Alas, Poor Yorick! I knew him, Horatio; a fellow of infinite jest, of most excellent fancy; he hath borne me on his back a thousand times; and now, how abhorred in my afterimage he is!
Well... that's what William Shakespeare's Hamlet might have said, had he been looking at a vintage Pears' Soap advertisement bearing court jester Yorick's skull, rather than holding an exhumed and rotting Danish cranium. Stare long enough at the skull in the ad, and it will be “burned” into your vision even after you look away.
Afterimages such as Yorick's skull help us understand how neurons in various areas of the brain adapt to the visual environment. Adaptation, in this case, is the process by which neurons habituate to, and eventually cease responding to, an unchanging stimulus. Once neurons have adapted, it takes a while for them to reset to their previous, responsive state: it is during this period that we see illusory afterimages. We see such images every day: after briefly looking at the sun or at a bright lightbulb or after being momentarily blinded by a camera flash, we perceive a temporary dark spot in our field of vision.
TO SEE OR NOT TO SEE...
To experience this antique illusion (left), stare at the X in Yorick's left eye socket for about 30 seconds. Then look away at a flat surface such as a piece of paper, wall, ceiling or sky, and you will see Yorick's afterimage as a ghostly apparition.
Vision scientists believe that the adaptation effect producing poor Yorick's ghost largely takes place in the neurons of the retina. How can we know? Close your right eye and stare at the X again. Then look at the wall again to see the afterimage, but this time switch back and forth between closing one eye and the other. Only the left eye—which was open during the adaptation period—will reveal Yorick's ghost. This means that the adaptation must have taken place only in neurons responding to stimulation from the left eye. If the adaptation had occurred in the binocular neurons of the brain (in the primary visual cortex and higher visual areas), you would see Yorick's ghost with either eye.
EVOLUTION AND ADAPTATION
In celebration of the 200th anniversary of Charles Darwin's birth, psychologists Rob Jenkins of the University of Glasgow in Scotland and Richard Wiseman of the University of Hertfordshire in England created an illusory homage to Darwin's evolutionary roots. Stare at the center of the image for 30 seconds, then look away at a white surface. The two monkeys turn into an afterimage of Darwin's portrait!
An afterimage is never as sharp as the original. Jenkins and Wiseman took ad- vantage of this difference in resolution to create an image that looks one way in “normal” high-resolution vision and a different way as a lower-resolution afterimage.
This illusion shows the interaction between color perception and afterimages. First, notice that the left image has a color imbalance to the right and left of the fly: the left side is bluish, and the right side has too much yellow. Now fixate your gaze on the fly in the right image for 30 seconds: this staring will make the neurons in your retina adapt to a blue hue on the left and yellow on the right. As a result, your left visual field will become less sensitive to blue and your right visual field less sensitive to yellow. Then look back at the fly on the cow's nose, and the image will appear to have a perfect color balance.
This illusion helps to explain why objects look the same color under different lighting conditions. For example, your shirt looks the same whether you are indoors or outdoors, even though light from a lamp and light from the sun have different color spectra. Your visual system adapts to the illumination and “discounts” it to maintain color constancy. Some of this processing happens in the retina rather than the brain.
Notice, too, that the color-selective adaptation is still constrained to a single eye: if you close one eye during the adaptation period and then switch eyes while looking at the cow, the color balance will revert to blue and yellow in the unadapted eye.
BIRDS IN A CAGE
When you stare at a color image, its afterimage takes on a shade of its own. For example, stare at the eye of the red parrot for 30 seconds, then immediately look at the center of the empty birdcage. You should see a ghostly blue-green parrot inside. Try the same thing with the green cardinal, and you should see a pink bird. This illusion is part of an exhibit at the Exploratorium museum in San Francisco.
Gazing at any colored surface can induce a vivid afterimage of the complementary color. For example, staring at the color red induces a blue-green afterimage because the cells in your retina that respond to red light adapt to the red environment by reducing their activity—to save energy and to prepare themselves for detecting any future changes in redness. When you look away to a white background, your retina remains adapted to the red environment for a few seconds. With the red “subtracted” from the white, you can see red's opposite: blue-green.
