Download Trichromatic theory of color vision

Chapter 3
Sensation and
Perception
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Sensing the World Around Us
• Learning Outcomes
– Define absolute thresholds
– Explain the difference threshold and Weber’s law
– Discuss sensory adaptation
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Absolute Thresholds: Detecting What’s
Out There
• Absolute threshold: the smallest intensity of a
stimulus that must be present for it to be
detected
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Sensation and Perception
Sensation can be thought of as an organism’s first encounter
with a raw sensory stimulus, whereas perception is the process
by which that stimulus is interpreted, analyzed, and integrated
with other sensory information.
For example, if we were considering sensation, we might ask
about the loudness of a ringing fire alarm. If we were
considering perception, we might ask whether someone
recognizes the ringing sound as an alarm and identifies its
meaning.
Terms
Terms essential to sensation and perception:
Sensation - The activation of the sense organs by a
source of physical energy.
Perception - The sorting out, interpretation, analysis,
and integration of stimuli by the sense organs and
brain.
Stimulus - Energy that produces a response in a sense
organ.
**Tip: Sensation is a physical response, while perception is a
psychological response
Psychophysics
Psychophysics is the study of the relationship between the
physical aspects of stimuli and our psychological experience of
them.
- Stimuli vary in both type and intensity. Different types of
stimuli activate different sense organs.
- Light activates the sense of sight and allows us to see colors.
- Sound activates the sense of hearing and allows us to hear musical
sounds.
- How intense does light need to be before it can be detected,
or how much perfume does a person need to wear before it is
noticed are questions related to stimulus intensity.
Thresholds: Noticing Distinctions
between Stimuli
Absolute threshold is the smallest intensity of a stimulus that must be
present for it to be detected (Aazh & Moore, 2007).
• Our senses are extremely responsive to stimuli. How loud does a noise
have to be before you can hear it? How bright does a light have to be
before you can see it? At what point can you feel a feather touching your
skin?
• Absolute thresholds vary due to something psychophysicists refer to as
“noise.” Noise is the background stimulation that interferes with the
perception of other stimuli, such as crowds of people, traffic jams,
television shows, or anything that interferes with the ability to perceive a
stimulus.
RECAP
• Sensation - The activation of the sense organs by a source of physical
energy. (Physical response)
• Perception - The sorting out, interpretation, analysis, and integration of
stimuli by the sense organs and brain. (Psychological response)
• Stimulus - Energy that produces a response in a sense organ.
• Psychophysics - The study of the relationship between the physical
aspects of stimuli and our psychological experience of them.
• Absolute Threshold - The smallest intensity of a stimulus that must be
present for it to be detected
• Noise - The background stimulation that interferes with the perception of
other stimuli.
Difference Thresholds: Noticing
Distinctions between Stimuli
Difference threshold (just noticeable difference) - The difference
threshold is the smallest level of added or reduced stimulation
required to sense that a change in stimulation has occurred. It is the
minimum change in stimulation required to detect the difference
between two stimuli, so it is also called the “just noticeable
difference” (Nittrouer & Lowenstein, 2007).
• The stimulus value that constitutes a just noticeable difference
depends on the initial intensity of the stimulus. The relationship
between changes in the original value of a stimulus and the degree
to which a change will be noticed forms one of the basic laws of
psychophysics: Weber’s law.
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Difference Thresholds: Noticing
Distinctions between Stimuli
Weber’s law – (pronounced “VAY-ber”) a basic law of
psychophysics stating that a just noticeable difference is in
constant proportion to the intensity of an initial stimulus.
– For example, if you work at the fragrance department of a department
store, you will notice the smells of the perfumes immediately. After
working there a month, you will not notice the smell any more. This
also explains why we often do not recognize the smells in our own
home, but we certainly recognize them in other people’s homes.
– Weber’s law helps explain why a person in a quiet room is more
startled by the ringing of a telephone than is a person in an already
noisy room. To produce the same amount of reaction in a noisy room,
a telephone ring might have to approximate the loudness of cathedral
bells. Similarly, when the moon is visible during the late afternoon, it
appears relatively dim—yet against a dark night sky, it seems quite
bright.
