OPTICAL ILLUSIONS. Matyas Molnar

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1 OPTICAL ILLUSIONS Matyas Molnar

2 More info, examples, sources Mohit Gupta: Understanding optical illusions

3 Preface Microscopy is a visualization technique the danger: we tend to believe what we see but humans are fooled by their vision in many different ways We see / don t see what we want / don t want to see learnt / didn t learn to se others expect / don t expect us to see Humans are not rational beings. Our perception and decisions are governed by our emotions and earlier experiences. We make decisions emotionally, only later we justify them with logical explanations. Biggest effects and limitations are in the following levels: Eyes Brain Environment Culture, religion, belief systems We cannot perceive the world objectively outside of our box Is there any objective world outside us?

4 Preface Many times there are no consensus on how an actual optical illusion works. Hard to explain optical illusions with one unified theory, numerous factors are involved There are various theories and counter theories, and sometimes it's also not clear if the illusion or type of illusion acts on the physical visual system or on the brain level. Studying optical illusions helps us to understand deeply how our vision and brain work

5 Brightness-contrast illusions

6 Brightness-contrast illusions Our visual system (eyes and/or brain) tries to compensate for differences in illumination We are responsive to gradients (changes) rather than absolute values Such as our body doesn t sense the temperature, it senses the change in temperature These illusions can happen at eye or brain level

7 A Brightness Illusion

8 Simultaneous Contrast Illusion

9 Craik-O Brien-Cornsweet Illusion Actual Profile Perceived Profile

10 Lightness Constancy

11 The Hermann grid illusion

12 Scintillating Grids: Straight and Curved Scincillating black dots No black dots

13 Color illusions

14 Color illusions colors in real Colors don t exist The colors we see are just products of our brain (emotion) In the physical world, periodic oscillations of electromagnetic radiation exist, we can characterize them by amplitude, frequency, wavelength and phase. Do I see blue the some way (same color) as you see?

15 Color illusions - limitations in our sensor We can sense EM radiation between in a limited way, nm We mainly see at around 550nm (green) We can only sense three colors: blue, green and red. To the different ratios of the RGB signals from our eye, our brain associate a sensation (emotion), that s how we perceive millions of color. Afterimage rods and cones are overstimulated and loses sensitivity (adaptation). Complementary color is burned in for a while. Our visual system (eyes and/or brain) tries to compensate for environmental differences (shades, tones, hues)

16 Which image is higher in intensity?

17 Same Color? Rubik s Cube Color Illusion

18 Rubik s Cube Color Illusion One of the most powerful color illusions. You know you are fooled But you still can t believe it. Try to flip back and forth.

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20 The two dogs have the same color

21 Both skulls are red We see purple and orange skulls. A type of Munker- White brightness illusion in color, the phenomenon causes an image to bias toward the color of the objects that surround it (left: blue lines; right: yellow lines).

22 A color illusion from our time What is the color of the dress? White-gold or blue-black?

23 Afterimage

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25 Bending the lines

26 Are the lines parallel? Straight?

27 Café Wall Illusion

28 A variant of the Café Wall Illusion

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30 The Fraser Spiral Illusion This one is an extremely strong illusion. Even if you trace it with your finger, you go down a spiral

31 Pinna s Intertwining Illusion Concentric or intersecting circles?

32 Moving illusions

33 Moving illusions Small, rapid eye movements are responsible for the illusion. If we focus on one spot, all movements will stop. Voluntary or involuntary eye movements (saccades) are mainly erased from our vision by the brain. When looking into the mirror, try to catch your eye movements. Most possible you can t see them as the brain removes them (saccadic masking), since it was taught to create a steady image and only concentrate on important, clear details. Many things that we physically see are removed or re-shaped by our brain. We see with our brain, earlier experiences, expectations, culture, etc Explanation with Moiré effect: similar, repetitive patterns merged together at slightly different angles, creating interference and a rippling effect Another possible explanation is that our visual system cannot bring the blurred boundaries within the image into focus, and our brain cannot adjust.

34 Spinning Wheel Illusion

35 Spinning seeds Illusion

36 Spinning colors

37 Works also in 3D

38 Seeing things that it s not there and the opposite

39 Where is the lake?

40 Kanizsa's Triangle

41 What does the sign say? Are you sure?

42 Size and perspective

43 Size and perspective The surroundings, environment and perspective can fool our visual system We cannot estimate correctly the size of an object if it s compared with other objects

44 Ebbinghaus illusion

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49 In or out?

50 M.C. Escher

51 Seeing faces

52 Seeing faces Face recognition is very special and complex; it s handled by the temporal lobe of the brain We don t just see faces, we recognize different patters, facial expressions, familiarities, emotions, etc; as they all play an important role in our society. Our brain unconsciously look for faces everywhere, as we gain major information from faces in our culture. Face recognition is affected by visual triggers and cultural experiences Sometimes due to brain damage, face perception can be lost prosopagnosia (face blindness)

53 The Mars face

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56 Double images

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58 Rubin s vase

59 Musician or a girl's face?

60 Young girl or old woman?

61 Double images based on earlier experiences

62 Double images based on earlier experiences We see with our brain, earlier experiences, expectations, culture, etc People with different earlier experiences and imprints see different images Possible factors: Culture Religion, belief systems Age Gender Environment Current emotions - You may be an undigested bit of beef (Dickens)

63 Old man's face or two lovers kissing?

64 Cultural

65 Environment

66 Age

67 Age, cultural

68 Don t forget your own filters and limitations, and remember that perhaps you are just fooled.