sensation and perception 6 “it’s life’s illusions i recall...” joni mitchell
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Sensation and perception
6
“It’s life’s illusions I recall...”
Joni Mitchell
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Perceptual Organization
How do we form meaningful perceptions from sensory information?
We organize it. Gestalt psychologists showed that a figure formed a “whole”
different than its surroundings.
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Organization of the visual field into objects (figures) that stand out from their
surroundings (ground).
Form Perception
Tim
e Savings S
uggestion, © 2003 R
oger Sheperd.
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Grouping
After distinguishing the figure from the ground, our perception needs to organize the figure into a meaningful form using
grouping rules.
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Grouping & Reality
Although grouping principles usually help us construct reality, they may occasionally lead us
astray.
Both photos by W
alter Wick. R
eprinted from G
AM
ES
Magazine. .©
1983 PCS G
ames L
imited Partnership
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Depth Perception
Visual Cliff
Depth perception enables us to judge distances. Gibson and Walk (1960) suggested that human infants (crawling age) have depth perception. Even newborn animals show depth perception.
Inne
rvis
ions
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Depth and distance perceptionBinocular cues: visual cues that require the use of both eyes
ConvergenceTurning inward of the eyes, which occurs when they focus on a nearby object
Retinal disparityThe slight difference in lateral separation between two objects as seen by the right and left eyes
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Binocular Cues
Retinal disparity: Images from the two eyes differ. Try looking at your two index fingers when pointing them
towards each other half an inch apart and about 5 inches directly in front of your eyes. You will see a
“finger sausage” as shown in the inset.
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Depth and distance perceptionMonocular cues: visual cues that can be used by one eye
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Monocular Cues
Relative Size: If two objects are similar in size, we perceive the one that casts a
smaller retinal image to be farther away.
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Monocular Cues
Interposition: Objects that occlude (block) other objects tend to be perceived as
closer.R
ene Magritte, The B
lank Signature, oil on canvas, N
ational Gallery of A
rt, Washington. C
ollection of M
r. and Mrs. Paul M
ellon. Photo by Richard C
arafelli.
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Monocular Cues
Relative Height: We perceive objects that are higher in our field of vision to be farther away than those that are
lower.
Image courtesy of S
haun P. V
ecera, Ph. D
., adapted from
stimuli that appered in V
ecrera et al., 2002
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Monocular Cues
Relative motion: Objects closer to a fixation point move faster and in opposing direction to those objects that are farther away from a fixation
point, moving slower and in the same direction.
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Monocular Cues
Linear Perspective: Parallel lines, such as railroad tracks, appear to converge in the distance. The more the lines converge, the
greater their perceived distance.
© T
he New
Yorker C
ollection, 2002, Jack Ziegler
from cartoonbank.com
. All rights reserved.
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Monocular Cues
Light and Shadow: Nearby objects reflect more light into our eyes than more distant objects. Given two identical objects, the dimmer one appears to
be farther away.
From
“Perceiving S
hape From
Shading” by V
ilayaur S
. Ram
achandran. © 1988 by S
cientific Am
erican, Inc. A
ll rights reserved.
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Perceptual Constancy
Perceiving objects as unchanging even as illumination and retinal
images change. Shape constancyLocation constancySize constancyBrightness constancyColor constancy
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Size-Distance RelationshipBoth girls in the room are of similar
height. However, we perceive them to be of different heights as they stand in the
two corners of the room.
Both photos from S. Schwartzenberg/ The Exploratorium
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Ames Room
The Ames room is designed to demonstrate the size-distance illusion.
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Lightness Constancy
The color and brightness of square A and B are the same.
Courte
sy E
dwar
d A
delso
n
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Color in context6
The way you perceive a color depends on the color surrounds.
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Perceptual Interpretation
Immanuel Kant (1724-1804) maintained that knowledge comes from our inborn ways of
organizing sensory experiences.
John Locke (1632-1704) argued that we learn to perceive the world through our
experiences.
How important is experience in shaping ourperceptual interpretation?
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Sensory Deprivation & Restored Vision
After cataract surgery, blind adults were able to regain sight. These
individuals could differentiate figure and ground relationships, yet they had difficulty distinguishing a circle
and a triangle (Von Senden, 1932).
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Facial Recognition
After blind adults regained sight, they
were able to recognize distinct features, but
were unable to recognize faces.
Normal observers also show difficulty in facial recognition when the
lower half of the pictures are changed.
Courtesy of R
ichard LeG
rand
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Kittens raised without exposure to horizontal lines later had difficulty
perceiving horizontal bars.
Blakemore & Cooper (1970)
Sensory Deprivation
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Perceptual Set
A mental predisposition to perceive one thing and not another. What you see in the center picture is influenced by flanking pictures.
From
Shepard, 1990.
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(a) Loch ness monster or a tree trunk; (b) Flying
saucers or clouds?
Perceptual Set
Other examples of perceptual set.
Frank Searle, photo Adam
s/ Corbis-Sygm
a
Dick R
uhl
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Is the “magician cabinet” on the floor or hanging from the ceiling?
Context Effects
Context can radically alter perception.
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To an East African, the woman sitting is balancing a metal box on her head, while the family is sitting
under a tree.
Cultural Context
Context instilled by culture also alters perception.