RECAP
• The difference threshold is the smallest level of added or
reduced stimulation required to sense that a change in
stimulation has occurred.
• The just noticeable difference is another term for the
“difference threshold.” It is the minimum change in
stimulation required to detect the difference between two
stimuli.
• Weber’s law states that a just noticeable difference is in
constant proportion to the intensity of an initial stimulus.
Rethink
Sensations have been described as the physical
responses to stimuli, while perceptions are the
psychological responses to those same stimuli.
Is it possible that people have different
perceptions based on the same sensations? On
other words, do people interpret sensations
differently? Why or why not?
Sensory Adaptation: Turning Down
Our Responses
• Sensory adaptation: an adjustment to sensory capacity when
stimuli in the environment are unchanging; “getting used to” a
sensory stimulus so that you no longer have the same
reaction to it as you initially did
– One example of adaptation is the decrease in sensitivity that occurs after
repeated exposure to a strong stimulus. If you were to hear a loud tone over
and over again, eventually it would begin to sound softer. Similarly, jumping
into cold water may be startling, but you eventually get used to the
temperature.
• Sensory nerve receptors cannot fire off messages to the brain indefinitely.
They get used to the constant stimulation, and the result is a decreased
sensitivity to constant sensory stimuli. Receptor cells are most responsive
to changes in stimulation.
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Critical Thinking Exercise
(Homework Due Wednesday May 4th)
Take a moment to consider the graphics in video games. They are designed to
stimulate our senses and imitate real experiences. Many people become so
focused on the game they are playing that they cannot hear a person in the
same room who calls their name.
1.
2.
3.
4.
How do video or virtual reality games appeal to our sense of sight?
How do they appeal to our sense of sound?
How do they appeal to our sense of touch?
Do people experience the bodily sensations of rapid breathing and rapid
heart beat when the game gets intense?
5. How do these games arouse emotion in people?
Vision: Shedding Light on the Eye
• Learning Outcomes
– Explain the basic structure of the eye
– Compare and contrast color vision with color
blindness
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Illuminating the Structure of the Eye
• Light passes through the cornea, pupil, and
the lens before reaching the retina: converts
the energy of the light to electrical impulses
for transmission to the brain
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VISION
Vision starts with light, the physical energy that stimulates the
eye. Light is a form of electromagnetic radiation waves, which
are measured in wavelengths. The sizes of wavelengths
correspond to different types of energy.
 The range of wavelengths that humans are sensitive to is
called the visual spectrum.
1. Light waves coming from some object outside the body are
sensed by the eye.
2. The eye converts light to a form that can be used by the
neurons that serve as messengers to the brain.
Illuminating the Structure of the Eye
Rods - are thin, cylindrical receptor cells that are highly sensitive
to light. They enable you to see in dim light. Rods play a key role
in peripheral and night vision.
Cones - are cone-shaped, light-sensitive receptor cells that are
responsible for sharp focus and color perception, particularly in
bright light.
Optic nerve - When light energy strikes the rods and cones,
it triggers a neural response that moves out of the back of the
eyeball and into the brain through a bundle of ganglion axons
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Feature Detection
Feature detection - Most processing of visual images takes place in the visual
cortex of the brain. Neurons in the cortex are extraordinarily specialized and
are activated only by visual stimuli of a particular shape or pattern.
• Different parts of the brain process nerve impulses in several individual
systems simultaneously. For instance, one system relates to shapes, one to
colors, and others to movement, location, and depth.
• Different parts of the brain are involved in the perception of specific kinds
of stimuli, showing distinctions, for example, between the perception of
human faces, animals, and inanimate stimuli.
RECAP
1.
2.
3.
4.
5.
6.
7.
Visual spectrum - the range of wavelengths that humans are able to see.
Cornea - the transparent front part of the eye that covers the pupil and
iris. The cornea works with the lens to refract light.
Pupil - the dark hole in the center of the iris.
Rods - thin, cylindrical receptor cells that are highly sensitive to light.
They allow you to see in dim light.
