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Virtual reality headsets contain two miniature screens for stereo vision. To
simulate reality, they are most realistic at short distances when they display separate
but overlapping images to each eye.
Drew Angerer / Staff/Getty
Origins of Depth Perception
With normal vision, interpreting the layout of objects in an environment is easy
and requires no conscious thought or effort. Why is depth perception so easy?
Clearly, a rich array of depth cues is available to one or both eyes—especially when
we are moving about (Gibson, 1979). But how do we know how to interpret these
cues? Are we born with these skills, or do we learn them from experience?
Perceptual Experience
The average person has an enormous amount of experience with depth
perception. Is this experience necessary? Case studies of blind people who had their
eyesight surgically restored during adulthood suggest that experience is critical to
depth perception. Sacks (1995) observed that Virgil sometimes stepped over
shadows so that he would not trip or failed to step up on a staircase that, for all he
knew, was a flat surface consisting of parallel and crossing lines. Richard Gregory
(1998) studied a similar patient by the name of S.B. and described his perception of
depth as “peculiar.” At one point, S.B. thought he could touch the ground below his
hospital window with his feet—even though his window was on the fourth floor. The
importance of perceptual learning and experience is also evident in cross-cultural
studies. We saw earlier that when a Pygmy named Kenge was taken from his dense
forest home to the open plain, he saw distant buffaloes as insects and a large boat as
a floating log. As described at the start of this chapter, research also shows that
people who lack exposure to three-dimensional representations in artwork find it
difficult to judge relative distance from pictures (Deregowski, 1989).
Depth Perception as Inborn
Experience may seem necessary, but studies of infants suggest otherwise. Infants
cannot tell us what they see, so Eleanor Gibson and Richard Walk (1960) devised the
visual cliff, a clever nonverbal test of depth perception. As demonstrated in Figure
3.24, the apparatus consists of a glass-covered table top, with a shallow one-inch
drop on one end and a steep “cliff” on the other end. Infants ages 6 to 14 months
were placed in the middle of the table, and their mothers tried to lure them into
crawling to one side or the other. The entire surface was covered by sturdy
transparent glass, so there was no real danger. The result: Six-month-old babies
would crawl to their mothers at the shallow end. But despite all the calling, clapping,
waving, and encouragement, most did not crawl out over the cliff. Clearly, they had
perceived the steepness of the drop.
visual cliff. An apparatus used to test depth perception in infants and animals.
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Description
Figure 3.24 The Visual Cliff Source: Levine, Laura E.; Munsch, Joyce, Child
Development From Infancy to Adolescence: An Active learning Approach, 1e, SAGE
Publications (2015).
Does the visual cliff experiment prove that depth perception is innate? Not
necessarily, argued critics. Perceptual learning begins at birth, so by the tender age
of 6 months an infant has already experienced over a thousand waking hours—and
has experienced plenty of perceptual practice. What about younger babies? They
may not be able to crawl, but their bodies can communicate to an astute researcher.
Accordingly, Campos, Langer, and Krowtiz (1970) moved 2-month-old infants from
one side of the glass top to the other and found that the infants exhibited a change in
heart rate when placed over the deep side but not over the shallow side. These
infants were too young to fear the situation as we would, but they “noticed” the
difference.
The development of this famous study did not happen overnight (Rodkey, 2015). It
took several animal studies conducted by Gibson and Walk to eventually evolve into
the visual cliff study we talk about today. The intriguing experiment that inspired the
visual cliff was one created by Gibson where rats were raised in an environment
completely absent of light (Rodkey, 2015).
In an attempt to replicate the surprising results with other animals, Gibson and
Walk also tested chicks, turtles, lambs, kid goats, pigs, kittens, dogs, and monkeys.
Kittens raised in the dark did not avoid the cliff. However, a mere 6-day exposure to
the light changed their response. Kittens exposed to light for this amount of time did
avoid the cliff, and they did not need to experience a fall to learn said avoidance.
Eventually, Walk (1981) published findings collected from other animals such as
newborn lambs, chicks, ducklings, pigs, cats, and rats.
This begs the question, is depth perception innate or is it the product of visual
experience? As the pieces of the puzzle have come together, it seems that both
factors are at work. Using binocular cues—and, later, monocular cues—infants are
capable of perceiving depth and dimension. But early experience is necessary for this
skill to emerge. Thus, formerly blind humans have trouble making judgments of depth
when their eyesight is surgically restored. As the saying goes, you have to “use it or
lose it.”
