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 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 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

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

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. 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