Psychology 3680
Illusion Lecture
What is Vision?
Levels of Analysis in Vision
Systems
Behaviour
Circuits
Neuron
Molecules
Micro-Circuits
Membranes, Ions, Biochemistry
Synapse
Vision
Light Sources
Everything Else
Scatter
Absorption
Reflection
Optics
Retina
Brightness
Colour
Perception
Point #1:
You “see” with
your brain.
Perception: How do we do it?
• Experimental Psychology / Neuroscience:
– Measurements, theories and phenomena (what)
– Biological mechanisms (how, why)
– Evolution (why)
• Also need to know more about the external world:
– Physics of light and material
– Statistics of the natural world. What is probable?
• For humans, “natural” means the hunter-gatherer environment before
~10,000 BCE.
Perception
•
•
•
•
Difficult to impossible for computers
Seems effortless to us
Also fast
Because:
– Short-cuts, assumptions and compromises
– Not aware of what we miss
• Big brain? Yes, and no.
– All animals cope, to some extent
Question:
• How do young
animals recognize
their parents?
• Genetic: Expensive
• Learned: Too slow
Imprinting:
• Identify parent as
first object that
moves after
hatching
• Will then follow
that shape
• Example of
simplifying
assumption
• Instinct & Learning
Frog Vision
• Few specialized shape detectors cells
• “bug” detectors
– Dark on light
– Small
– Moving
• But just about any shape. Can’t see legs.
• No detector, not “seen”
• Abstraction, simplification
Supernormal Stimulus
• An exaggerated stimulus that is
elicits a larger (more frequent,
preferential) response than does
the normal (natural) stimulus
• Oystercatcher egg retrieval
Supernormal Stimuli for Humans?
•
Joan Miró, Bleu II
•
Geisha, Zanatta
Lessons from Ethology
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•
•
•
•
•
Some things are innate; some learned
Rely on environmental consistencies
Make simplifying assumptions
Ignore some data
Favour speed as much as accuracy
“Odd” behaviour results when stimuli
exceed natural limits.
Problems for Humans
• Must deal with theoretical infinities:
– 2D from 3D images
– Shadows vs absorption
– Colour
– Video/Film motion
– Stereopsis
• Size and orientation variations
• Motion, self and object
• Multiple objects & backgrounds
• Individuals and category
Basic Assumptions
• Objects do not change:
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–
–
–
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Identity
Color
Albedo
Size
Shape
– Etc.
(object constancy)
(color constancy)
(lightness constancy)
(size constancy)
(shape constancy)
Example of Our Assumptions
• Light is coming from 3D objects in a 3D world
• Light travels in a straight line
• Objects lie in the direction the light comes from
• Needed because it is a 3D world and we only get 2D
retinal images
Ames
Room
Ames Room
Infinite Ways to Produce a 2D image
Ellipse or Circle?
Coincidences
• Cooincidences are rare.
• Cooincidences are hard to detect.
• Brain does not waste time checking for them.
• Object constancy:
– We expect an identified object to be the “same
thing” as we, and it, move around.
Modules
• Not one big brain, but big collection of
small brains (modules)
• A module does a simple calculation
• Limited input; ignorant of many things
• Makes assumptions about the world
• No conscious access into internal workings.
Bottom-Up Processing
• Start at low level modules
– Brightness, color, edges, motion etc.
•
•
•
•
Pass information up hierarchy
Automatic, quick
No effort required
Always working
Top-Down Processing
• High-level modules
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–
–
–
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Knowledge & Memory
Attention
Context, Expectations
More general
Large scale
• High level modules influence low level modules
– Affect how simple elements are perceived
• “Active” versus “Passive”
• Attention etc.
– May require “effort”
– Limited capacity, so maybe elsewhere.
Top-Down Effects
Ignoring Information
Ames Room Illustrates:
• There is more than one way in the 3D world
to create any 2D image.
• Mostly low-level, bottom-up processing.
– Makes assumptions (angles are square)
– Ignores things (sizes of women)
– Illusion is fast and automatic.
• We assume the view point is not critical.
– Object constancy
– Do not check for coincidences
Thompson Effect - RMD
Thompson Effect - RMD
That’s not the
half of it!
Quic kTime™ and a GIF dec ompres sor are needed to s ee this pic ture.
Your visual system creates something
out of nothing!
QuickTime™ and a Photo - JPEG dec ompres sor are needed to see this pic ture.