Cones - cone-shaped, light-sensitive receptor cells that are responsible
for sharp focus and color perception, particularly in bright light.
Optic nerve – the nerve that transmits visual information from the retina
to the brain.
Feature detection – the process explaining that neurons in the cortex are
extraordinarily specialized and are activated only by visual stimuli of a
particular shape or pattern.
Color Vision and Color Blindness: The
Seven-Million-Color Spectrum
Trichromatic theory of color vision: This theory suggests that there are three
kinds of cones in the retina, each of which responds to a specific range of
wavelengths.
• One is most responsive to blue-violet colors, one to green, and the third to
yellow-red (Brown & Wald, 1964). According to trichromatic theory
perception of color is influenced by the relative strength with which each
of the three kinds of cones is activated.
• The trichromatic theory does not explain all aspect of color vision. If you
stare at a picture for thirty seconds and then look at a piece of blank,
white paper, you will see a distorted image of the picture you were just
staring at.
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Trichromatic theory of color vision
Trichromatic theory of color vision
• The phenomenon you experienced is called afterimage. It
occurs because activity in the retina continues even when you
are no longer staring at the original picture.
• Now try this while closing one eye. Notice that your eyes
cannot transfer an afterimage back and forth. If one eye is
shut while you stare at the black light, the shut eye will not
see the afterimage. The only eye that will see the afterimage
is the eye that was exposed to the image.
Color Vision & Color Blind
A person with normal color vision is capable of distinguishing no less than
seven million different colors, but approximately 50 in 5,000 men and 1 in
5,000 women are color-blind. For most people with color blindness, the world
looks quite dull. Red fire engines, green grass, and the three colors of a traffic
light all look yellow.
In the most common form of color blindness, all red and green objects are
seen as yellow. There are other forms of color blindness as well, but they are
quite rare. In yellow-blue blindness, people are unable to tell the difference
between yellow and blue, and in the most extreme case an individual
perceives no color at all. To such a person the world looks something like the
picture on a black-and-white television set.
http://colorvisiontesting.com/ishihara.htm#plate with 10
Color Vision and Color Blindness: The
Seven-Million-Color Spectrum (cont.)
Stare at the dot in this flag for about a minute and then look at a piece of plain white paper. What
do you see? Most people see an afterimage that converts the colors in the figure into the
traditional red, white, and blue U.S. flag. If you have trouble seeing it the first time, blink once
and try again.
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Color Vision and Color Blindness: The
Seven-Million-Color Spectrum
Opponent-process theory of color vision: This theory suggests that receptor cells for
color are linked in pairs, working in opposition to each other. There are specific
pairings:
1. blue-yellow pairing
2. red-green pairing
3. black-white pairing
If an object reflects light that contains more blue than yellow, it will stimulate the firing of the
cells sensitive to blue, while discouraging or inhibiting the firing of receptor cells sensitive to
yellow. The outcome is that the object will appear blue. If a light contains more yellow than
blue, the cells that respond to yellow will be stimulated to fire while the blue ones are inhibited,
and the object will appear yellow (D. N. Robinson, 2007).
- Both opponent processes and trichromatic mechanisms are at work in allowing us to see
color. However, they operate in different parts of the visual sensing system:
 1. Trichromatic processes work within the retina itself.
 2. Opponent mechanisms operate both in the retina and at later stages of neuronal
processing.
(Gegenfurtner, 2003; Chen, Zhou, & Gong, 2004; Baraas, Foster, & Amano, 2006).
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RECAP
1. The trichromatic theory of color vision suggests that
there are three kinds of cones in the retina, each of
which responds to a specific range of wavelengths.
2. Afterimage – after staring at a picture activity in the
retina continues causing you to see an image of
distorted colors when looking at a white space.
3. The opponent-process theory of color vision
suggests that receptor cells for color are linked in
pairs, working in opposition to each other, blueyellow, red-green, and black-white.
Hearing and the Other Senses
• Learning Outcomes
– Describe how we sense sound
– Discuss smell and taste
– Distinguish the skin senses
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Sensing Sound
The outer ear acts as a reverse megaphone, designed to collect and bring sounds into
the internal portions of the ear, where sounds are processed. The outer ears are
located on different sides of the head, which helps to identify the direction from
which a sound is coming. Wave patterns in the air enter each ear at a slightly different
time, and the brain uses the discrepancy as a clue to the sound’s point of origin.
