Sensation and Perception
CHAPTER 3 OUTLINE
I. Introduction: What Are Sensation and Perception?
A. Sensation refers to the detection and basic sensory experience of environmental
stimuli, such as sounds, objects, and odors.
B. Perception occurs when we integrate, organize, and interpret sensory
information in a way that is meaningful.
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C. Basic Principles of Sensation
1. Sensation involves stimulation of sensory receptors and
transduction.
a. All sensation is a result of the stimulation of specialized cells
called sensory receptors by some form of energy.
b. Transduction is the process by which a form of physical energy
is converted into a coded neural signal that can be processed by
the nervous system.
2. Sensory Thresholds
A threshold is the point at which a stimulus is strong enough to be
detected because it activates a sensory receptor cell.
a. The absolute threshold refers to the smallest possible strength
of a stimulus that can be detected half the time.
b. The difference threshold is the smallest possible difference
between two stimuli that can be detected half the time; this is
also called the just noticeable difference (jnd).
c. Weber’s law is a principle of sensation that holds that for each
sense, the size of the just noticeable difference is a constant proportion
of the size of the initial stimulus.
3. Science Versus Pseudoscience: Subliminal Perception
a. Subliminal perception refers to the perception of stimuli that
are below the threshold of conscious perception or awareness.
b. The mere exposure effect refers to the fact that when people
are repeatedly exposed to a novel stimulus, their liking for that
particular stimulus will increase.
c. There is no evidence that subliminally presented stimuli can
change behavior or personality in any long-lasting or significant
way.
4. Sensory adaptation is the gradual decline in sensitivity to a constant
stimulus; that is, our experience of sensation is relative to the
duration of exposure.
II. Vision: From Light to Sight
A. What We See: The Nature of Light
1. Wavelength is the distance from one wave peak to another.
2. Humans are capable of visually detecting only a minuscule portion of
the electromagnetic energy range.
B. How We See: The Human Visual System
1. Vision involves a complex chain of events. Light waves reflected from
an object enter the eye, passing through the cornea, pupil, and lens.
a. The cornea is a clear membrane covering the front of the eye
that helps gather and direct incoming light. The sclera, the
white portion of the eye, is a tough, fibrous tissue that covers the
eyeball except for the cornea.
b. The pupil is the black opening in the middle of the eye.
c. The iris, the colored structure that determines eye color, is actually
a ring of muscles that controls the size of the pupil and thus
the amount of light entering the eye.
d. The lens is a transparent structure located behind the pupil that
actively focuses, or bends, light as it enters the eye.
(1) In a process called accommodation, the lens thins or thickens
to focus incoming light so that it falls on the retina.
(2) If the eyeball is abnormally shaped, the lens may not properly
focus the incoming light on the retina, resulting in a visual
disorder such as nearsightedness (myopia), farsightedness
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(hyperopia), or astigmatism. Presbyopia, another form of farsightedness,
often occurs during middle age.
2. The Retina: Rods and Cones
The retina is a thin, light-sensitive membrane located at the back of
the eye that contains the sensory receptors for light: the rods and
cones (the photoreceptors).
a. Rods and cones are shaped differently—rods are long and thin,
with blunt ends; cones are shorter and fatter, with one end that
tapers to a point.
b. The eye contains far more rods (about 125 million) than cones
(about 7 million).
c. Rods and cones are specialized for different visual functions—
rods are more sensitive to light than cones and are primarily
responsible for peripheral vision and night vision. Cones are
responsible for color vision and for vision in bright light (visual
acuity).
d. Rods and cones react differently to changes in the amount of
light; rods take about 30 minutes to reach maximum sensitivity
to light, whereas cones take about 5 minutes to do so.
e. Most of the cones are concentrated in the fovea, where visual
information is most sharply focused. Rods are most prevalent in
the periphery of the retina.
3. The blind spot
a. The optic disk is the point at which the fibers that make up the
optic nerve leave the back of the eye and project to the brain.
b. Because there are no photoreceptors in the optic disk, there is a
tiny hole, or blind spot, in your field of vision.
C. Processing Visual Information
Visual information is mostly processed in the brain after it undergoes
some preliminary processing in the retina.
