to “figurize” objects that are close to us,
novel, intense, loud, and moving rather than still. Even ethnicity and culture are
believed to play a role in what is perceived as figure versus ground (Valarmathi et al.,
2021). As in the reversible figure in Figure 3.18, however, the image in Figure 3.20
demonstrates that we can mentally flip-flop the figure and ground from one moment
to the next. It is as if each of us is shining a spotlight on a portion of the sensory field
—and can move that spotlight if necessary.
Description
Figure 3.20 Figure and Ground iStock.com/Martin Janecek
Gestalt Laws of Grouping
Another principle of perceptual organization is that we tend to group collections of
shapes, sizes, colors, and other features into perceptual wholes. The natural
grouping tendencies are not arbitrary; rather, they follow simple rules like those
shown in Figure 3.21. The Gestalt psychologists argued that these tendencies are
inborn, and they may have been right. Research shows that even young infants
“group” stimulus objects in the predicted ways (Bhatt & Quinn, 2011; Quinn, Burke, &
Rush, 1993; White, Jubran, Heck, Chroust, & Bhatt, 2018). Some of these laws of
grouping are as follows:
Proximity. The closer objects are to one another, the more likely they are to
be perceived as a unit.
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Similarity. Objects that are similar in shape, size, color, or any other feature
tend to be grouped together.
Good Continuation. People perceive the contours of straight and curved
lines as continuous flowing patterns.
Closure. When there are gaps in a pattern that resembles a familiar form,
people mentally “close” the gaps and perceive the object as a whole. This
tendency enables us to recognize imperfect representations in hand drawings,
written material, and so on.
Common fate. Extending on the static grouping principles of proximity and
similarity, we find that objects moving together in the same direction, or sharing a
“common fate,” are perceived as belonging to a single group. Examples include
marching bands, schools of fish, flocks of birds, and sports fans sending the
“wave” around a stadium.
Description
Figure 3.21 Gestalt Laws of Grouping
The principles of Gestalt psychology describe how people transform raw visual
input—lights, shadows, lines, points, shapes, and colors—into meaningful displays.
More recent research has focused as well on the question of how our brains combine
these simple features into larger units, enabling us to identify common objects such
as chairs, airplanes, bottles, and so on. According to Irving Biederman (1987), people
can recognize common objects from a quick glance, based on exposure times as
brief as one-tenth of a second. The reason, he says, is that we perceive objects by
breaking them down into simple, three-dimensional component shapes called geons
(“geometric ions”) and then matching the unique pattern of shapes to “sketches”
stored in memory. Biederman has identified 36 geons that, when combined (as
illustrated in Figure 3.22), enable us to identify the essential contours of all objects—
the way that an alphabet of 26 letters can be used to form thousands of words.
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Description
Description
Figure 3.22 Identifying Objects
Theories of how people organize visual information and identify common objects
help to explain the effortless nature of perception—and the confusion that results
when figures are concealed from view through camouflage. But what happens to our
perception of an object when its retinal image changes from one moment to the next?
How do we know that objects have depth when the images projected on the retina
are flat and two-dimensional? How are interpretations of input influenced by
characteristics of the perceiver? As we will see, people are highly adept, yet often
fooled, by disparities between sensation and perception.
Ponder the following question: Is a zebra a white horse with black stripes or a
black horse with white stripes? What would you say? Neither color is figural in Gestalt
terms, and answers tend to be split.
iStock.com/pchoui
Perceptual Constancies
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Unlike a camera or microphone stationed on a tripod, the human perceiver is
active and mobile. And unlike the portrait or landscape hanging on a wall, many of
the objects that humans perceive are likewise active and mobile. As the perceiver
and perceived move about, the image projected on the retina may change in size,
shape, brightness, color, and other properties. But this is not a problem. Thanks to
perceptual constancies, perceptions remain stable despite radical changes in sensory
input (Dosher & Lu, 2017; Rock, 1997). As an example, consider size constancy, the
tendency to view an object as constant despite changes in the size of its image on
the retina. This is a common phenomenon. Imagine an observer watching from the
ground as an airplane pokes its nose through a cloud to descend for a landing. As the
plane approaches, its image looms larger and larger. Or another observer watches a
friend walk away. As the friend walks away, the image gets smaller and eventually
fades into the distance. The changing sensations might lead the observers to think
that the airplane was growing and that the friend was shrinking right before their
eyes.
size constancy. The tendency to view an object as constant in size despite
changes in the size of the retinal image.
But we know better—for two reasons. One has to do with experience and
familiarity. We know that airplanes are bigger than people and that people are bigger
than insects, so our perceptions remain stable despite variations in retinal image size.