Perceptual Set
At any given moment, the interpretation of sensory input can be influenced by
prior experiences and expectations, which create a perceptual set. To illustrate,
review Figure 3.25. The middle drawings in this series are ambiguous: They can be
seen as either a man’s face or the figure of a kneeling young woman. Do people tend
to see the man’s face, or the kneeling young woman? It turns out that interpretations
are biased by prior experience. Subjects who were first shown the drawing on the far
left saw the middle pictures as a man’s face, whereas those who were first shown the
drawing on the far right saw the same pictures as a kneeling woman (Fisher, 1968).
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This finding highlights an important point about perception: At times, we see what we
expect to see (Bruner & Potter, 1964).
perceptual set. The tendency to perceive or notice some aspects of the available
information due to our past experiences or the context
Description
Figure 3.25 Perceptual Set Adapted from Heuer Jr., Richards J., Psychology of
Intelligence Analysis. Center for the Study of Intelligence. 1999.
Perceptual sets are established not only by past experience but also by the
context in which a stimulus is perceived. In Figure 3.26, for example, the same
physical pattern of black and white is used for the letter B as for the number 13.
Close inspection shows that the B and 13 are physically identical. Which of the two is
“seen” depends on whether the surrounding context consists of letters or numbers.
Description
Figure 3.26 Context Effects Jerome S. Bruner & A. Leigh Minturn (1955)
Perceptual Identification and Perceptual Organization, The Journal of General
Psychology, 53:1, 21-28, DOI: 10.1080/00221309.1955.9710133
The same phenomenon can influence perceptions of color. Review the shadow
illusion presented in Figure 3.27. Two squares are marked with an “X.” These two
squares appear to be different shades of gray, even though they are the same shade
of gray. The colors seen depend in part on the broader context in which they appear
(Hoffman, 1998).
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Description
Figure 3.27 The Shadow Illusion iStockphoto.com/PeterHermesFurian
The World of Illusions
The brain’s capacity to transform sensations into accurate perceptions of reality is
impressive. Without conscious thought, effort, or instruction, we often manage to
perceive size, shape, depth, and other properties in an accurate manner. But the
mind also plays tricks on us. Magicians, ventriloquists, and artists count on it. So do
perception psychologists. Over the years, researchers have learned a great deal
about how people perceive the world by probing the systematic ways in which they
also misperceive the world. From the mirage that glistens as wet on a highway to the
fastball that looks as if it is rising, and whether we should avoid wearing horizontal
stripes, perceptual illusions are all around us (Rodgers, 1998; Thompson &
Mikellidou, 2011; Wade, 1990).
perceptual illusions. Patterns of sensory input that give rise to misperceptions.
What is interesting about perceptual illusions is that they often stem from the
overapplication of rules that normally serve us well. Review the two vertical lines on
the left side of Figure 3.28. Which is longer? Most people believe that the line on the
right is slightly longer than the one on the left. Measure them, however, and you will
discover that they are the same length. As devised by Franz Müller-Lyer, in 1889,
these comparisons illustrate the classic and pervasive Müller-Lyer illusion.
Müller-Lyer illusion. An illusion in which the perceived length of a line is altered
by the position of other lines that enclose it.
Description
Figure 3.28 The Müller-Lyer Illusion
Why is the Müller-Lyer illusion so compelling? There are several possible
explanations (Nijhawan, 1991). One is that the arrowed tips trick us into overapplying
the linear-perspective depth cues and the principle of size constancy. In Figure 3.28,
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the vertical configuration depicted on the left resembles the near outside corner of a
room or building, whereas the configuration on the right resembles a far inside corner.
Because both lines cast equal-size retinal images, we assume that the farther one
must be larger. Part of the problem, then, is that people mistakenly apply a rational
rule of three-dimensional depth perception—that distance decreases image size—to
a flat two-dimensional figure (Gregory, 1998). Interestingly, however, the illusion is not
purely visual. It is also found in blindfolded people who make the line judgments by
running their fingers along raised plastic lines with arrowed tips (Millar & Al-Attar,
2002).
