Visual Perception and Illusions

Optical Illusions Which of the above gray rectangles is darker?

Optical Illusions Do you see gray spots at the intersections?

Structure of the Retina Rods Neurons Pattern of incoming light What your eye tends to see (edges are important) Intensity (as “seen” by brain) Direct (black) and indirect (blue) connections

Lateral Inhibition Indirect (lateral) rod signals tend to cancel out (inhibit) the direct connection signals Result: Changes in intensity (amount of incident light) are most important for the visual system.

Direct connections Indirect connections Mach Bands 0+1.6

Mach Bands Intensity we perceive

Lateral Inhibition Parts of retina responding to gray rectangle on the right are receiving strong inhibitory signals, fatiguing the retina and making it less sensitive (so the gray part seems darker).

Lateral Inhibition

The point “a” is surrounded on four sides by white area. Inhibitory signals from receptors responding to “white” tell receptors responding to the point “a” to become less sensitive. Point “b” is surrounded by white on two sides and black on two sides; there are not as many signals from receptors trying to “desensitize” the rods responding to the area at “b.” 100,000,000 rods and cones (sensors) lead to 1,000,000 neurons - the brain is throwing away 99% of the input!!

Stratton Article 1. What was the point of Stratton's experiments? 2. Can the eye + brain be fooled into making a major new adaptation as described by Stratton? 3. What is the optical instrument Stratton uses to alter his vision?

Negative Afterimages "Successive lightness contrast"

Negative Afterimages Stare at this for 30 sec., then look at a white surface

Positive Afterimages Time response, or “persistency” You see a white image if there was a white stimulus  camera flash

Persistency Movies – frames flash by faster than your eye can change its response Strobe lights – a flashing light can make a moving object appear stationary

Binocular Vision Eyes in front or eyes on the side? How do we judge distances? Overlapping “field-of-view gives us the possibility of depth perception

Binocular Vision Lions need binocular vision to help them in catching dinner. Rabbits need a large field of view to avoid being dinner.

Depth Perception Accomodation What if all parts of a scene are at a distance for which the eye is relaxed? Rene MagritteRene Magritte,

Depth Perception Convergence

Depth Perception Parallax Different views of a scene from different positions. Driving a car - nearby objects “move past” quickly; distant objects seem to follow.

Depth Perception Impossible figures and ambiguous depth cues

Ambiguous Depth Cues

Depth Perception Why might one wish to “play games” with depth perception and visual cues? Look at drawings and woodcuts by M.C. Esher such as “Ascending and Descending” or “Relativity.”“Relativity.”

Poggendorf Illusion