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

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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.

• 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

Gestalt Laws of Perceptual Grouping – Top-Down

• How do we group objects perceptually?

• Several principles:

– Similarity– Proximity– Continuation– Simplicity– Closure

• 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

SimilaritySimilarity

Similarity grouping suggests horizontal columns

Similarity grouping suggests horizontal columns

Proximity

Proximity Grouping Suggests Vertical Columns

ContinuationContinuation

We see a square even though its corners are missing

We see a square even though its corners are missing

SimplicitySimplicity

The lines are naturally grouped as a 3D cube.

The lines are naturally grouped as a 3D cube.

The lines are naturally grouped as a 2D pentagon.

The lines are naturally grouped as a 2D pentagon.

ClosureClosure

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 shapeModeled 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.

Lines will appear to draw closer together as they go farther into the distance.

Arranged by Gordon Vessels 2005

Muller-Lyer illusion: which line is longer?

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

All lines below are straight; distorted pattern created by the tiny squares

Click here to see inversion illusions on website

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