A second illusion that seems to stem from depth-related cues is the most
spectacular but also the most puzzling, as illustrated in Figure 3.29. A full moon
appears larger when it is low on the horizon than when it is high in the sky. The moon
is the moon, of course. It does not change in size or in its distance from the earth. So,
what causes this moon illusion?
moon illusion. The tendency for people to see the moon as larger when it is low
on the horizon than when it is overhead.
Description
Figure 3.29 The Moon Illusion iStock.com/mokee81
Anne Clark / Alamy Stock Photo
LEARNING CHECK
Perceptive Perspective
From the column on the right, choose the perceptual phenomenon that most
closely answers each question:
(Answers: 1. g; 2. d; 3. b; 4. c; 5. f; 6. a; 7. e.)
Throughout history, scholars have tried to understand this phenomenon. Then in
1962, Lloyd Kaufman and Irvin Rock brought it to the attention of perception
psychologists, which stimulated many theories and explanations. Some psychologists
claim the illusion is caused by earth-bound depth cues that make the moon seem
farther away and thus trick us into “seeing” a larger object. Indeed, if you peer at the
low moon through a tube, apart from surrounding cues, it will appear smaller. Others
have found that people sometimes perceive the horizon moon as closer, not more
distant (Coren & Aks, 1990), and that the illusion does not occur when the target
object is a star instead of the moon (Reed & Krupinski, 1992). Then there are those
who have demonstrated that the illusion persists even when the moon is projected at
different angles without depth cues, as in the total darkness of a planetarium (Suzuki,
1991), and that the illusion can also be created indoors by projecting a point of light
straight ahead, horizontally, or elevated at an upward angle (Suzuki, 1998). Ralph
Weidner and colleagues believe the explanation could be the unique firing of neural
patterns. Their study, which employed fMRI, found that certain parts of the brain were
involved in combining retinal size and distance when viewing a digital moon in a
three-dimensional virtual environment (Weidner et al., 2014). To this day, the moon
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illusion—and other psychological phenomena—remains something of a perceptual
mystery (Weidner et al., 2014). Psychology has come a long way to help us
understand why we perceive the things we do, but there is so much more we still do
not know.
Thinking Like a Psychologist About Sensation and
Perception
Sensation and perception are processes by which we make sense of the world
around us. The process begins with the raw stimuli that impinge on various sensory
receptors, sending signals through neural pathways to specialized regions of the
brain. In this way, light is converted to vision, vibration to sound, odorant molecules
into smell, and so on. That is the physiological part. But the human mind is active,
and people do not perceive stimuli the way a photocopy machine reproduces an
image. As perceivers, people select, organize, and interpret input from the world in
ways that are sensible and adaptive. Thus, perception is an active and constructive
mental process.
Human beings are sometimes remarkable in their ability to convert raw sensations
into an accurate representation of reality. Without conscious thought, effort, or
instruction, we can perceive sizes, shapes, depths, distances, colors, the location of
sounds, subtle odors, tastes, touches to the skin, and other properties of our
surrounding environment. Yet at times the mind plays tricks on us, fooling us into
misperceiving reality, sometimes in predictable ways—as shown in the many
demonstrations of perceptual illusions. This dual portrait of human beings is evident
in other chapters too. In many ways, we are supremely competent and yet subject to
bias and distortion. The trick is to recognize our biases and remember that much of
what we perceive is all in our heads.
SUMMARY
Through sensation we absorb raw energy with our sense organs. Transduction
converts this energy into neural signals to the brain, and then we select, organize,
and interpret the signals through perception. Sensation and perception are
interconnected. But they involve different processes.
Measuring the Sensory Experience
Psychophysics uses special measuring procedures to study the link between
physical stimuli and the sensations they arouse.
Absolute Thresholds
The absolute threshold is defined as the smallest amount of stimulation an
organism can detect 50 percent of the time.
Signal Detection Theory
The original work on absolute thresholds assumed that the stimulus alone
determined the threshold. But signal-detection theory takes into account the
subject’s response bias as well.
Difference Thresholds
Researchers also measure the ability to detect differences between two levels of a
stimulus. The smallest detectable change is called the difference threshold, or just
noticeable difference (JND). According to Weber’s law, the JND is a constant
proportion of the stimulus, so it increases as the stimulus increases.