1. Visual processing in the retina
a. Information from the sensory receptors, the rods and cones, is
collected by specialized neurons, called bipolar cells, which
then funnel the information to other specialized neurons called
ganglion cells.
b. Each ganglion cell receives information from photoreceptors in its
receptive field, then combines, analyzes, and encodes this information
before transmitting it to the brain.
c. A single ganglion cells receives information from only one or two
cones, but it may receive information from a hundred or more
rods.
2. From eye to brain
a. Information travels from the ganglion cells to the brain via the
optic nerve, a thick nerve made up of axons of the ganglion
cells that exits from the back of the eye at the optic disk and
extends to the visual cortex of the brain.
b. After leaving the back of the eyes, the left and right optic nerves
meet at the optic chiasm.
(1) The fibers of each optic nerve split in two, with one set of
axons crossing over to the opposite side of the brain and the
other set continuing along the same side of the brain.
(2) From the optic chiasm, most of the optic-nerve axons project
to the thalamus. A smaller number of axons first detour to
areas in the midbrain.
Chapter 3 Sensation and Perception 7
(3) From the thalamus, the signals are sent to the visual cortex,
where they are decoded and interpreted.
(4) Receiving neurons in the visual cortex of the brain are highly
specialized and are called feature detectors, because they
detect, or respond to, particular features or aspects of more
complex visual stimuli.
(5) Recognition of images involves additional levels of processing
in the visual cortex and other brain regions, including the
frontal lobes.
3. Focus on Neuroscience: Visual Experience and the Brain
Neuroscientists using fMRI have found that certain perceptual abilities
depend on experience. Although areas devoted to color and
motion perception develop early in infancy, brain areas specialized in
processing faces require experience.
D. Color Vision
1. The Experience of Color: What Makes an Orange Orange?
a. Our experience of color involves three properties of light waves:
(1) Hue is the property of wavelengths of light known as color.
Different wavelengths correspond to our subjective experience
of different colors.
(2) The saturation, or purity, of the color corresponds to the
purity of the light wave.
(3) Brightness, or perceived intensity, corresponds to the amplitude
of the light wave; the higher the amplitude, the greater
the degree of brightness.
b. The color of an object is determined by the wavelength of light
that the object reflects.
2. How we see color
a. The trichromatic theory
(1) According to the trichromatic theory of color vision,
there are three types of cones in the retina, each especially
sensitive to certain wavelengths: red light (long wavelengths),
green light (medium wavelengths), or blue light
(short wavelengths).
(2) When a color other than red, green, or blue strikes the
retina, it stimulates a combination of cones.
(3) The trichromatic theory explains the most common form of
color blindness, red–green color blindness, in which people
cannot discriminate between red and green.
b. The opponent-process theory
(1) An afterimage is a visual experience that occurs after the
original source of stimulation is no longer present.
(2) According to the opponent-process theory of color
vision, there are four basic colors that are divided into two
pairs of color-sensitive neurons: red–green and blue–yellow;
when one member of a color pair is stimulated, the other
member is inhibited. In addition, black and white act as an
opposing pair.
c. An integrated explanation of color vision
Both the trichromatic theory and the opponent-process theory of
color vision are accurate. Each theory explains color vision at a
different level of visual processing; trichromatic theory explains
processing in the cones in the retina, whereas opponent-process
theory explains processing in the ganglion cells, the thalamus,
and the visual cortex.
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III. Hearing: From Vibration to Sound
Audition, or the sense of hearing, results when sound waves are collected in
the outer ear, amplified in the middle ear, and transduced, or transformed
into neural messages, in the inner ear.
A. What We Hear: The Nature of Sound
1. Sound waves are the physical stimuli that produce the sensory experience
of sound.
2. Loudness is determined by the intensity, or amplitude, of a sound
wave and is measured in units called decibels.
3. Pitch, the relative “highness” or “lowness” of a sound, is determined
by the frequency of a sound wave, which is the rate of vibration, or
the number of sound waves per second. Frequency is measured in
units called hertz, which refers to the number of wave peaks per
second.
4. Timbre, the distinctive quality of a sound, is determined by the
complexity of the sound wave.