A BIRD OF A DIFFERENT FEATHER
Positive afterimages can be captured from a complementary surrounding color, as in this demonstration of an uncolored bird that captures the reddish color of its background. Stare for about 30 seconds at the “target” on the bird in the left panel. Then look immediately at the same spot on the bird in the right panel.
In this illusion, created by vision scientists Yuval Barkan and Hedva Spitzer of Tel Aviv University in Israel, the red background in the left panel causes the bird to fill in with a complementary blue-green color, which gives rise to a surprisingly strong and long-lasting red afterimage of the bird once the red background is removed.
This illusion won second prize in the 2009 Best Illusion of the Year Contest. To experience an even more striking version of this illusion with a “flying” bird, visit http://illusioncontest.neuralcorrelate.com/2009/color-dove-illusion
RED AND YELLOW BEADS
Gaze at the cross between the upper and lower squares for about 30 seconds. Then look immediately at the cross on the right, and you should see 18 colored beads: nine red beads above the cross and nine yellow beads below the cross. The beads are all gray in the image itself, but the surrounding colors induce colored afterimages.
THE SPANISH INQUISITION
This incredible afterimage illusion shows just how powerful a little color can be. First, fixate your gaze on the tiny black spot in the center of the left image. Notice that no contours are present in the image, just splotches of pure color. Once you have adapted your retina for about 30 seconds, look at the black-and-white image of the Manzanares el Real Castle, near Madrid, Spain, on the right (again, fixate on the black spot in the center). ¡Olé! The Spanish castle has gone from black-and-white to glorious color.
This illusion demonstrates that the brain can assign color to monochromatic objects, even when the color is from an afterimage. The illusion is effective only when the afterimage lines up perfectly with the actual image—otherwise the actual image dominates, and the color is suppressed. The neural details of this complex process are largely unknown.
In this illusion a single colored image produces two afterimages of different colors, depending on the shapes you look at afterward. Fixate your gaze on the black dot between the colored stars in the middle panel and stare at it for 30 seconds without moving your eyes. Then look at the empty outlines in the top panel. The left one fills in with a ghostly blue-green, and the right one looks reddish. When you look at the bottom panel, the colors are reversed.
How does one image produce two afterimages of different colors? And how does the shape of the outline determine the filled-in color? The creators of this illusion, Rob van Lier and Mark Vergeer of Radboud University Nijmegen in the Netherlands, suggest that patches of an afterimage can spread and merge to fill the contours of an outlined shape. The shape at the upper right takes on a reddish hue because it has the same outline as the complementary blue-green patches in the original color image. Likewise, the blue-green-tinged shape on the upper left matches the red patches in the color image.
NOTHING IS THE SAME
These incredible illusions by Abigail E. Huang, Alice J. Hon, Christopher W. Tyler and Eric L. Altschuler of New Jersey Medical School and the Smith-Kettlewell Eye Institute show that objects of the same apparent color can look like different colors in an afterimage and that differently colored objects can appear to be the same color in an afterimage. Gaze at the white spot between the yellow letters M and P in the upper image. Hold your gaze for 30 seconds and then look at a white wall. You will see an afterimage of the letters, which are now magenta (M) and purple (P).
In the lower image there are two Y's, one blue and one purple. Look at the white spot between them for 30 seconds and then move your eyes to a white wall. You will now see that the Y's are the same shade of yellow in the afterimage.
In fact, the M and P in the upper image are different colors that only look the same shade of yellow because of the effect of the red and black backgrounds. In the afterimage, the complementary background colors—blue-green and white—have the opposite effect: they make the M and P look more different than they really are. The Y's are also different from each other, in the real image, but the complemen-tary background colors in the afterimage make them look the same color.
THE LEAST COLORFUL PLACE ON EARTH
Afterimage color assignment works very well with objects to which humans are exquisitely well tuned, such as faces. Gaze for 30 seconds at this reverse-color portrait of John Bortniak, commander in the National Oceanic and Atmospheric Administration Corps, at the South Pole (left) and then look at the image on the right to see it in color.