Sensation
Humans have several distinct sensory modalities—more than the so-called five
senses.
Vision
The light we see is only a small band in the spectrum of electromagnetic radiation.
The physical properties of light waves—length, amplitude, and purity—correspond,
respectively, to our sensations of color, brightness, and saturation.
The human eye translates light waves into neural impulses. Light passes through
the cornea, which bends the light to focus it. Behind the cornea, the ring-shape iris
controls the size of the pupil, the hole through which light enters the eye. The lens
continues the task of focusing the light, becoming rounder for nearby objects and
flatter for remote ones—a process called accommodation. After passing through the
vitreous humor, the light hits the retina, a multilayer screen of photoreceptor cells.
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The rods in the retina are responsible for black-and-white vision in dim surroundings.
The cones, which provide for color vision, are concentrated in the fovea, the center
of the retina. With millions of photoreceptors, the eye can adjust to lighting changes
through dark adaptation and light adaptation.
The rods and cones stimulate bipolar and ganglion cells that integrate the
information they receive and pass it on to the optic nerve, composed of the axon
fibers of the ganglion cells. Because the area where the optic nerve enters the eye
has no rods or cones, each eye has a blind spot.
The two optic nerves meet at the optic chiasm, where the axons split up so that
fibers from inside half of each eye cross to the opposite side of the brain. The fibers
travel through the thalamus to the visual cortex. There the image is processed by
specialized neurons called feature detectors.
There are two theories of color vision. According to the trichromatic theory, the
human eye has three types of cones, sensitive to red, green, and blue. However, this
theory cannot explain afterimages, the visual sensations that linger after prolonged
exposure to a stimulus. The opponent-process theory also assumes that there are
three types of photoreceptors, but it contends that each kind responds to a pair of
“opponent” colors. Both theories are correct. The retina contains the types of cones
described by the trichromatic theory, but neurons in the thalamus operate in
accordance with the opponent-process theory.
Hearing
The stimulus for audition, or hearing, is sound—vibrations in air molecules
caused by movement of an object. Our sensations of pitch, loudness, and timbre
derive from the frequency, amplitude, and complexity of sound waves. White noise is
the hissing sound we hear when all frequencies of the sound spectrum are combined.
Collected by the outer ear, sound waves travel through the auditory canal to
vibrate the eardrum. The vibration continues through the bones in the middle ear, the
oval window of the inner ear, the fluid of the cochlea, and the membrane that excites
hair cells, which activate the auditory nerve. Like visual impulses, auditory signals
cross to the opposite side of the brain and pass through the thalamus before reaching
the auditory cortex.
The remarkable faculties of human hearing include auditory localization, our
ability to judge a sound’s direction. There are two types of hearing loss: conductive
hearing loss (caused by damage to the eardrum or middle-ear bones) and
sensorineural hearing loss (resulting from damage to the inner ear).
Other Senses
Our sense of smell derives from the olfactory system. Odor-causing molecules
dissolve and become trapped by receptors in the upper nasal passages, triggering
the olfactory nerve. Instead of passing through the thalamus like other sensory
information, the impulse goes straight to the olfactory bulb, which distributes the
information to the cerebral cortex and to limbic structures. Researchers are
investigating whether humans secrete pheromones, chemicals that transmit signals
to other humans.
Like smell, taste is a chemical sensation. The gustatory system begins with
taste buds in the mouth, which absorb molecules in food or drink and trigger neural
impulses to the thalamus and cortex. There are five primary tastes: sweet, salty, sour,
umami, and bitter. The flavor of food depends in part on the number of taste buds that
dot the tongue and in part on other factors such as odor.
Touch is based in the skin, the body’s largest organ. Touch involves many sensory
systems and the sensations of pressure, warmth, cold, and pain. Active touch, as
used by Braille readers, provides much more information than passive touch.
Temperature is a sensation with two unusual aspects: It is generally relative to a
person’s current state, and it entails two separate sensory systems—one for signaling
warmth and the other for signaling cold.
Pain is a subjective sensation with no single stimulus. The gate-control theory
suggests that pain signals to the brain can be blocked when they become too
intense. This theory explains why pain can often be eased by a competing sensation.
Endorphins, the body’s natural pain relievers, can also help control pain, as does the
psychological technique of distraction.
