Stare at the red dot in the center of the figure for a minute or two. Before long, the green ring will disappear—it simply seems to fade into the white background. There are no tricks: This is a simple, static image file. The effect has been known for more than two centuries and is named for its discoverer, Ignaz Paul Vital Troxler (1780–1866), a Swiss physician and philosopher. “Troxler fading” is actually related to what you experience when you get “dizzy”: You become so habituated to a phenomenon (spinning in a circle or seeing a green ring in your peripheral vision) that you stop noticing it’s there. Or, rather, you don’t realize that your perceptual system has begun actively ignoring it. It’s only when your circumstances change that you see what the phenomenon has done to your perceptual system. When you stop spinning, the world seems to continue, in reverse. When you look away from the green ring, you see a red ring in the same part of your visual field.
Occasionally an illusion attracts widespread notice online, perhaps because it was posted by a popular blogger, but it’s rare that we see a scientific explanation of how the illusion works. That’s beginning to change. Each year, at the meeting of the Visual Science Society, the Neural Correlate Society holds a contest where vision scientists share their latest, greatest optical illusions. This year’s winner is entitled “The Break of the Curveball” and was created by Arthur Shapiro, Zhong-Lin Lu, Emily Knight, and Robert Ennis. Shapiro, an associate professor of psychology at American University, is also a blogger and an avid baseball fan. (He once referred to pro football games in the early fall during the baseball playoffs as “preseason games that count.”) He has a full explanation of the illusion on his blog, but to my mind an even more impressive illusion is this one. (Click and watch this before continuing!)
When you shift your focus from the red dot to the yellow dot, the motion of the balls rotating around the dots appears to reverse. Again, Shapiro explains the effect on his blog. As with Troxler fading, the effect is due to our perceptual system’s limited ability to process information outside of a central focal region. When you look directly at the red dot, you can see that the surrounding circles are moving in one direction as the shaded patterns inside the circles are moving the opposite direction. But you can’t process all the information about the circles ringing the yellow dot in your peripheral vision, so the pattern moving inside the circles dominates.
But our perception of the world doesn’t rely solely on vision. We use all our senses to build a representation of what the world is really like. Many illusions occur because what we perceive with one sense conflicts with another. Varun Sreenivasan, a graduate student at the EPFL in Lausanne, Switzerland, has written an amazing account of a 2003 study on the “rubber hand effect.” The basic premise is this: If your real hand is hidden behind a screen and you see a fake hand in its place, then you can “feel” it when a researcher touches the fake hand. Neuroscientists K. Carrie Armel and V.S. Ramachandran wanted to see when the illusion broke down.
First, they asked volunteers to place one hand behind a screen. An experimenter scratched the table in front of the screen while either scratching or not scratching the hidden hand. The participants reported “feeling” a scratch on their real hand whether or not it was actually being scratched.
They also experimented with an extremely unrealistic rubber hand and arm, much longer than a real arm. The researchers bent one of the fake fingers back to what would be a very painful position while lifting the volunteer’s real (hidden) finger only slightly. The participants said they felt real pain, which was only slightly less intense with the extra-long rubber arm than with a realistic rubber arm. A measure of skin conductance showed a dramatic a physiological response in the volunteers as well. Clearly the effect of seeing a finger being bent contributes greatly to our experience of pain.
These illusions are not only fascinating to observe and experience, they also tell us a great deal about how our perceptual system functions. We receive so many inputs from the environment that the brain must prioritize which inputs to trust. Illusions represent the boundaries between conflicting inputs to the perceptual system, and by uncovering them—and often explaining them on their blogs—researchers can also uncover how the brain itself works. You can follow that conversation at ResearchBlogging.org.
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