Gestalt psychology contends that the human brain organizes what the eyes see based on traits such as similarity, common background, and proximity. But a new illusion that took second place in the 2011 Best Illusion of the Year Contest - a competition held annually by the Neural Correlate Society - illustrates that our brains can also organize what we see based on changes in contrast.

Arthur Shapiro, professor of psychology at American University, Erica Dixon, a first-year PhD student at AU, and Kai Hamburger, a researcher at Universität Giessen (Germany), created the illusion called "Grouping by Contrast."

The illusion brings a new dimension to understanding how our brains organize what we see.

"We perceive a world that can be divided into objects with boundaries," said Shapiro. "According to Gestalt psychology, the objects constitute the foreground, while the rest of the world acts likes the background for these objects. Our illusion illustrates that the visual system can organize the world based on the transition between the foreground and the background."

Helping to Understand Vision and Visual Problems

While the illusions in the contest are fascinating for all people to test out, they serve an important purpose.

"Illusions like these help us better understand the visual system and how we perceive the world," said Shapiro, who in 2010 received a National Institutes of Health grant for his project "Separating the Visual Response to Color from the Visual Response to Color Contrast." The techniques described in the grant proposal can be applied to the study of congenital stationary night blindness and visual decline associated with the normal aging process.

In Grouping by Contrast, two disks at the top of a solid grey background flash together from yellow to black. Simultaneously, two disks the same size at the bottom flash together from black to yellow. When the disks at the top are yellow, the disks at the bottom are black, and vice-versa.

But once the background is changed from solid grey to a split background (yellow on the right half; black on the left), it appears that the top and bottom disk sets no longer flash in synch or "group" together. Instead, it appears that the disks flash in synch diagonally, but in the opposite colors (when top right disk is yellow, lower left disk is black.)

According to the Gestalt law of proximity, the bottom two dots - which are closer together than the two dots at the top - should flash together, but with the split background they do not. The law of common region says the dots over the black half should group together, as should the dots over the yellow half - again, they do not. The law of similarity dictates that the top two and bottom two dots should group together - which they did over the solid grey background - but don't over the split background.

"While we may think of the world in terms of objects and their background, the visual brain also cares about the important information that cuts across those boundaries," Shapiro said.

Harry Potter magic? Not so fast.

The Grouping by Contrast illusion wasn't the only impressive Best Illusion of the Year entry from Shapiro - a second one called "The Exchange of Features, Textures, Faces" was also one of 10 finalists that was selected by an international panel of judges from the 170 entries that were submitted to the contest.

The illusion, a collaboration of Shapiro and University of Nevada-Reno psychology professor Gideon Caplovitz, features a stop-motion animation video involving figures of Harry Potter and house elf Dobby. In the video, Harry and Dobby move from separate sides of a surface (Harry on right; Dobby on left) toward each other and meet in the middle.

When you focus your eyes on the place where Harry and Dobby meet, it appears as if the two figures bounce off each other and return to their respective original places.

But if after watching Harry and Dobby collide a few times, you focus your eyes just above the video (say, on the wall slightly above the top of the monitor on which the video is playing) - all the while keeping the video in your peripheral vision - it appears that Harry and Dobby now pass through each other (Harry ends up on the left where Dobby began, and vice-versa).

Shapiro says this peripheral Harry-Dobby switcharoo isn't Hogwarts magic, but another instance of our muggle brains tricking us as Harry's and Dobby's features seem to bind to each other. (For non-Harry Potter fans, "muggle" means non-magical people).

Shapiro and Caplovitz created this illusion to explore a fundamental issue in neuroscience called "the binding problem." The term refers to the fact that different parts of our brain simultaneously process the information our eyes gather. For instance, when we see a red ball rolling across the floor, one part of our brain analyzes color, another shape, another motion, but all of the information combines to be perceived as a whole.

"In this illusion, we are exploring the limits of the brain's ability to bind together features and objects," Shapiro said. "We created conditions where colors appear to detach from one object and then appear to reattach to another object. The illusions illustrate that while we see a world composed of objects, our perception of the features of the objects is not necessarily as stable as we would like to believe. The ability of keeping a feature and object together seems to depend on the relationship of the object to its background and whether we watch what is going on centrally or peripherally."

Shapiro and Caplovtiz created another video to further illustrate the same concept - this one uses the faces of Prince William and Kate Middleton. It appears that William's and Kate's faces circle each other, but in reality, they are simply moving along parallel lines (toward each other from opposite directions) over and over again.

Shapiro is no stranger to the Best Illusion of the Year Contest. In 2009, he and Zhong-Lin Lu, William M. Keck Chair in Cognitive Neuroscience at the University of Southern California, won the top prize for their illusion on the curve ball's break (it does curve, but not as dramatically as we think it does). In 2010, their study was published in the journal PLoS ONE. Shapiro's illuisions also won first prize in 2005 - the illusion contest's inaugural year - and third prize in 2007.

Maggie Barrett
American University