Our sense of coordination derives from the kinesthetic system. Receptors in the
joints, tendons, and muscles, linked to motor areas of the brain, help us register the
body’s position and movements. The related vestibular system includes structures
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in the inner ear that monitor the head’s tilt and location in space, giving us our sense
of equilibrium.
Keeping the Signals Straight
In the welter of sensations that confront us, three factors help us keep our signals
straight. First, the different senses have different receptors—though some people
have synesthesia, a very rare condition in which one sensory modality triggers
sensations in another sensory modality. Second, our senses are built to detect
novelty rather than sameness. As a result of sensory adaptation, our sensitivity to a
stimulus declines as a result of constant exposure. Third, selective attention allows us
to focus on one input and to block out the rest.
Perception
Perception is an active, “constructive” process. As simple reversible figures
demonstrate, perception involves selecting, organizing, and interpreting sensory
information.
Perceptual Organization
Based on the idea that the whole (perception) is different from the sum of its parts
(sensation), Gestalt psychology studies the way we construct meaningful
perceptions. In any perceptual field, we focus on the figure rather than the
background. We also group features into perceptual wholes according to the rules of
proximity, similarity, good continuation, closure, and common fate.
Perceptual Constancies
Although sensory inputs are always changing, perceptual constancies keep our
perceptions stable. Because of size constancy, we see an object as retaining its
size even when its retinal image grows or diminishes.
Depth and Dimension
Through depth perception, flat images on the retina are used to perceive
distances in three-dimensional space. One binocular depth cue is convergence, the
turning inward of the eyes when objects get closer. Another cue is binocular
disparity, the difference in retinal image between the two eyes. The closer the object
is, the greater the disparity. There are also monocular depth cues that permit depth
perception, including relative image size, linear perspective, interposition, and
atmospheric perspective.
Experiments with the visual cliff indicate that the capacity for depth perception
may be inborn. But case studies of blind people whose eyesight was restored and
cross-cultural evidence suggest as well that experience is needed to interpret depth
cues correctly.
Perceptual Set
Our prior experience and expectations often create a perceptual set that leads us
to see what we expect to see.
The World of Illusions
Despite the brain’s astonishing feats of perception, it falls prey to various
perceptual illusions. In the Müller-Lyer illusion, the perceived length of a line is
changed by the position of other lines that enclose it. And in the moon illusion, the
full moon looks larger when it is close to the horizon than when it is high in the sky.
Critical Thinking
Thinking Critically About Sensation and Perception
1. Distinguish sensation and perception. Can we have sensation without
perception? How about perception without sensation?
2. Gestalt psychologists assume that the whole is greater than the sum of its
parts. What exactly does this mean, and what does it have to do with
perception?
3. Children have more taste buds than adults, and we all have more taste buds
at the tip of our tongues than at the center. From an evolutionary perspective,
how might these differences be adaptive?
4. Suppose a friend wants to get a tattoo but is worried about the pain involved.
Your friend decides to deal with the pain by trying to ignore it. Is this a good
strategy for managing pain? What other strategies would you advise?
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Career Connection: Education
Childcare Worker
Daycare centers, nursery schools, preschools, after-school programs, and even
individual families may employ childcare workers. These caregivers attend to the
basic needs of children as well as enable children’s social and emotional growth
through play and learning. Administrators at care centers establish overall objectives
and standards, provide supervision of the staff and children in their care, and resolve
any disputes or disciplinary issues that may arise. Childcare workers are also needed
in mental health settings, aiding children and their families in outpatient settings.
Although some positions in daycare centers do not require a college degree, a
background in childhood development or psychology is increasingly desirable.
