OPTICAL ILLUSIONS Matyas Molnar

• Mohit Gupta: Understanding optical illusions

• https://www.eyebuydirect.com/understanding-perception-optical-illusions

• https://www.rd.com/culture/optical-illusions/

• https://www.thisisinsider.com/classic-optical-illusions-2018-1#this-is-troxlers-fading-circle-if-you-stare-the-dot-for-at-least-20-seconds-the-circle-will-completely-fade-away-20

• https://interestingengineering.com/11-puzzling-optical-illusions-and-how-they-work

• https://www.collective-evolution.com/2017/07/26/ex-nasa-scientists-share-concealed-information-about-the-face-pyramid-found-on-mars/

• https://www.buzzfeed.com/arielknutson/people-who-found-jesus-in-their-food

More info, examples, sources

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?

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

Brightness-contrast illusions

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

A Brightness Illusion

Simultaneous Contrast Illusion

Craik-O’Brien-Cornsweet Illusion

Actual Profile

Perceived Profile

Lightness Constancy

The Hermann grid illusion

Scintillating Grids: Straight and Curved

Scincillating black dots No black dots

Color illusions

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?

Color illusions - limitations in our sensor • We can sense EM radiation between in

a limited way, 400-700nm • 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)

Which image is higher in intensity?

Rubik’s Cube Color Illusion

Same Color?

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.

The two dogs have the same color

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).

A color illusion from our time

What is the color of the dress? White-gold or blue-black?

Afterimage

Bending the lines

Are the lines parallel? Straight?

Café Wall Illusion

A variant of the Café Wall Illusion

The Fraser Spiral Illusion

This one is an extremely strong illusion. Even if you trace it with

your finger, you go down a spiral

Pinna’s Intertwining Illusion

Concentric or intersecting circles?

”Moving” illusions

”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.

Spinning Wheel Illusion

Spinning seeds Illusion

Spinning colors

Works also in 3D

Seeing things that it’s not there and the opposite

Where is the lake?

Kanizsa's Triangle

Are you sure?

What does the sign say?

Size and perspective

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

Ebbinghaus illusion

In or out?

M.C. Escher

Seeing faces

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)

The Mars face

Double images

Rubin’s vase

Musician or a girl's face?

Young girl or old woman?

Double images based on earlier experiences

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)

Old man's face or two lovers kissing?

Cultural

Environment

Age

Age, cultural

Don’t forget your own filters and limitations,

and remember that perhaps you are just fooled.