Deceptions of the senses are the truths of perception. - J. Purkinje

Structure of the human eye

The following schematic illustration shows the human eye and how images are formed on the retina.

Note that the images are upside-down (inverted).

An important question to consider: does the human brain come pre-wired to turn the inverted image around (so that we see things non-inverted) or does the brain learn how to invert images?

Kohler I, Experiments with Goggles, Scientific American May 1962

Drawing of retina from Hubel DH Eye, Brain, and Vision Scientific American Library p.38


Color addition demo:

Look through a diffraction grating at the colored lines below. The "white" line is actually made from three colors: red, green, and blue. Depending upon your computer monitor, each of the primary colors may, in turn, be made of other colors. On my monitor, the red is made of both red and a little orange.

When opposite colors, such as yellow and blue, are added together, they combine to form white. Likewise, if yellow is removed from white, the resultant color is blue. In an afterimage, persistent exposure to a given color causes the retina to become "tired" of that color. The retina subsequently removes that color. When the color stimulus is removed and the eye is exposed to white light, then the complementary color is perceived for a brief period of time.

Here's an afterimage from

Move the mouse away from the picture and stare at the center dot. Continue to stare at the center dot and move the mouse back over the picture.

Adjust the contrast in the Lilac Chaser illusion to about 20%-30%. Fixate on the center cross. The magenta spots will slowly vanish, but the green afterimage will persist.

Lilac Chaser illusion

McCollough Effect

Focus intently on this video for about 5 minutes. Unlike the afterimage effect, you don't need to fixate on a specific location. However, you may want to look at the center of the movie for the best effect.

Once you've finished with the movie, look at the patterns below. The top pattern should show faint green and red shading, while the bottom pattern should show faint blue and yellow shading.

Unlike afterimages, the color shading should last for several minutes to several hours. In some people, the effect will linger for a few days.

The site below has a nice computer animation which works a bit better than my movie, but only encodes red and green stripes.

For more information on the McCollough effect, see McCollough C, Color adaptation of edge-detectors in the human visual system , Science 149 pp.1115-1116 (1965)

Craik-O'Brien-Cornsweet illusion

The Cornsweet illusion

O'Brien V, Contrast by contour-enhancement, American Journal of Psychology 72 pp.299-300 (1959)

The Cornsweet illusion can be encountered in medical x-rays. The white blob in this man's chest looks very menacing at first glance.

This image was caused by a fold of skin (which is not uncommon in elderly men). Covering the border of the blob reveals that there is, in fact, no blob at all.

A related illusion is the watercolor effect: the region between the square frames appears to have a faint color, but is actually white.

So, why is there a Cornsweet illusion?

From Why We See What We Do Purves D, Lotto RB, Nundy S American Scientist 90(3):236-243 (2002)

Adelson's checker square illusion

Adelson's corrugated plaid illusion

Shaded diamond illusion

More information at

Fechner colors and Neural coding of color

Benham's wheel (also known as Benham's top and Benham's disk) is a black and white disk that shows colored patterns when rotated at a speed of approximately 4 revolutions per second.

Note that the colors will still appear on a black and white computer monitor.

It is also possible to make a video of the spinning disk using a black and white video camera, and show it on a black and white TV: the colors will still be visible.

So, why are there colored patterns?

Color constancy and Land's Retinex theory

While trying to perfect instant color photography, Edwin Land made a remarkable discovery. He initially worked with three monochrome (black and white) images, each representing the red, green, and blue portions of a single color image. Each image would be exposed with an appropriate color filter in front of the camera (red, green, or blue). Each monochrome slide was developed, then each was placed in a projector with the appropriate color filter in front of the projector. The resulting image was composed of three colors in much the same manner that color TV and computer monitors display color.

One day, someone knocked the green filter off the green projector with the result that the "green" image was now being projected as white light. To Land's surprise, there was almost no change in the resulting image. Land experimented further and discovered that he could turn off the blue projector and still see an almost completely normal image. Since the image was now being formed from only white and red light, Land might have expected to see only shades of pink. Instead, he saw a full color image.

The image below is composed of two different interleaved images. The odd scan lines are all shades of red: they faithfully represent the "red" component of the image. The even scan lines are all shades of gray: they are formed by taking the "green" component of the image and converting the green into white. The resulting combination is not as vivid as Land's original demonstration, but it does show more than shades of pink.

The image below is made using a checkerboard with alternating red-gray squares (much like a real checkerboard). The red squares display the red component of the color image. The gray squares are made by changing the green component of the image to gray.

