62 Optical Illusions & Visual Phenomena

http://www.michaelbach.de/ot/index.html

62 Optical Illusions & Visual Phenomena
(Visual Illusion · Optische Täuschung)

by Michael Bach

These pages demonstrate visual phenomena and »optical illusions« or »visual illusions«. The latter is more appropriate, because most effects have their basis in the visual pathway, not in the optics of the eye. I selected these based on relative novelty and interactivity. When I find the time I will expand the explanations , to the degree that these phenomena are really understood; any nice and thoughtful comment welcome.

Most visitors of this site are no vision scientists, so you might find the explanatory attempts too highbrow. That is not on purpose, but vision research just is not trivial, like any science. So, if the explanation sounds like rubbish, simply enjoy the phenomenon ;–).

»Optical illusion« sounds pejorative, as if exposing a malfunction of the visual system. Rather, I view these phenomena as bringing out particular good adaptations of our visual system to standard viewing situations. These adaptations are »hard-wired« into our brains, and thus under some artificial manipulations can cause inappropriate interpretations of the visual scene. Or, as Purkinje put it: »Illusions of the senses tell us the truth about perception« (cited by Teuber, 1960).

Before we delve in, I’d like to express my thanks to the many visitors who went out of their way to give overwhelmingly positive feedback, adding constructive advice on content, style and typography; please do keep it coming! I have also been able to improve explanations when convincing scientific evidence was sent ;-). Yes, it does take sizable resources to set up and maintain this, but it is also very rewarding, bringing me into contact with so many charming fellow human beings. Should you want to give back, simply link to this page or use the donation link ;-). Any advice is appreciated – and, yes, for a non-native speaker, English does represent sizable challenges.

[Für Deutsche: »Optische Illusionen« halte ich für eine Fehlübersetzung aus dem Englischen, aber ich mache mir keine Illusionen [sic], dass der treffendere Begriff »Täuschung« sich wieder durchsetzen könne… ;-)]

Motion & Time

• »Stepping feet« illusion 1 – strong, beautiful and (relatively) easy to understand
• Motion Induced Blindness – an unexplained optical illusion of disappearing dots
• Stereokinetic Phenomenon – an optical illusion of 3D from motion (aka »kinetic depth effect«)
• »Rotating Snake« Illusion – with a new explanatory twist
• Pinna-Brelstaff Illusion
• Motion Aftereffect – also known as »waterfall effect«
• Spoke Illusion – a motion illusion + an unexpected (?) aftereffect
• 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
• Eye Jitter – more on eye movements, involuntary ones
• 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 by Isia Leviant
• Reverse Phi Motion – something for the specialist
• »Stepping feet« illusion 2 – radial version of »Stepping feet 1«, weaker
• Motion-Bounce Illusion – sound influencing perception of motion (crossmodally)
• Motion Binding
• Missing-fundamental motion inversion
• Frequency-doubling illusion

Luminance & Contrast

• Hermann’s Grid – fleeting luminance illusions
• Hermann’s Grid bent – a new twist, refuting the classical explanation
• Scintillating Grid – strong fleeting luminance illusions
• Induced grating – classical brightness contrast
• Craik-O’Brien-Cornsweet – collaboration of retina & cortex
• Wertheimer-Koffka-Ring – pitting Gestalt vs. lateral inhibition
• 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
• Contrast Constancy – demonstrating space-average-based contrast gain control
• Contrast Gain Control – demonstrating temporal contrast gain control
• Visual Acuity ↔ Hyperacuity

Colour

• »Lilac Chaser« – a rapid aftereffect, you pursue colours which aren't really there
• Benham’s Top – colour from spatio-temporal patterns
• Neon Colour Spreading
• Structure from motion – fails at equiluminance

Geometric- & Angle Illusions

• Hering Illusion – and other angular interactions
• Poggendorff Illusion – a classical geometric optical illusion with a new twist
• Fraser’s Spiral
• »Café Wall« Illusion

Space, 3D & Size Constancy

• Müller-Lyer Illusion – test your bisection accuracy
• Moon illusion
• T-illusion
• Luminance Looming
• Shepard: “Turning the Tables”
• Shepard: “Subterranean Terror”
• Missing Corner Cube

Cognitive- / Gestalt Effects

• Blur & picture content – spatial filtering (or high frequency masking)
• Hidden message – blur & figure-ground discrimination
• Kanisza’s Triangle – depends on context
• Blotted letters – Gestalt completion
• Hidden Figures – Dalmatian Dog with 2 helpers
• Rotation – changes the interpretation
• Impossible Objects – draw your own impossible object

What’s special about faces?

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

Some sources

Donations