Key skills for this role that psychology students learn to develop:
Interpersonal relationship development
Self-efficacy and self-regulation
Innovative and integrative thinking and problem solving
Key Terms
absolute threshold (p. 89)
accommodation (p. 93)
afterimages (p. 99)
audition (p. 102)
auditory localization (p. 104)
binocular disparity (p. 120)
blind spot (p. 95)
conductive hearing loss (p. 105)
cones (p. 93)
convergence (p. 120)
cornea (p. 92)
dark adaptation (p. 95)
depth perception (p. 120)
feature detectors (p. 96)
fovea (p. 93)
gate-control theory (p. 112)
Gestalt psychology (p. 116)
gustatory system (p. 108)
iris (p. 92)
just noticeable difference (JND) (p. 90)
kinesthetic system (p. 113)
lens (p. 93)
light adaptation (p. 95)
monocular depth cues (p. 120)
moon illusion (p. 125)
Müller-Lyer illusion (p. 125)
olfactory system (p. 107)
opponent-process theory (p. 99)
optic nerve (p. 95)
perception (p. 87)
perceptual illusions (p. 124)
perceptual set (p. 124)
pheromones (p. 108)
psychophysics (p. 89)
pupil (p. 92)
receptive field (p. 96)
retina (p. 93)
reversible figure (p. 115)
rods (p. 93)
sensation (p. 87)
sensorineural hearing loss (p. 105)
sensory adaptation (p. 114)
signal-detection theory (p. 90)
size constancy (p. 118)
synesthesia (p. 114)
taste buds (p. 108)
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transduction (p. 87)
trichromatic theory (p. 99)
vestibular system (p. 113)
visual cliff (p. 122)
visual cortex (p. 96)
Weber’s law (p. 90)
white noise (p. 102)
Descriptions of Images and Figures
Back to Figure
The test features 2 illustrations as follows.
The upper picture shows a hunter with a spear on the left. The landscape consists
of low rolling hills. There is a tree at the top of the hill in the background. An elephant
stands to the right of the tree. An antelope stands to the right of the picture.
The lower picture shows a hunter with a spear on the left. The landscape consists
of flat land divided in half by a road that disappears into the distance. An elephant
stands on the road. A tree is positioned to the right of the elephant. An antelope
stands to the right of the picture.
The positions of the hunter and the animals have not changed. The landscape
and the position of the tree have changed. This alters the viewer’s perception of the
image.
Back to Figure
An author introduction reads as follows. Perception is the process of converting
physical stimuli, such as light and sound energy, into neural signals within our
sensory organs.
The diagram is divided into 2 illustrated examples. Part A includes the following
introduction.
Light is reflected off the petals of the flowers and into the eyes. The eyes then
transduce this light into a neural signal to be sent to the brain.
The illustration shows a woman crouching down in her garden. She is looking at
some pink and yellow tulips growing in a flowerbed. The second part of the illustration
shows how the eye translate this information into a neural signal. The image of the
tulips is inverted within the eye, so it appears upside down.
The diagram is divided into 2 illustrated examples. Part B includes the following
introduction.
Sound is produced by the bird and reaches our ears. Special hair cells in the
cochlea of the ear transduce the sound into a neural signal to be sent to the brain.
In the illustration a bird is shown perched on a branch and singing. A man listens
to the birdsong. The cochlea translates the sounds into a neural signal which is sent
to the brain.
Back to Figure
An author introduction reads as follows. The top part of the figure shows the
visible spectrum of light on a continuum of electromagnetic radiation. The figure
makes it clear that visible light is just a small range on this continuum. The bottom
part of the figure shows the distribution of wavelengths across natural sunlight,
incandescent light bulbs, and fluorescent bulbs.
Section 1. The Spectrum of Electromagnetic Radiation.
The horizontal continuum ranges from 10 superscript minus 11 meters to 10
superscript 17 meters, at intervals of 2. The wavelength is measured in nanometers.
The continuum is divided into the following sections.
Gamma rays, from 10 superscript minus 11 meters to 10 superscript minus 1
meters.
X-rays, from 10 superscript minus 1 meters to 10 superscript 0.5 meters.
U, V rays, from 10 superscript 1 meters to 10 superscript 2 meters.
The visible spectrum is positioned between the U, V and Infrared radiation
regions. The visible spectrum is explored in section 2.
Infrared radiation, from 10 superscript 3 meters to 10 superscript 6 meters
Microwaves, from 10 superscript 6 meters to 10 superscript 8 meters.
Radio waves, from 10 superscript 8 meters to 10 superscript 17 meters.
Section 2. The Visible Spectrum of Electromagnetic Radiation.
A line chart visualizes the relative intensity of the visible spectrum. The
wavelength in nanometers is plotted on the X-axis, with a range from 400 to 700, at
increments of 100. The relative intensity is plotted on the Y-axis, with a range from
zero 6. 4 light examples are presented on the graph. These are as follows.
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