The Neuroscience of Yoricks's Ghost and Other Afterimages
This is the eighth article in the Mind Matters series on the neuroscience behind visual illusions.
The Neuroscience of Yoricks's Ghost and Other Afterimages
- Twinkle: A Dynamic Aftereffect That Helps to Diagnose Blindness So far, the afterimages in this slide show have been filled in with colors and brightness. But what happens when you adapt to a dynamic stimulus? The Twinkle effect shows that when a static object adapts to a dynamic surround, the static areas may appear as dynamic. Fixate your gaze on the center point to adapt to the snow (vision scientists call it “dynamic noise”). Notice that the gray square is slowly replaced with dynamic noise, until the entire background appears as dynamic noise. When the dynamic noise in the surround is then extinguished, you will see a twinkling afterimage where the gray square used to be: this is the Twinkle aftereffect. This illusion shows the amazing extent to which the brain works to adapt to the world around us. Even in the presence of constant change, our visual system produces complementary and continually changing adaptation effects. Various aspects of this effect were discovered by Vilayanur S. Ramachandran (of the University of California San Diego), Richard Gregory (University of Bristol, U.K.), Lothar Spillmann and A. Kurtenbach (Frieburg University, Germany). Vision scientists Michael Crossland, Steven Dakin, and Peter Bex have used the Twinkle illusion to characterize blind spots caused by retinal damage in patients with macular degeneration (the most common form of blindness for patients over age 50). IMAGE COURTESY OF JORGE OTERO-MILLAN, XOANA TRONCOSO, STEPHEN MACKNIK AND SUSANA MARTINEZ-CONDE
- Robin's Illusory Red Breast Click here to view this illusion Afterimages can be captured from the complementary surround color, as in this demonstration of an uncolored bird that captures the reddish color of its background, before taking flight. In this illusion, created by vision scientists Yuval Barkan and Hedva Spitzer of Tel-Aviv University, Israel, the red background causes the bird to fill-in with a greenish complementary color, which gives rise to a surprisingly strong and long-lasting red afterimage once the red background, is extinguished (creating a green background afterimage). IMAGE COURTESY OF YUVAL BARKAN AND HEDVA SPITZER
- Lilac Chaser Click here to view this illusion
Afterimages can form quickly and, as the adaptation deepens, so, too, does the intensity of the afterimage. Jeremy L. Hinton, of BAE Systems Technology Centre in Bristol, U.K., developed a compelling illusion that highlights these effects. Fixate your gaze on the center point and hold your eyes as still as possible. After a few seconds, the pink dots will start to adapt and fade. As the pink dots fade, a green afterimage replaces each dot and becomes more and more salient, until it appears to be as bright as the original pink dots. IMAGE COURTESY OF JEREMY L. HINTON - Bovine Fly 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. Now, fixate your gaze on the fly in the right image for 10 to 30 seconds: this staring will selectively adapt the blue versus yellow color channels of your retina in your left versus right visual field. 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 environments. For example, your shirt looks the same under incandescent lighting and under sunlight (these two light sources have different color spectra): part of the system that produces color constancy in our perception also plays a role in this slide’s illusion. 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/yellow in the unadapted eye. IMAGE COURTESY OF GIANNI SARCONE [http://www.archimedes-lab.org/]
- Evolution and Adaptation In celebration of Darwin’s birthday, psychologists Rob Jenkins of the University of Glasgow and Richard Wiseman of the University of Hertfordshire created an illusory homage to Darwin’s evolutionary roots. Stare at the center of the image for 10 to 30 seconds and look away at a white surface. The monkeys turn into an afterimage of Darwin’s portrait! IMAGE COURTESY OF ROB JENKINS AND RICHARD WISEMAN
- Yoricks's Ghost 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. 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.
