2002/02/12 PSYC202, Term 2, Copyright Jason Harrison 2002

1

Illusions

tricking the processes that estimate properties of the world

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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”

“Memory”

Images

HypothesisGenerator

Hypothesis(percept)

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Example• Perception of 3D depth (dented surface)

from shading pattern in imageMemory:

Lighting is usually from above

HypothesisGenerator

Perception of 3D dent in surface

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Two possible hypotheses

• Corresponds to physical reality– veridical perception (“true perception”)– occurs most of the time

• Does not correspond to physical reality– visual illusion

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Four kinds of illusions

1. Distortions2. Ambiguities3. Paradoxes4. Hallucinations

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1. Distortions

• Perception is not accurate• e.g., incorrect size or shape

• Example Ponzo Illusion

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1. Distortions

• Perception is not accurate• e.g., incorrect size or shape

• Example Ponzo Illusion

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Explanation of Ponzo Illusion• “inappropriate” use of perspective and

size constancy

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How versus What pathways

• distortion illusions affect “what” pathway• but not the “How” pathway

– e.g., perception confused, action not confused

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2. Ambiguities

• percept is not stable (alternates)• Example 1: Necker cube

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2. Ambiguities

• percept is not stable (alternates)• Example 1: Necker cube

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2. Ambiguities

• percept is not stable (alternates)• Example 1: Necker cube

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2. Ambiguities

• percept is not stable (alternates)• Example 1: Necker cube

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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

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Example 2: Rabbit-duck (Jastrow)

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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

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If interpreted as 3D, not possible forthese cubes to existin the world

3. Paradoxes• No hypothesis possible -- no consistency• Example 1: Impossible figure (Reuterswärd)

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If interpreted as 3D, not possible forthis box to existin the world

Example 2: Impossible figure (McAllister)

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If interpreted as 3D, not possible forthis city to existin the world

Example 3: Impossible figure (Escher)

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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

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Perception ofoccluding triangle,even thoughit’s not really there

4. Hallucinations (fictions)

• Hypothesis independent of reality– e.g., “seeing” things that aren’t there

• Example 1: Illusory figure (Kanisza)

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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

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Example 2: Vegetable Man (Arcimboldo)

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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

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Four kinds of illusions1. Distortions2. Ambiguities3. Paradoxes4. HallucinationsOne explanation Hypotheses formation via

– bottom-up information from images on retinas

– top-down knowledge from “memory”