B. How We Hear: The Path of Sound
1. The outer ear includes the pinna, the ear canal, and the eardrum.
a. The pinna is that oddly shaped flap of skin and cartilage that’s
attached to each side of your head; its primary role is to catch
sound waves and funnel them into the ear canal.
b. The eardrum, which separates the outer ear from the middle
ear, is a tightly stretched membrane at the end of the ear canal
that vibrates when hit by sound waves.
2. The middle ear amplifies the vibrations of the eardrum; it consists
of three small bones, the hammer, the anvil, and the stirrup (so
named because of their shapes). Each bone sets the next bone in
motion.
a. The innermost bone, the stirrup, transmits the amplified vibration
to the oval window; if the tiny bones of the middle ear are
damaged or become brittle, as in old age, conduction deafness
may result.
b. Like the eardrum, the oval window is a membrane, but it is
many times smaller than the eardrum and it separates the middle
ear from the inner ear.
3. The inner ear is the part of the ear where sound is transduced into
neural impulses. It consists of the cochlea and the semicircular
canals.
a. The cochlea is the fluid-filled tube that is coiled in a spiral. The
fluid in the cochlea ripples in response to vibrations from the
oval window.
b. The vibrations from the rippling fluid are transmitted to the
basilar membrane. The basilar membrane runs the length of
the cochlea and is embedded with hair cells, the sensory receptors
for sound.
c. Hair cells have tiny, projecting fibers; if damaged, the result
can be nerve deafness, which cannot be helped by a hearing aid.
d. Transduction finally occurs: as the hair cells bend, they stimulate
the cells of the auditory nerve, which carries the neural
information to the thalamus and the auditory cortex in the
brain.
Chapter 3 Sensation and Perception 9
C. Distinguishing Pitch
1. The frequency theory explains how low-frequency sounds are
transmitted to the brain through vibration of the basilar membrane
at the same frequency as a sound wave.
2. According to the place theory, different frequencies cause larger
vibrations at different locations along the basilar membrane.
3. Both the frequency theory and the place theory explain our discrimination
of pitch.
IV. The Chemical and Body Senses: Smell, Taste, Touch, and Position
Chemical stimuli produce the sensations of smell (olfaction) and taste
(gustation), while pressure and other stimuli are involved in touch, pain,
position, and balance sensations.
A. How We Smell (Don’t Answer That!)
1. The sensory stimuli that produce our sensation of an odor are molecules
in the air. The molecules encounter millions of olfactory receptor
cells located high in the nasal cavity, which are constantly being
replaced. Odor receptors are present on hairlike fibers of the olfactory
neurons.
2. Each odor receptor seems to be specialized to respond to molecules of
a different chemical structure. When these receptor cells are stimulated,
a neural message is created, which travels along their axons,
bundles of which make up the olfactory nerves. The brain identifies
odors by interpreting the pattern of receptors that are stimulated.
3. The olfactory nerves directly connect to the olfactory bulb in the
brain, which is actually the enlarged end of the olfactory cortex at
the front of the brain. Axons from the olfactory bulb form the olfactory
tract, which projects to different brain areas, including the temporal
lobes and structures in the limbic system. Olfactory neurons are
also the only neurons that directly link the brain and the outside
world.
4. Sensory adaptation to odors generally occurs in less than one minute
5. In Focus: Do Pheromones Influence Human Behavior?
Pheromones are chemical signals that affect the behavior of other
animals of the same species. The search for human chemosignals has
narrowed to chemicals found in steroid compounds that occur in
sweat, armpit hair, blood, and semen. Rather than affecting behavior,
these chemosignals may be social signals, affecting social interactions
and relationships.
B. Taste
1. Our sense of taste, or gustation, results from the stimulation of special
receptors in the mouth called taste buds. Taste buds are located
on the tongue, on the insides of the cheeks, on the roof of the mouth,
and in the throat. Each taste bud contains about 50 receptor cells.
2. When activated, the receptor cells in the taste buds send neural
messages along neural pathways to the thalamus in the brain,
which, in turn, directs the information to several regions in the cortex.
