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.
This article was originally published with the title "Yorick's Ghost and Other Afterimages" in SA Special Editions 22, 3s, 30-33 (September 2013)
ABOUT THE AUTHOR(S)
Stephen L. Macknik is a professor of opthalmology, neurology, and physiology and pharmacology at SUNY Downstate Medical Center in Brooklyn, N.Y. Along with Susana Martinez-Conde and Sandra Blakeslee, he is author of the Prisma Prize-winning Sleights of Mind. Their forthcoming book, Champions of Illusion, will be published by Scientific American/Farrar, Straus and Giroux. Follow Stephen L. Macknik on Twitter Credit: Sean McCabe
Susana Martinez-Conde is a professor of ophthalmology, neurology, and physiology and pharmacology at SUNY Downstate Health Sciences University in Brooklyn, N.Y. She is author of the Prisma Prize–winning Sleights of Mind, along with Stephen Macknik and Sandra Blakeslee, and of Champions of Illusion, along with Stephen Macknik. Follow Susana Martinez-Conde on Twitter Credit: Nick Higgins