This image is best viewed in a darkened room with the monitor brightness turned up.


Color constancy accounts for much of the color in the above images. Color constancy relates to our ability to see the "true" color of an object regardless of the illuminating light color. For example, a yellow banana illuminated with blue light still looks yellow, even though the "color" of the illuminated banana is green.

See the demonstration at and set the "Red Component value to about 50. The odd banana on the right looks green, but the same banana on the left looks yellow. Click the "Mask" button to show that they are the same color.

The relation between color constancy and the red-white Land effect is that the eye subtracts the constant red light level from the color image, and white minus red equal green. By mixing red, white, black, and green, the eye can construct a wealth of color.

Munker-White illusion

Look closely at the stripes that form the colored squares in the image below.

All the squares are the same color (gray).

Munker H Doctoral Thesis (1970)

White M A new effect on perceived lightness Perception 8 pp. 413-416 (1979)

Hermann grid illusion

In the classic Hermann grid, smudges can be seen at the lattice intersections except when you focus directly on an intersection.

Janos Geier has investigated variations on the Hermann grid.

Surrounding patterns

Click here to reveal the hidden pattern

The image above (and the one on the link below) is hard to recognize unless you can see the surrounding occluding pattern.

Look closely at the image below. The four circles on the left look dark, while the four circles on the right look bright.

If you look closely, you'll see that the circles are identical.

An animated version of this illusion is found at

For more information, read Anderson BL, Winawer J Image segmentation and lightness perception Nature 434 pp. 79-83 (2005)

Anderson has a nice set of optical illusions demos at and there is a rebuttal to the Nature article at

Blind Spot

For more imformation on blindspots, read Ramachandran VS, Gregory RL Perceptual filling in of artificially induced scotomas in human vision Nature 350 pp. 699-702 (1991)

Motion can cause "blindness." Look at the image below using red-blue (or red-green) glasses. Look steadily at Mona Lisa's smile. As the colored circles whirl around, her face disappears leaving only her smile (and a bit of her nose).

This illusion is inspired by the Exploratorium's cheshire cat illusion.

In this illusion, the motion causes the yellow dots to disappear abruptly.

Michael Bach has a version of this demonstration that permits you to adjust the colors.

Motion can also blind you to other visual changes.

For more information on motion-induced blindness, see Bonneh YS, Cooperman A, Sagi D, Motion-induced blindness in normal observers Nature 411 pp. 798-801 (2001)

We also have a temporal "blind spot." Try looking at your eyes in a mirror. Look first at the left eye and then at the right eye: can you see your eyes move? When your eyes move, your brain temporarily stops processing visual information so you don't perceive anything for a short instant. Your brain fills in this small gap so you are never aware of it.

A more dramatic illusion showing how your brain processes temporal information is shown in the flash-lag effect.

Structure and Form

View Muller-Lyer image explanation

(explanation is from Gregory RL Visual Illusions, Scientific American November 1968)

Ambiguous figures

Necker's cube was discovered in the middle of the 19th century by the Swiss crystallographer Louis Albert Necker while preparing technical drawings of crystals.

There are many good demos at Project LITE

Click here to reveal the ambiguous image

Anaglyphs and Impossible Figures

The impossible triangle. Each piece is logically consistent, yet the whole figure is impossible to construct.

Here is a 3-D computer image of the impossible triangle.

In the center of the Belgian village of Ophoven is a sculpture showing an impossible triangle.

Go to to find out how they did this.

A copy of M. C. Escher's lithograph Ascending and Descending (1960)

Andrew S. Lipson has created a sculpture of Ascending and Descending using legos.

You can see how he did this at

He has also created a lego version of Escher's Waterfall

and "McWholles" has created a video of a working Escher waterfall which can be seen here.

The picture below has been posted on many different internet sites by someone who had too much time on their hands to play around with Photoshop

An anaglyph of Theodore Roosevelt from

An anaglyph showing the Empire State building from

An anaglyph of Denali National Park from

Copyright 1994 by Jeffrey L. Cooper

An anaglyph showing the surface of the planet Mars from

Links to more anaglyph images can be found at

The figure below shows several 3D curves. When viewed separately, these curves merge together but when viewed through red-blue (or red-green) glasses, the 3D curves clearly separate into distinct objects. (This is a test pattern for some 3D medical imaging research that I'm doing.)

A shadow anaglyph from

An anaglyph of an impossible triangle.

Some good references: