152 Visual Phenomena & Optical Illusions with explanations by Michael Bach

Optical illusions don’t “trick the eye”, nor “fool the brain”, nor reveal that “our brain sucks”, … but are fascinating!

They also teach us about our visual perception and its limitations. My selection here emphazises beauty and interactive experiments; I also attempt explanations of the underlying visual mechanisms where possible. Returning visitor? Check →here for History/News.

»Optical illusion« sounds derogative, as if exposing a malfunction of the visual system. Rather, I view these phenomena as highlighting particular good adaptations of our visual system to its experience with standard viewing situations, confronted with an atypical situation. These experiences are based on normal visual conditions, and thus under unusual contexts can lead to inappropriate interpretations of a visual scene (=“Bayesian interpretation of perception”).

If you are not a vision scientist, you might find my explanations too highbrow. That is not on purpose, but vision research simply is not trivial, like any science. So, if an explanation seems gibberish, simply enjoy the phenomenon 😉.

Motion & Time

• »Stepping feet« illusion 1 – strong
• Motion Aftereffect – also known as »waterfall effect«
• Reverse Phi Motion – something for the specialist
• »Rotating Snakes« Illusion – with a new explanatory twist
• Snake ad lib – optimising this motion illusion
• “Rotating Snakes” – with luminance control
• Motion Induced Blindness – an illusion of disappearing dots
• Stereokinetic Phenomenon – an optical illusion of 3D from motion (aka »kinetic depth effect«)
• Motion silences hue changes – winner of the illusion contest 2011
• Pinna-Brelstaff Illusion
• Pinna-Brelstaff Illusion № 2
• Kinegram (“Scanimation”) – akin to a flip book
• Roget's Palisade Illusion
• Spoke Illusion – a motion illusion + an unexpected (?) aftereffect
• Wagon-wheel effect – closely related to stroboscopic artifacts
• Wagon-wheel with colours
• Stroboscopic Artifacts – making a teaching point out of a nuisance
• Sigma Motion – an optical illusion due to an interaction of image dynamics and eye movements
• Spine Drift illusion – more on eye movements
• Eye Jitter – and even more on involuntary eye movements
• Your eye movements evoke relative motion illusions
• Biological Motion – for me the most beautiful class of visual illusions
• Flash-lag Effect – moving and stationary objects perceived at different times
• Ramp Aftereffect – an aftereffect in time
• Enigma (Isia Leviant)
• Freezing Rotation Illusion – a “Best Illusion of the Year” winner
• »Stepping feet« illusion 2 – circular version of »Stepping feet 1«
• Motion Binding
• Tusi Motion
• Breathing Square
• Missing-fundamental motion inversion
• Frequency-doubling illusion
• »Kaleidoscope Motion« Illusion
• Ternus Display
• Stroboscopic alternative motion
• “Pigeon Neck” illusion
• “Psychokinematic” objects
• Dotted Lines motion illusion
• Spiral Motion Aftereffect – more effective for some

Luminance/Brightness & Contrast

• Hermann’s Grid – fleeting luminance illusions
• Hermann’s Grid bent – a new twist
• Scintillating Grid – strong fleeting luminance illusions
• 12 Vanishing Dots – No variation of the above
• Translational Moiré Patterns
• Rotatory Moiré Patterns
• Pulfrich effect – luminance turning time into displacement → stereo
• Classic simultaneous brightness contrast probed
• Induced grating – cortical(!) brightness contrast
• Shaded-diamond illusion – more brightness contrast
• Craik-O’Brien-Cornsweet – collaboration of retina & cortex
• Wertheimer-Koffka-Ring – pitting Gestalt vs. lateral inhibition
• Mach Bands
• Simultaneous Contrast – here dynamic
• Pyramid Illusion – Vasarely revealed
• Munker-White Illusion – contrary to lateral inhibition
• Adelson’s »Corrugated Plaid« – context affecting brightness
• Adelson’s »Checker Shadow« illusion – more context affecting brightness
• Saccadic Suppression – eye movements interact with visibility
• Contour Adaptation – a new case of contrast gain control
• Contrast Constancy – demonstrating space-average-based contrast gain control
• Contrast Gain Control – demonstrating temporal contrast gain control
• Lazy Shadow – Interaction of luminance & time
• Visual Acuity ↔ Hyperacuity
• Grid Masking – What hides behind the grating?
• »Shape from shading« – strong
• Ehrenstein Illusion (disks)

