Alas! Poor Yorick. I knew him well. 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 Hamlet would have said, had he been holding the vintage Pear’s Soap advertisement bearing Yorick’s skull in the accompanying slide, rather than a dug up and rotting Danish cranium. In this antique illusion, you can stare at the X in Yorick’s left eye socket for about 10 to  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.

Afterimages such as this one help us to 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 and eventually cease responding to an unchanging stimulus. Once neurons have adapted, it takes a while for them to reset to their previous, unadapted state: it is during this period that we see illusory afterimages. We see afterimages every day: after briefly looking at the sun, at a bright light bulb or after being momentarily blinded by a camera flash, we perceive a temporary dark spot in our field of vision.

Vision scientists believe that the adaptation effect producing poor Yorick’s afterimage 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 result means that the adaptation must have taken place only in neurons responding to stimulation from the left eye. If the binocular neurons of the brain (in the primary visual cortex and higher visual areas) had been adapted, you would see Yorick’s ghost with either eye, despite having adapted only one eye.

This month’s slide show features several afterimage illusions. Each of them illustrates the perceptual effects of neural adaptation at a particular stage of visual processing.

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ABOUT THE AUTHOR(S)
Stephen L. Macknik is director of the Laboratory of Behavioral Neurophysiology at the Barrow Neurological Institute and earned a Ph.D. in neurobiology from Harvard University. Susana Martinez-Conde is director of the Laboratory of Visual Neuroscience at the Barrow Neurological Institute in Phoenix. She holds a Ph.D. in medicine and surgery from the University of Santiago de Compostela in Spain.