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 incominglight

What your eyetends to see(edges areimportant)

Inte

nsit

y (a

s “s

een”

by

brai

n)Direct (black) andindirect (blue) connections

Lateral InhibitionIndirect (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.

+0.2 +0.4 +0.8Direct connections

Indirect connections -0.05 -0.1 -0.2

Mach Bands

0 +1.6

Mach BandsIntensity we perceive

0.4

0.2

Lateral Inhibition

Parts of retina responding to gray rectangleon the right are receiving strong inhibitorysignals, fatiguing the retina and making itless sensitive (so the gray part seems darker).

Lateral Inhibition

a

b

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 lesssensitive.

Point “b” is surrounded by white on two sides and blackon two sides; there are not as many signals from receptorstrying to “desensitize” the rods responding to the area at “b.”

100,000,000 rods and cones (sensors) lead to 1,000,000neurons - 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 youreye 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 givesus the possibility of depth perception

Binocular Vision

Lions need binocular vision to help themin 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 Magritte,

Depth Perception

Convergence

Small angle

Big angleL

L

R

R

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 ambiguousdepth 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.”

Poggendorf Illusion