Sound: is the movement of air molecules brought about by a source of vibration.
Sounds arrive at the outer ear in the form of these wavelike vibrations. They are
funneled into the auditory canal, a tube-like passage that leads to the eardrum.
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Sensing Sound
When an auditory message leaves the ear, it is transmitted to the auditory cortex of
the brain through a complex series of neural interconnections. As the message is
transmitted, it is communicated through neurons that respond to specific types of
sounds.
– Some neurons respond to clicks and whistles
– Some neurons respond to specific patterns such as steady tones.
– Some neurons respond to intermittent patterns.
– Other neurons transfer information about the location of the sound.
The auditory cortex provides us with a “map” of sound frequencies, so we are able to
identify the sound.
Balance
Semicircular canals: of the inner ear consist of three tubes containing fluid
that sloshes through them when the head moves, signaling rotational or
angular movement to the brain. The pull on our bodies caused by the
acceleration of forward, backward, or up-and-down motion, as well as the
constant pull of gravity, is sensed by the otoliths, tiny, motion-sensitive
crystals in the semicircular canals. When we move, these crystals shift like
sands on a windy beach. The brain’s inexperience in interpreting messages
from the weightless otoliths is the cause of the space sickness
experienced by two-thirds of all space travelers
(Flam, 1991; Stern & Koch, 1996).
Smell
• Smell (olfaction)
– Molecules enter the nasal passages and pass over
olfactory cells (receptor neurons); responses sent to
brain, where they are combined for recognition of
particular smells
– Humans are able to detect more than ten thousand
separate smells. We also have a good memory for smells,
and long-forgotten events and memories can be brought
back with the mere whiff of an odor associated with a
memory
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SMELL
• Results of “sniff tests” have shown that women generally have
a better sense of smell than men do. People also have the
ability to distinguish males from females on the basis of smell
alone. People can distinguish happy from sad emotions by
sniffing underarm smells, and women are able to identify
their babies solely on the basis of smell just a few hours after
birth
SMELL
• More than a thousand receptor cells, known as olfactory cells, are spread
across the nasal cavity. The cells are specialized to react to particular
odors.
• The sense of smell is sparked when the molecules of a substance enter the
nasal passages and meet olfactory cells, the receptor neurons of the nose,
which are spread across the nasal cavity. More than a thousand different
types of receptors have been identified on those cells so far. Each of these
receptors is so specialized that it responds only to a small band of different
odors. The responses of the separate olfactory cells are then transmitted
to the brain, where they are combined into recognition of a particular
smell (Murphy et al., 2004; Zho & Buck, 2006; Marshall, Laing, & Jinks,
2006).
SMELL
Humans have seven primary odors that help them determine objects. Listed
below are the seven odors.
Camphoric = Mothballs
Musky = Perfume/Aftershave
Floral = Roses
Pepperminty = Toothpaste
Etheral = Dry Cleaning Fluid
Pungent = Vinegar
Putrid = Rotten Eggs
SMELL
• Dogs have 1 million smell cells per nostril and their smell cells
are 100 times larger than humans!
• Humans use insect warning chemicals, called pheromones, to
keep away pesky insects!
• People who cannot smell have a condition called Anosmia.
• If your nose is at its best, you can tell the difference between
4000-10,000 smells!
• As you get older, your sense of smell gets worse.
• Children are more likely to have better senses of smell than
their parents or grandparents.
http://www.senseofsmell.org/
TASTE
The sense of taste is referred to as gustation. It involves
receptor cells that respond to four basic stimulus qualities:
sweet, sour, salty, and bitter. The receptor cells for taste are
located in roughly ten thousand taste buds, which are
distributed across the tongue and other parts of the mouth and
throat. The taste buds wear out and are replaced every ten days
or so. That’s a good thing, because if our taste buds were not
constantly reproducing, we’d lose the ability to taste after
accidentally burning our tongues.