Colour

• Hinton's »Lilac Chaser« – a strong aftereffect
• Watercolor Illusion – long-range colour induction
• Chromostereopsis demonstration
• Context affects colour – different eyes?
• Colour Fan – more aftereffects
• Benham’s Top – colour from spatio-temporal patterns
• Benham’s Top “unrolled”
• Colour Assimilation
• Neon Colour Spreading
• Xmas Colours – a seasonal variant of colour assimilation
• Munker Illusion
• Structure from motion – fails at equiluminance
• Loss of contour in colour
• Gestalt loss in equilumance
• Equiluminance – Heterochromatic flicker fusion & minimum motion
• Colour mixing – additive and multiplicative
• Colour matching – a game
• Colour Phi
• The Dress – the Necker cube of color vision
• Red Strawberries that are not red – strong demo of colour constancy
• McCollough Effect – an aftereffect that can last months
• Color Change illusion through eye movements
• Color from contour flankers

Geometric & Angle Illusions

• Skye’s Oblique Grating – No Café Wall variant
• Oblique Grating adjustable
• Kinetic Orbison Illusion – motion improves…
• Tilted Table – a cute Zöllner variant
• Hering Illusion – and other angular interactions
• Poggendorff Illusion – a classical geometric optical illusion with a new twist
• Disjointed Arch Illusion – a strong Poggendorff variant
• Fraser’s Spiral
• Tilt Illusion
• »Café Wall« Illusion
• Oblique grating simplest
• Checker Bulge
• Bourdon Illusion
• Jastrow Illusion – a classic
• Peripheral curved lines – from 2021
• Triangle Puzzle – a puzzle

Space, 3D & Size Constancy

• Müller-Lyer Illusion – test your bisection accuracy
• Moon illusion
• T-illusion
• Sine Illusion
• Schäffel's Luminance Looming
• Shepard’s “Turning the Tables”
• Shepard’s “Subterranean Terror”
• Size Constancy
• Reverspective – reverse perspective
• Necker Cube – the mother of reversible figures
• Missing Corner Cube – a cube with multiple ambiguities
• Ames Window
• Ambiguous Silhouette Illusion
• Ambiguous Wiredframe / Silhouette Illusion
• Frankfurter Illusion – binocular rivalry & supression
• Anamorphosis
• Leaning Towers Illusion

Cognitive / Gestalt Effects

• Blind Spot – intelligent fill-in
• Darwin Illusion – the afterimage creates a recognisable Gestalt
• Blur & picture content – spatial filtering (or high frequency masking)
• Hidden message – blur & figure-ground discrimination
• Demonstration of blur in the diamonds vs. squares illusion
• Kanizsa’s Triangle – depends on context
• Blotted letters – Gestalt completion
• Hidden Figure – Dalmatian Dog with 2 helpers
• Hidden Bird – more camouflage
• Hidden object in brick wall
• Rotation – changes the interpretation
• Impossible Objects – draw your own impossible object
• Hallucii – An “impossible” movie
• Impossible waterfall – Another Escher inspiration
• Numerosity adaptation
• Delboeuf Illusion – with an application to eating
• Ebbinghaus Illusion
• Dynamic Ebbinghaus Illusion
• Pop-out in visual search

Specialties with faces

• Rotating face mask – hollow faces
• Find the Face (in the Beans) – a picture puzzle
• The face on Mars – imaginations of perception
• Church or chicken?
• The »Thatcher Illusion« – inverted faces lose their emotional message
• »Ghostly Gaze« Illusion
• Dr. Angry and Mr. Smile – two messages in two spatial frequency bands
• The Lincoln Effect

Auditory Illusions

• Shepard Tone
• Motion-Bounce Illusion – sound influences perception of motion (crossmodally)
• Missing Fundamental Illusion – our brain reconstructs the missing first harmonic