Illusions
tricking the processes that
estimate properties of the world
2002/02/12
PSYC202, Term 2, Copyright Jason Harrison 2002
1
Task of visual perception
• estimate properties of the world
– i.e., construct a hypothesis
• Hypotheses formed via
– bottom-up information
from images on retinas
– top-down knowledge
from “memory”
2002/02/12
“Memory”
Hypothesis
Generator
Hypothesis
(percept)
Images
PSYC202, Term 2, Copyright Jason Harrison 2002
2
Example
• Perception of 3D depth (dented surface) from
shading pattern in image
2002/02/12
Memory:
Lighting is usually
from above
Hypothesis
Generator
Perception of
3D dent in surface
PSYC202, Term 2, Copyright Jason Harrison 2002
3
Two possible hypotheses
• Corresponds to physical reality
– veridical perception (“true perception”)
– occurs most of the time
• Does not correspond to physical reality
– visual illusion
2002/02/12
PSYC202, Term 2, Copyright Jason Harrison 2002
4
Four kinds of illusions
1.
2.
3.
4.
Distortions
Ambiguities
Paradoxes
Hallucinations
2002/02/12
PSYC202, Term 2, Copyright Jason Harrison 2002
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1. Distortions
• Perception is not accurate
• e.g., incorrect size or shape
• Example Ponzo Illusion
2002/02/12
PSYC202, Term 2, Copyright Jason Harrison 2002
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1. Distortions
• Perception is not accurate
• e.g., incorrect size or shape
• Example Ponzo Illusion
2002/02/12
PSYC202, Term 2, Copyright Jason Harrison 2002
7
Explanation of Ponzo Illusion
• “inappropriate” use of perspective and size
constancy
2002/02/12
PSYC202, Term 2, Copyright Jason Harrison 2002
8
How versus What pathways
• distortion illusions affect “what” pathway
• but not the “How” pathway
– e.g., perception confused, action not confused
2002/02/12
PSYC202, Term 2, Copyright Jason Harrison 2002
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2. Ambiguities
• percept is not stable (alternates)
• Example 1: Necker cube
2002/02/12
PSYC202, Term 2, Copyright Jason Harrison 2002
15
2. Ambiguities
• percept is not stable (alternates)
• Example 1: Necker cube
2002/02/12
PSYC202, Term 2, Copyright Jason Harrison 2002
16
2. Ambiguities
• percept is not stable (alternates)
• Example 1: Necker cube
2002/02/12
PSYC202, Term 2, Copyright Jason Harrison 2002
17
2. Ambiguities
• percept is not stable (alternates)
• Example 1: Necker cube
2002/02/12
PSYC202, Term 2, Copyright Jason Harrison 2002
18
Explanation of Necker cube
• multiple high-level interpretations are
compatiable with image
• brain attempts to find (remember) structures
compatible with data
• if more than one is found, the percept
alternates
– not a blend of alternatives
– alternation much like binocular rivalry
2002/02/12
PSYC202, Term 2, Copyright Jason Harrison 2002
19
Example 2: Rabbit-duck (Jastrow)
2002/02/12
PSYC202, Term 2, Copyright Jason Harrison 2002
20
Explanation of Rabbit-duck
• multiple high-level interpretations are
compatiable with image
• brain attempts to find (remember) structures
compatible with data
– memory biased towards “favourite” interpretation
• if more than one is found, the percept
alternates
– not a blend of alternatives
– alternation much like binocular rivalry
2002/02/12
PSYC202, Term 2, Copyright Jason Harrison 2002
21
3. Paradoxes
• No hypothesis possible -- no consistency
• Example 1: Impossible figure (Reuterswärd)
2002/02/12
If interpreted as 3D,
not possible for
these cubes to exist
in the world
PSYC202, Term 2, Copyright Jason Harrison 2002
22
Example 2: Impossible figure (McAllister)
2002/02/12
If interpreted as 3D,
not possible for
this box to exist
in the world
PSYC202, Term 2, Copyright Jason Harrison 2002
23
Example 3: Impossible figure (Escher)
2002/02/12
If interpreted as 3D,
not possible for
this city to exist
in the world
PSYC202, Term 2, Copyright Jason Harrison 2002
24
Explanation
• no hypothesis can account for the entire image
• brain can find local interpretations (e.g. cubes)
based on rules such as T-junctions, shading,
etc.
• interpretation dependant on local area and path
of attention through image
• Result: paradoxical percept
– different hypothesis for each part of the image
2002/02/12
PSYC202, Term 2, Copyright Jason Harrison 2002
25
4. Hallucinations (fictions)
• Hypothesis independent of reality
– e.g., “seeing” things that aren’t there
• Example 1: Illusory figure (Kanisza)
2002/02/12
Perception of
occluding triangle,
even though
it’s not really there
PSYC202, Term 2, Copyright Jason Harrison 2002
26
Explanation of illusory figure
• a triangle is “imagined” since it is the simplest
account of image pattern
– visual completion
• brain hypothesizes such structures
– must be no evidence against the interpretation
– Charles Bonnet syndrome
• Note: no replacement of image properties
– no filling in of triangular occulder
2002/02/12
PSYC202, Term 2, Copyright Jason Harrison 2002
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Example 2: Vegetable Man (Arcimboldo)
2002/02/12
PSYC202, Term 2, Copyright Jason Harrison 2002
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Explanation of illusory figure
• a man is “imagined” since it is the simplest
account of image pattern
– abstract level -- overall form
• brain hypothesizes such structures
– even if details don’t fit exactly
– day to day differences in your friends and family
• Note: no replacement of image properties
– vegetables are still seen
2002/02/12
PSYC202, Term 2, Copyright Jason Harrison 2002
29
Four kinds of illusions
1. Distortions
2. Ambiguities
3. Paradoxes
4. Hallucinations
One explanation Hypotheses formation via
– bottom-up information from images on retinas
– top-down knowledge from “memory”
2002/02/12
PSYC202, Term 2, Copyright Jason Harrison 2002
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