TASTE
TASTE BUD
TASTE
• Everyone's tastes are different. In fact, your tastes will change as you get
older. When you were a baby, you had taste buds, not only on your
tongue, but on the sides and roof of your mouth. This means you were
very sensitive to different foods. As you grew, the taste buds began to
disappear from the sides and roof of your mouth, leaving taste buds
mostly on your tongue. As you get older, your taste buds will become even
less sensitive, so you will be more likely to eat foods that you thought
were too strong as a child.
• What if you could not taste anything? Things like medications, smoking,
not getting enough of the right vitamins, injury to the head, brain tumors,
chemical exposure, and the effects of radiation can cause taste disorders.
TASTE
The sense of taste differs significantly from one person to another, largely as a
result of genetic factors:
• Supertasters are highly sensitive to taste; they have twice as many taste
receptors as “nontasters.” More females are “supertasters”. Supertasters
find sweets sweeter, cream creamier, and spicy dishes spicier. Weaker
concentrations of flavor are enough to satisfy any cravings they may have.
• Nontasters are relatively insensitive to taste. They may seek out relatively
sweeter and fattier foods in order to maximize the taste. As a
consequence, they may be prone to obesity.
• Average tasters lie somewhere between supertasters and nontasters.
TASTE FACTS
• We have almost 10,000 taste buds inside our
mouths; even on the roofs of our mouths.
• Insects have the most highly developed sense of
taste. They have taste organs on their feet, antennae,
and mouthparts.
• Fish can taste with their fins and tail as well as their
mouth.
• In general, girls have more tastebuds than boys.
• Taste is the weakest of the five senses.
RECAP
1. Olfaction – the human sense of smell.
2. Olfactory cells - the receptor neurons of the
nose, which are spread across the nasal cavity.
3. Gustation – the sense of taste.
4. Supertasters - people who are highly
sensitive to taste.
5. Nontasters - people who are relatively
insensitive to taste.
The Skin Senses: Touch, Pressure,
Temperature, and Pain
• Skin senses: are identified as touch, pressure, temperature, and pain.
They play a critical role in our survival, making us aware of potential
danger to our bodies. Most of these senses operate through nerve
receptor cells located at various depths throughout the skin, distributed
unevenly throughout the body.
– For example, some areas, such as the fingertips, have many more
receptor cells that are sensitive to touch. These areas are notably
more sensitive than other areas of the body.
• The lower the average threshold is, the more sensitive a body part is. The
fingers and thumb, lips, nose, cheeks, and big toe are the most sensitive.
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TOUCH & PAIN
• The most extensively researched skin sense is pain. People consult
physicians and take medication for pain more than for any other symptom
or condition. Losses to U.S. business productivity due to employee pain is
more than $60 billion a year, and overall pain costs $100 billion a year in
the United States alone.
• Pain is a response to a great variety of different kinds of stimuli. A light
that is too bright can produce pain, and a sound that is too loud can be
painful. One explanation is that pain is an outcome of cell injury. When a
cell is damaged, it releases a chemical called substance P that transmits
pain messages to the brain.
SKIN & TOUCH
TOUCH & PAIN
• Some people are more susceptible to pain than others.
– For example, women experience painful stimuli more intensely than
men. These gender differences are associated with the production of
hormones related to menstrual cycles.
– In addition, certain genes are linked to the experience of pain, so that
we may inherit our sensitivity to pain.
• However, the experience of pain is not determined by biological factors
alone.
– For example, women report that the pain experienced in childbirth is
moderated to some degree by the joyful nature of the situation. In
contrast, even a minor stimulus can produce the perception of strong
pain if it is accompanied by anxiety (like a visit to the dentist). Clearly,
then, pain is a perceptual response that depends heavily on our
emotions and thoughts
Gate-Control Theory of Pain
According to the gate-control theory of pain, particular nerve receptors in the spinal
cord lead to specific areas of the brain related to pain. When these receptors are
activated because of an injury or problem with a part of the body, a “gate” to the brain
is opened, allowing us to experience the sensation of pain.
