1. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 20021 Illusions tricking the processes that estimate properties of the world

2. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 20022 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)

3. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 20023 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

4. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 20024 Two possible hypotheses Corresponds to physical reality –veridical perception (“true perception”) –occurs most of the time Does not correspond to physical reality –visual illusion

5. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 20025 Four kinds of illusions 1.Distortions 2.Ambiguities 3.Paradoxes 4.Hallucinations

6. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 20026 1.Distortions Perception is not accurate e.g., incorrect size or shape Example Ponzo Illusion

7. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 20027 1.Distortions Perception is not accurate e.g., incorrect size or shape Example Ponzo Illusion

8. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 20028 Explanation of Ponzo Illusion “inappropriate” use of perspective and size constancy

9. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 20029 Example 2: Mueller-Lyer illusion “wings-out” configuration seen as larger

10. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 200210 Example 2: Mueller-Lyer illusion “wings-out” configuration seen as larger

11. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 200211 Explanation of Mueller-Lyer illusion Inappropriate use of perspective and size constancy

12. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 200212 Explanation of Mueller-Lyer illusion Inappropriate use of perspective and size constancy

13. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 200213 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)

14. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 200214 How versus What pathways distortion illusions affect “what” pathway but not the “How” pathway –e.g., perception confused, action not confused

15. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 200215 2. Ambiguities percept is not stable (alternates) Example 1: Necker cube

16. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 200216 2. Ambiguities percept is not stable (alternates) Example 1: Necker cube

17. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 200217 2. Ambiguities percept is not stable (alternates) Example 1: Necker cube

18. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 200218 2. Ambiguities percept is not stable (alternates) Example 1: Necker cube

19. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 200219 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

20. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 200220 Example 2: Rabbit-duck (Jastrow)

21. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 200221 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

22. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 200222 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)

23. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 200223 If interpreted as 3D, not possible for this box to exist in the world Example 2: Impossible figure (McAllister)

24. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 200224 If interpreted as 3D, not possible for this city to exist in the world Example 3: Impossible figure (Escher)

25. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 200225 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

26. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 200226 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)

27. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 200227 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

28. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 200228 Example 2: Vegetable Man (Arcimboldo)

29. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 200229 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

30. 2002/02/12PSYC202, Term 2, Copyright Jason Harrison 200230 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”