3. The five basic taste categories—sweet, salty, sour, bitter, and
umami—combine to form all other tastes. Each taste bud shows
maximum sensitivity to one particular taste quality and lesser
degrees of sensitivity to other tastes.
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4. Taste is one aspect of flavor, which also involves the aroma, temperature,
texture, and appearance of food.
C. The Skin and Body Senses
The skin senses provide essential information about our physical status
and our physical interaction with objects in the environment. The body
senses keep us informed as to our position and orientation in space.
1. Touch
a. The skin is the largest (covers about 20 square feet of surface
area) and heaviest (weighs about six pounds) sense organ.
b. Located beneath the skin, the Pacinian corpuscle is an important
receptor involved in the sense of touch. When stimulated by
pressure, it converts the stimulation into a neural message that
is relayed to the brain.
c. Sensory receptors are distributed unevenly among different
areas of the body. Sensitivity to touch and temperature sensations
varies because some areas, such as the hands, face, and
lips, are much more densely packed with sensory receptors than
are other areas.
2. Pain
a. Pain is the sensation of physical discomfort or suffering that can
occur in varying degrees of intensity.
b. Nociceptors are the body’s pain receptors; they are actually
small sensory fibers called free nerve endings in the skin, muscles,
and internal organs. They transmit their messages to the
spinal cord.
c. Fast and slow pain systems
(1) The myelinated A-delta fibers transmit the sharp, intense,
but short-lived pain of the immediate injury
(2) The smaller, unmyelinated C fibers transmit the longerlasting
throbbing, burning pain of injury
(3) Most C fibers produce substance P, a pain enhancer that
stimulates free nerve endings at the injury site and increases
the pain messages within the spinal cord.
(4) Most messages cross to the other side of the spinal cord, then
to the brain.
(5) Fast pain messages travel to the thalamus, then the
somatosensory cortex.
(6) Slow pain messages go to the hypothalamus and thalamus,
then to the limbic system structures, such as the amygdala.
d. Factors that influence pain “gates”
According to the gate-control theory, pain is controlled by a
series of spinal “gates” that open and close.
(1) Depending on how the brain interprets the pain experience,
it regulates pain by sending signals down the spinal cord
that either “open” or “close” the gates. If, because of
psychological, social, or situational factors, the brain signals
the gates to open, pain is experienced or intensified; if the
brain signals the gates to close, pain is reduced.
(2) Psychological factors also influence the release of endorphins
and enkephalins, the body’s natural painkillers. In the brain
and spinal cord, they inhibit the transmission of pain signals.,
including the release of substance P.
(3) A person’s mental or emotional state can intensify or reduce
the perception of pain. Pain is also influenced by genetic factors,
social and situational factors
Chapter 3 Sensation and Perception 11
e. Sensitization: Unwarranted pain
Sometimes pain continues even after the injury is healed, as in
phantom limb pain. During the process of sensitization, pain
pathways become increasingly more responsive over time.
Eventually, pain occurs in the absence of any sensory input.
Such sensitization can result in chronic pain.
3. Movement, position, and balance
a. The kinesthetic sense involves the location and position of
body parts in relation to one another. Specialized sensory
neurons, called proprioceptors, are located in the muscles and
joints; these constantly communicate information to the brain
about changes in body position and muscle tension.
b. The vestibular sense provides a sense of balance, or equilibrium,
by responding to changes in gravity, motion, and body
position.
(1) Vestibular sensory information comes from the ear’s semicircular
canals and vestibular sacs. These fluid-filled structures
are lined with hairlike receptor cells that shift in response to
motion, changes in body position, or changes in gravity.
(2) Maintaining our equilibrium also involves information from
other senses, particularly vision. When information from the
eyes conflicts with information from the vestibular system,
the result can be dizziness, disorientation, and nausea.
V. Perception
Perception refers to the process of integrating, organizing, and interpreting
sensory information into meaningful representations.
1. Bottom-up processing, or data-driven processing, refers to
information processing that emphasizes the importance of the
sensory receptors in detecting the basic features of a stimulus in
the process of recognizing a whole pattern.
2. Top-down processing, or conceptually driven processing, refers to
information processing that occurs when the observer draws on his
or her knowledge, expectations, and other cognitive processes in
arriving at meaningful perceptions.