Another set of neural receptors can, when stimulated, close the “gate” to the brain, thereby
reducing the experience of pain. The gate can be shut in two different ways.
1. Other impulses can overwhelm the nerve pathways relating to pain, which are spread
throughout the brain. In this case, nonpainful stimuli compete with and sometimes displace
the neural message of pain, thereby shutting off the painful stimulus.
2. Psychological factors account for the second way a gate can be shut. Depending on an
individual’s current emotions, interpretation of events, and previous experience, the brain
can close a gate by sending a message down the spinal cord to an injured area, producing a
reduction in or relief from pain. Thus, soldiers who are injured in battle may experience no
pain. The lack of pain probably occurs because a soldier experiences such relief at still being
alive that the brain sends a signal to the injury site to shut down the pain gate .
PAIN MANGEMENT
Here are some of the strategies that psychologists and medical specialists have
devised to fight pain:
1. Medication – painkilling drugs are used to fight pain and reduce swelling on
painful joints. The most recent innovation is to pump pain medication right into
the spinal cord .
2. Nerve and brain stimulation – a low-voltage electric current is passed through a
specific part of the body that is in pain. Electrodes can also be surgically implanted
into the brain. Another process known as transcutaneous electrical nerve
stimulation (TENS) uses a handheld battery pack to stimulate nerve cells providing
direct relief of pain.
3. Light therapy - one of the newest forms of pain reduction involves exposure to
specific wavelengths of red or infrared light. Certain kinds of light increase the
production of enzymes that may promote healing.
4. Hypnosis – this technique of deep relaxation can be used to manage pain.
PAIN MANAGEMNT
5. Biofeedback and relaxation techniques – processes where people learn to
control “involuntary” functions such as heartbeat and respiration. They
can also learn to manage pain through progressive relaxation, the
relaxation of isolated parts of the body one at a time.
6. Surgery - nerve fibers that carry pain messages to the brain can be cut
surgically. Still, because of the danger that other bodily functions will be
affected, surgery is a treatment of last resort, used most frequently with
dying patients.
7. Cognitive restructuring – the use of positive ways of thinking as a
substitute for negative statements like, “This pain will never stop,” or “This
pain is ruining my life.” When people increase their sense of control, they
can reduce the pain they feel.
TOUCH FACTS
• You have more pain nerve endings than any other type.
• The least sensitive part of your body is the middle of your
back.
• The most sensitive areas of your body are your hands, lips,
face, neck, tongue, fingertips and feet.
• Shivering is a way your body has of trying to get warmer.
• There are about 100 touch receptors in each of your
fingertips.
• Rattlesnakes use their skin to feel the body heat of other
animals.
RECAP
1. The skin senses are identified as touch, pressure, temperature,
and pain.
2. People consult physicians and take medication for pain more
than for any other symptom or condition.
3. Some people are more susceptible to pain than others.
4. Certain genes are linked to the experience of pain, so that we
may inherit our sensitivity to pain.
5. Pain is a perceptual response that depends heavily on our
emotions and thoughts.
6. Gate-control theory of pain - The theory that particular nerve
receptors lead to specific areas of the brain related to pain.
Perceptual Organization: Constructing Our
View of the World
• Learning Outcomes
– Explain the gestalt laws of organization
– Identify top-down and bottom-up processing
– Define perceptual constancy
– Explain depth perception
– Relate motion perception to daily life
– Determine the importance of perceptual illusions
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Perceptual Organization: Constructing Our
View of the World (cont.)
• The gestalt laws of organization are a series of principles that
focus on the ways we organize bits and pieces of information
into one whole picture, pattern, or piece of information.
These principles were set forth in the early 1900s by a group
of German psychologists, led by Max Wertheimer, who
studied patterns, or gestalts.
Figure-ground organization: we
usually perceive objects as a figure
standing out against a background
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The Gestalt Laws of Organization
Principles that describe how we organize pieces
of information into meaningful wholes (gestalts = patterns)
– Closure - when an object is incomplete, but enough of the
shape is indicated for people to fill in the gaps and
perceive the whole picture. Even though parts of this
panda bear are missing, you are able to perceive the whole
picture:
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The Gestalt Laws of Organization
– Proximity - when elements are placed close
together, they tend to be perceived as a group. If
the squares below were spread out over this page,
the perception would be very different.
