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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.
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
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 longlasting 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
(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
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.
(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.
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.
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 outsideworld.
4. Sensory adaptation to odors generally occurs in less than one
minute
5. In Focus: Do Pheromones Influence Human Behavior?
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.
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
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
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.
5. 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.
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?
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.
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 threedimensional 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.
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 reticular 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.