2002/02/12PSYC202, Term 2, Copyright Jason Harrison 20021 Illusions tricking the processes that estimate properties of the world.

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Presentation transcript:

2002/02/12PSYC202, Term 2, Copyright Jason Harrison Illusions tricking the processes that estimate properties of the world

2002/02/12PSYC202, Term 2, Copyright Jason Harrison 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 Hypothesis Generator Hypothesis (percept)

2002/02/12PSYC202, Term 2, Copyright Jason Harrison Example Perception of 3D depth (dented surface) from shading pattern in image Memory: Lighting is usually from above Hypothesis Generator Perception of 3D dent in surface

2002/02/12PSYC202, Term 2, Copyright Jason Harrison Two possible hypotheses Corresponds to physical reality –veridical perception (“true perception”) –occurs most of the time Does not correspond to physical reality –visual illusion

2002/02/12PSYC202, Term 2, Copyright Jason Harrison Four kinds of illusions 1.Distortions 2.Ambiguities 3.Paradoxes 4.Hallucinations

2002/02/12PSYC202, Term 2, Copyright Jason Harrison Distortions Perception is not accurate e.g., incorrect size or shape Example Ponzo Illusion

2002/02/12PSYC202, Term 2, Copyright Jason Harrison Distortions Perception is not accurate e.g., incorrect size or shape Example Ponzo Illusion

2002/02/12PSYC202, Term 2, Copyright Jason Harrison Explanation of Ponzo Illusion “inappropriate” use of perspective and size constancy

2002/02/12PSYC202, Term 2, Copyright Jason Harrison Example 2: Mueller-Lyer illusion “wings-out” configuration seen as larger

2002/02/12PSYC202, Term 2, Copyright Jason Harrison Example 2: Mueller-Lyer illusion “wings-out” configuration seen as larger

2002/02/12PSYC202, Term 2, Copyright Jason Harrison Explanation of Mueller-Lyer illusion Inappropriate use of perspective and size constancy

2002/02/12PSYC202, Term 2, Copyright Jason Harrison Explanation of Mueller-Lyer illusion Inappropriate use of perspective and size constancy

2002/02/12PSYC202, Term 2, Copyright Jason Harrison Explanation of Mueller-Lyer illusion Inappropriate use of perspective and size constancy Lines have same angle in image (s/d the same) - wings-out: line further away (larger d) -> interpreted as larger (larger s)

2002/02/12PSYC202, Term 2, Copyright Jason Harrison How versus What pathways distortion illusions affect “what” pathway but not the “How” pathway –e.g., perception confused, action not confused

2002/02/12PSYC202, Term 2, Copyright Jason Harrison Ambiguities percept is not stable (alternates) Example 1: Necker cube

2002/02/12PSYC202, Term 2, Copyright Jason Harrison Ambiguities percept is not stable (alternates) Example 1: Necker cube

2002/02/12PSYC202, Term 2, Copyright Jason Harrison Ambiguities percept is not stable (alternates) Example 1: Necker cube

2002/02/12PSYC202, Term 2, Copyright Jason Harrison Ambiguities percept is not stable (alternates) Example 1: Necker cube

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

2002/02/12PSYC202, Term 2, Copyright Jason Harrison Example 2: Rabbit-duck (Jastrow)

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

2002/02/12PSYC202, Term 2, Copyright Jason Harrison If interpreted as 3D, not possible for these cubes to exist in the world 3. Paradoxes No hypothesis possible -- no consistency Example 1: Impossible figure (Reuterswärd)

2002/02/12PSYC202, Term 2, Copyright Jason Harrison If interpreted as 3D, not possible for this box to exist in the world Example 2: Impossible figure (McAllister)

2002/02/12PSYC202, Term 2, Copyright Jason Harrison If interpreted as 3D, not possible for this city to exist in the world Example 3: Impossible figure (Escher)

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

2002/02/12PSYC202, Term 2, Copyright Jason Harrison Perception of occluding triangle, even though it’s not really there 4. Hallucinations (fictions) Hypothesis independent of reality –e.g., “seeing” things that aren’t there Example 1: Illusory figure (Kanisza)

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

2002/02/12PSYC202, Term 2, Copyright Jason Harrison Example 2: Vegetable Man (Arcimboldo)

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

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