3. Gestalt psychology was founded by German psychologist Max
Wertheimer in the early 1900s. Gestalt psychologists emphasized
that we perceive whole objects or figures (gestalts) rather than isolated
bits and pieces of sensory information.
4. Culture and Human Behavior: Ways of Seeing
a. People in individualistic cultures tend to emphasize the needs of
the individual. In collective cultures, people take a more interdependent
perspective. Research has shown that these differences
influence visual perception and memory.
b. Further research showed that people from different cultures use
the same neural processes (brain functions) to make perceptual
judgments, but they notice different things and think differently
about what they see.
4. Critical Thinking: ESP: Can Perception Occur Without Sensation?
ESP, or extrasensory perception, refers to the perception of information
by some means other than through the normal processes of
sensation.
a. Forms of ESP include telepathy, clairvoyance, psychokinesis, and
precognition. The general term for these abilities is paranormal
phenomena.
12 Chapter 3 Sensation and Perception
b. Parapsychology refers to the scientific investigation of claims
of paranormal phenomena.
c. Possible explanations for claims of ESP experiences include coincidence
and the fallacy of positive instances.
d. Although some psychologists argue that studies using the
ganzfeld procedure demonstrate that ESP exists, many disagree.
To date, no experiment claiming to show evidence of the existence
of ESP has been replicated successfully.
A. The Perception of Shape: What Is It?
Although to some degree we rely on size, color, and texture to determine
what an object might be, we rely primarily on an object’s shape to
identify it.
1. The figure–ground relationship is an important perceptual principle
that states that we automatically separate the elements of a
perception into the feature that clearly stands out (the figure) and
its less distinct background (the ground). The perception of an image
in two different ways is called a figure–ground reversal.
2. Perceptual grouping is the way we actively organize elements to try
to produce the stable perception of well-defined, whole objects. These
“laws,” or principles, include similarity, closure, good continuation,
and proximity. The law of Prägnanz, or the law of simplicity, which
encompasses all the other Gestalt principles, states that when several
perceptual organizations of an assortment of visual elements are
possible, the perceptual interpretation that will occur will be the one
that produces the “best, simplest, and most stable shape.”
B. Depth Perception: How Far Away Is It?
Depth perception refers to the use of visual cues to perceive the distance
or three-dimensional characteristics of objects.
1. Monocular cues are distance or depth cues that can be processed
by either eye alone. They are used by artists to convey distance or
depth. These pictorial cues include
a. Relative size: If two or more objects are assumed to be similar in
size, the object that appears larger is perceived as being closer.
b. Overlap (or interposition): When one object partially blocks the
view of another object, the partially blocked object is perceived
as being farther away.
c. Aerial perspective: Faraway objects often appear hazy or slightly
blurred by the atmosphere.
d. Texture gradient: As a surface with a distinct texture extends
into the distance, the details of the surface texture gradually
become less clearly defined.
e. Linear perspective: Parallel lines seem to meet in the distance.
The closer together the lines appear to be, the greater the perception
of distance.
f. Motion parallax: When you are moving, you use the speed of
passing objects to estimate the distance of the objects. Nearby
objects seem to zip by faster than do distant objects.
g. Accommodation, another monocular cue, utilizes information
about changes in the shape of the lens of the eye to help us
estimate distance.
2. Binocular cues for distance or depth perception require
information from both eyes.
a. Convergence is the degree to which muscles rotate your eyes to
focus on an object.
Chapter 3 Sensation and Perception 13
b. Binocular disparity occurs because our eyes are set a couple of
inches apart, causing a slightly different image of an object to be
cast on the retina of each eye.
c. A stereogram is a picture that uses the principle of binocular
disparity to create the perception of a three-dimensional image.
C. The Perception of Motion: Where Is It Going?
1. As we follow a moving object with our gaze, the image of the object
moves across the retina; we compare the moving object to the background,
which is usually stationary.
2. When the retinal image of an object enlarges, we perceive the object
as moving toward us; our perception of its speed is based on our
estimate of the object’s rate of enlargement.
3. First studied by Gestalt psychologist Karl Duncker in the 1920s,
induced motion is the illusion of motion that occurs because we have
a strong tendency to assume that the background is stationary.
