visual perception and illusions. optical illusions which of the above gray rectangles is darker?
TRANSCRIPT
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Visual Perception and Illusions
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Optical Illusions
Which of the above gray rectangles is darker?
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Optical Illusions
Do you see gray spots at the intersections?
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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
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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.
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+0.2 +0.4 +0.8Direct connections
Indirect connections -0.05 -0.1 -0.2
Mach Bands
0 +1.6
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Mach BandsIntensity we perceive
0.4
0.2
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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).
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Lateral Inhibition
a
b
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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!!
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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?
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Negative Afterimages
"Successive lightness contrast"
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Negative Afterimages
Stare at this for 30 sec., then look at a white surface
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Positive Afterimages
Time response, or “persistency”
You see a white image if there was a white stimulus camera flash
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Persistency
Movies – frames flash by faster than youreye can change its response
Strobe lights – a flashing light can make a moving object appear stationary
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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
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Binocular Vision
Lions need binocular vision to help themin catching dinner.
Rabbits need a large field of view to avoid being dinner.
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Depth Perception
Accomodation
What if all parts of a scene are at a distance for which the eye is relaxed?
Rene Magritte,
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Depth Perception
Convergence
Small angle
Big angleL
L
R
R
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Depth Perception
Parallax
Different views of a scene from different positions.
Driving a car - nearby objects “move past” quickly; distant objects seem to follow.
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Depth Perception
Impossible figures and ambiguousdepth cues
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Ambiguous Depth Cues
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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.”
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Poggendorf Illusion