Sources of Illusions
• Illusions occur because our perceptual
system is structured to function as though
our assumptions and hypotheses about the
world around us are true.
• In situations where our assumptions
appear to be true but are actually not, we
automatically construct perceptions that
bring some sense of meaning and order.
These misperceptions are called illusions.
Then we realize that there are alternative,
incompatible, and equally valid perceptions
in addition to the one we constructed.
Written by Gordon Vessels 2005
Visual Perception & Illusions
Visual Perception & Illusions
Slide arrangement by Gordon Vessels 2005
Dark Adaptation
• Rods & cones contain photopigments
• Photopigments are bleached or burned
out by light
• They eventually regenerate
– However, if you stare at a bright light
such as the sun for long enough, some
will burn out permanently
• Cones regenerate quicker (10 min)
than rods (about 30 min)
• Eyes adapted to the dark are 100,000
times more sensitive to light.
Written by Gordon Vessels 2005
Gestalt Theory of Perception
• We impose
order and
structure on
what we see.
• Our
expectancies
affect how we
interpret
sensory input.
• Hence, Gestalt
is a top-down
theory of
perception.
Modeled after a ppt slide created by Dr. Kevin Richardson in 1998 and made available through the American Psychological Society
Figure-Ground Differentiation
• Major Gestalt
principle of
perceptual
organization
• We divide world
into 2 parts
1.Figure
•
Has shape &
location in space
2.Ground
•
Has no definite
shape and seems
to continue
beyond figure
Modeled after a ppt slide created by Dr. Kevin Richardson in 1998 and made available through the American Psychological Society
Bill or Monica?
Find the Hidden Star
Click Here to
Go to Website
about illusions
Click here to
go to website
about illusions
Gestalt Laws of Perceptual
Grouping – Top-Down
• How do we group objects
perceptually?
• Several principles:
–
–
–
–
–
Similarity
Proximity
Continuation
Simplicity
Closure
Modeled after a ppt slide created by Dr. Kevin Richardson in 1998 and made available through the American Psychological Society
Similarity
Similarity
grouping
suggests
horizontal
columns
Proximity
Proximity Grouping Suggests Vertical
Columns
Continuation
We see a square even though its
corners are missing
Simplicity
The lines are
naturally grouped
as a 3D cube.
The lines are
naturally grouped
as a 2D pentagon.
Closure
Closure
Closure
Perceptual Constancies
• Does our perception of
objects around you change if
objects get closer or farther
away, or if they are moving?
• No! No! No! No! No! No! No!
• Why?
– Because of perceptual
constancies
Modeled after a ppt slide created by Dr. Kevin Richardson in 1998 and made available through the American Psychological Society
Size Constancy
What happens when our
perception fails?
• Illusions (incorrect perceptions)
2 major categories:
– Illusions of size
• size constancy at work (e.g., St. Louis
Gateway, Ponzo Illusions) – objects
higher in our visual field
are perceived as more distant
• learning and past experience
(e.g., Muller-Lyer illusion)
– Illusions of shape
Modeled after a ppt slide created by Dr. Kevin Richardson in 1998 and made available through the American Psychological Society
Distance and depth perception
• How do we judge distance and depth?
• We utilize a variety of cues!
Monocular cues:
- size
- linear perspective ─ (parallel lines)
- texture gradient ─ (farther = smoother)
- atmospheric perspective ─ (i.e., quality of discernment)
- overlap
- height ─ (in reference to horizon)
- motion parallax ─ (when moving, distant objects move
in the same direction while close objects move in the
opposite direction, or at least appear to do so)
Binocular cues:
- convergence ─ (eyes turn inward to see closer objects)
- retinal disparity ─ (stereovision)
Arranged by Gordon Vessels 2005
Ponzo Illusion: line in the distance appears larger
Click here to see many
other famous illusions
Arranged by Gordon Vessels 2005
Depth: Textural Gradient
Surfaces appear to have a finer texture
as they recede into the distance.
Arranged by Gordon Vessels 2005
Depth: Convergence / Perspective
Lines will appear to draw closer together
as they go farther into the distance.
Arranged by Gordon Vessels 2005
Muller-Lyer illusion:
line is longer?
which
Click here to see many
other famous illusions
An illusion invented by the German psychologist Wilhelm Wundt in the 19th
century. In the figure above, the two red horizontal lines are both straight, but
they look as if they are bowed inwards. The distortion is induced by the crooked
lines on the background.
Arranged by Gordon Vessels 2005
Illusions of shape: the Moon illusion
Just plain cool illusions!
Click Here
Click here to see inversion
illusions on website
All lines below are straight; distorted
pattern created by the tiny squares
Café wall illusion
Click here to go to a website
about motion perception
•
•
•
•
•
•
•
•
•
Motion sensing
four-stroke motion
motion aftereffect
RDKs
second-order
motion
2-D integration
motion capture
direction repulsion
plaid motion
• 3-D
interpretation
• kinetic depth
• stereokinetic
motion
• biological motion
• shadow motion
• transformational
motion
Arranged by Gordon Vessels 2005