4. Stroboscopic motion, another illusion of apparent motion, was first
studied by Gestalt psychologist Max Wertheimer in the early 1900s.
It occurs when a light briefly flashes at one location, followed about
a tenth of a second later by another light briefly flashing at a second
location. The perception of smooth movements in a movie is also due
to stroboscopic motion.
D. Perceptual Constancies
Perceptual constancy is the tendency to perceive objects, especially
familiar objects, as constant and unchanging despite changes in sensory
input.
1. Size and shape constancy
a. Size constancy is the perception that an object remains the
same size despite its changing image on the retina. If the retinal
image of an object does not change, but the perception of its distance
increases, the object is perceived as larger.
b. Shape constancy is the tendency to perceive familiar objects as
having a fixed shape regardless of the image they cast on our
retinas.
VI. Perceptual Illusions
A perceptual illusion is the misperception of the true characteristics of an
object or an image. The following two illusions are misapplications of the
principle of size constancy.
A. The Müller-Lyer Illusion
1. The Müller-Lyer illusion involves the misperception of the identical
length of two lines, one with the arrow pointed inward, one with
the arrow pointed outward.
2. Culture and Human Behavior: Culture and the Müller-Lyer Illusion:
The Carpentered-World Hypothesis
Research has supported the carpentered-world hypothesis that
people living in urban industrialized environments are more susceptible
to the Müller-Lyer illusion because they have more experience
judging lines, corners, edges, and other rectangular manufactured
objects.
B. The Moon Illusion
1. The moon illusion involves the misperception that the moon is
larger when it is on the horizon than when it is directly overhead.
The retinal size of the full moon is the same in all positions; if you
watch the moon rise from the horizon to the night sky, however, it
does appear to shrink in size.
14 Chapter 3 Sensation and Perception
Chapter 3 Sensation and Perception 15
2. Perceptual illusions underscore the fact that what we see is not
merely a simple reflection of the world, but rather our subjective
perceptual interpretation of it.
3. Mike and perceptual illusions
The Shepard Tables capitalize on our automatic use of depth
perception cues to perceive a two-dimensional drawing as a threedimensional
object.
VII. The Effects of Experience on Perceptual Interpretations
1. Past experiences often predispose us to perceive a situation in a particular
way, even though other perceptions are possible.
2. Perceptual set is the influence of prior assumptions and expectancies
on perceptual interpretations.
VIII. Application: Strategies to Control Pain
1. Self-Administered Strategies
a. Distraction: Actively focus your attention on some nonpainful
stimulus.
b. Imagery: Create a vivid mental image of a pleasant scenario.
c. Relaxation: Learn relaxation strategies.
d. Counterirritation: Create a strong competing sensation that is
mildly stimulating or irritating.
e. Positive self-talk: Make positive coping statements or redefine
the pain.
2. Can Magnets Relieve Pain?
a. Magnets are a popular alternative treatment for pain, but scientific
evidence of their efficacy is inconclusive.
3. Strategies Pain Specialists Use
a. Hypnosis
b. Painkilling drugs
c. Biofeedback: Learning control over largely automatic body
functions.
d. Acupuncture: Inserting tiny needles at specific locations on
the body to relieve pain.
Summary of the Senses
Sense Stimulus Sense Organ Sensory Receptor Cells
Hearing Sound waves Ear Hair cells in cochlea
(audition)
Vision Light waves Eye Rods and cones in retina
Color vision Different wavelengths Eye Cones in retina
of light
Smell Airborne odor molecules Nose Hairlike receptor cells
(olfaction) at top of nasal cavity
Taste Chemicals dissolved in Mouth Taste buds
(gustation) saliva
Touch Pressure Skin Pacinian corpuscle
Pain Tissue injury or damage; Skin, muscles, Nociceptors
varied stimuli and organs
Movement Movement of the body None; muscle Proprioceptors in
(kinesthetic and joint muscle and joint tissue
sense) tissue
Balance Changes in position, Semicircular Hairlike receptor cells
(vestibular gravity canals and in semicircular canals
sense) vestibular and vestibular sacs
sacs