Distance cues provide a second source of information. As objects move around in
space, we perceive the change in distance and adjust our size perceptions
accordingly. In other words, we know that the closer an object is, the larger the image
it casts on the retina, so we make the adjustment.
This skill is so basic that it can be observed in infants at 4 months of age. Carl
Granrud (2006) habituated 4-month-olds to an object: either a disk 6 centimeters in
diameter at a distance of 18 centimeters or a disk 10 centimeters in diameter at a
distance of 50 centimeters. After habituation, both objects were presented side by
side to the infants at a distance of 30 centimeters. The goal was to determine if
infants were able to differentiate between physical size and retinal size. Thus, during
the objects’ presentation, one test object had the same physical size that it did during
habituation, but a novel retinal size, while the other test object had the same retinal
size but a novel physical size. Infants looked longer at the object with the novel
physical size, demonstrating that it was not the retinal image they were attending to.
As demonstrated by Granrud (2006), the capacity for size constancy may be
present in infancy, but cultural and environmental experiences also play a role. In
1961, anthropologist Colin Turnbull studied Pygmies who lived in a densely wooded
central African forest. At one point, he took a Pygmy named Kenge for a Jeep ride
out of the forest. It was Kenge’s first trip away from home—and he was disoriented.
Standing on a mountain overlooking miles of open plain, Kenge saw buffaloes and
thought they were insects. Then he saw a fishing boat in the middle of a lake and
thought it was a floating piece of wood. The problem? Turnbull (1961) came to realize
that “in the forest the range of vision is so limited that there is no great need to make
an automatic allowance for distance when judging size” (p. 252).
Depth and Dimension
Perceptual constancies enable us to identify objects despite changes in sensory
input. But there’s another problem: How do we know that objects in three-dimensional
space have depth, and how do we perceive distance when images projected on each
retina are flat and only two-dimensional? Two types of information are used in depth
perception: binocular cues and monocular cues.
depth perception. The use of visual cues to estimate the depth and distance of
objects.
Binocular Depth Cues
With eyes on the sides of their heads, deer, sheep, and other prey can use
peripheral vision to see predators sneaking up from behind. By contrast, the eyes of
lions, owls, and other predators are squarely at the front of the head, an arrangement
that maximizes depth perception and enables them to track their prey. Human eyes
are the eyes of a predator. Our binocular (two-eyed) vision, in turn, allows us to use
two binocular depth cues: convergence and binocular disparity.
Convergence refers to the fact that the eyes turn in toward the nose or
“converge” as an object gets closer, and move outward or “diverge” to focus on
objects farther away. Hold your finger up at arm’s length and slowly move it toward
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your nose. As you refocus, you can actually feel your eye muscles contracting. This
signals the brain about the object’s distance from the eyes.
convergence. A binocular cue for depth perception involving the turning inward of
the eyes as an object gets closer.
The second cue is binocular disparity. With our eyes set about 2.5 inches apart
on the face, each retina receives a slightly different image of the world. To
demonstrate, hold your finger about 4 inches from your nose and shut your right eye.
Then shut only your left eye and look at the finger. Right. Left. As you switch back
and forth, you will see that each eye picks up the image from a slightly different
vantage point. Now hold up your finger farther away, say at arm’s length, and repeat
the routine. This time you will see less image shifting. The reason: Binocular disparity
decreases with distance. Special neurons located in the visual cortex use this retinal
information to “calculate” depth, distance, and dimensionality (Cumming & DeAngelis,
2001).
binocular disparity. A binocular cue for depth perception whereby the closer an
object is to a perceiver, the more different the image is in each retina.
If two eyes combine to give a three-dimensional look at the world, can flat pictures
do the same? In the 19th century, British physicist Charles Wheatstone invented the
first stereoscope—an optical instrument that brought two-dimensional pictures to life.
To create the illusion, Wheatstone photographed a scene twice, using two cameras
spaced inches apart. He then mounted both pictures side by side on the device,
using mirrors to overlap the images. This technique underlies the View-Master—a toy
that shows three-dimensional scenes in double-view cardboard slides. It is also used
in virtual reality (VR) systems and the Nintendo 3DS (Tidbury, Black, & O’Connor,
2015).
Note that these cupcakes are the same size even though the images they project
on your retina shrink with distance.
FOODSTUFF / Alamy Stock Photo
Monocular Depth Cues
Binocular depth cues are useful at short distances. But for objects that are farther
away, convergence and binocular disparity are uninformative. At such times, we can
utilize monocular depth cues, which enable us to perceive depth, quite literally with
one eye closed. These are cues that many artists use to bring a flat canvas to life.
What are they? Figure 3.23 describes the cues and illustrates them with
accompanying images.
monocular depth cues. Distance cues, such as linear perspective, that enable
us to perceive depth with one eye.
Figure 3.23 Monocular Depth Cues
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