The Gestalt Laws of Organization
- Similarity - when objects look similar, people perceive
them as a group or pattern.
The Gestalt Laws of Organization
(cont.)
- Simplicity – when we observe a pattern, we
perceive it in the most basic, straightforward
manner that we can.
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Top-Down and Bottom-Up Processing
• Top-down processing: perception is guided by
higher-level knowledge, experience, expectations,
and motivations
Can you figure out the meaning of this sentence?
Ca- yo- re-d t-is -en-en-e, w-ic- ha- ev-ry -hi-d l-tt-r m-ss-ng?
You were probably able to figure out the meaning of the sentence with the
missing letters because of your prior reading experience, and because written
English contains redundancies. Not every letter of each word is necessary to
decode its meaning. Your expectations played a role in your being able to
read the sentence.
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Top-Down and Bottom-Up
Processing
• Bottom-up processing: processing information by progressing
from the individual elements of a stimulus and moving up to
the perception of the whole
• We would make no headway in our recognition of the sentence without
being able to perceive the individual shapes that make up the letters.
Some perception, then, occurs at the level of the patterns and features of
each of the separate letters.
• Top-down and bottom-up processing occur simultaneously, and interact
with each other, in our perception of the world around us. Bottom-up
processing permits us to process the fundamental characteristics of
stimuli, whereas top-down processing allows us to bring our experience to
bear on perception.
Perceptual Constancy
• Physical objects are perceived as unvarying and
consistent despite changes in appearance or changes
in the physical environment
– Ex.: the image on your retina of a person far away
from you is very small, but you understand
(perceive) her to be of “normal” size
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Depth Perception:
Translating 2-D to 3-D
Depth perception: the ability to view the world in three dimensions and to perceive
distance
 The ability to view the world in three dimensions and to perceive distance is due
largely to the fact that we have two eyes. Because there is a certain distance
between the eyes, a slightly different image reaches each retina. The brain
integrates the two images into one composite view, but it also recognizes the
difference in images and uses it to estimate the distance of an object from us. The
difference in the images seen by the left eye and the right eye is known as
binocular disparity.
 If we view two objects at the same time and one is a lot closer than the other is,
the retinal disparity will be large and we will have a greater sense of depth
between the two. However, if the two objects are a similar distance from us, the
retinal disparity will be minor, and we will perceive them as being a similar
distance from us.
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Monocular Cues
Certain cues permit us to obtain a sense of depth and distance with just one
eye. These cues are known as monocular cues:
1.
Motion parallax - the change in position of an object on the retina
caused by movement of your body relative to the object.
 For example, if you are a passenger in a moving car, focus your eye on
a stable object such as a tree. Objects that are closer than the tree
will appear to move backward, and the nearer the object is, the more
quickly it will appear to move. In contrast, objects beyond the tree
will seem to move at a slower speed, but in the same direction as you
are. Your brain is able to use these cues to calculate the relative
distances of the tree and other objects.
Monocular Cues
2. Binocular disparity - is the difference in the images seen by the left eye and
the right eye.
3. Relative size - when two objects are the same size, the one that makes a
smaller image on the retina is farter away than is the one that provides a
larger image.
4. Texture gradient – the quality and detail of the image helps to judge the
distance of that object.
5. Linear perspective – the perspective that distant objects appear to be
closer together than nearer objects. For example, follow a pair of railroad
tracks to the horizon. They seem to join together as you look farther away.
Motion Perception: As the World Turns
• How do we perceive motion?
– Movement of an object across the retina is perceived
relative to an unmoving background
– If a stimulus is coming toward you, the image on the
retina will expand in size, filling more of the visual
field, but we assume the stimulus is approaching
rather than it’s growing in size
– We factor information about our head and eye
movements with information about changes in the
retinal image
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Perceptual Illusions:
The Deceptions of Perceptions
• Visual illusions: physical stimuli that
consistently produce errors in perception
– Muller-Lyer illusion
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