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Tricks the Brain Plays

Scientists at the California Institute of Technology have developed a visual illusion that they believe will help explain how human brains make sense of the world. The computer simulation shows how various parts of the brain act together to make a coherent visual image.

NPR's Joe Palca talks with NPR's Michele Norris, host of All Things Considered, about the research, which appears in the current issue of Nature.

Do the Illusion

Step 1: Click on the "Illusion Animation" link in the sidebar on the left.

Sit so your eyes are about 8 inches from the screen. Look at the center of the animation and pay careful attention to the direction the red dots appear to moving.

Step 2: Still sitting with your eyes about 8 inches from the animation, now look at a point about 1 inch from the left hand edge of the screen and note which direction the red dots are now moving.

What You Should See: In step 1, all the red dots in the entire animation appear to be moving down, and the green dots appear to be moving up. In reality, the screen is broken into three columns. In the center column, the red dots are indeed moving down, and the green dots up. But in the other two columns, to the left and right of center, the direction of the dots is reversed.

The Illusion Explained: What's happening is an example of a "binding problem" in the brain. Typically, color and movement are thought to be processed by different parts of the brain. But a red ball rolling across a table looks like a red ball rolling across a table because the brain puts the movement and color information together to form a coherent perception.

The brain is trying to do that in this illusion; it's incorrectly binding color and motion so it can tell us that all the red dots are moving in the same direction throughout our "world," in this case the animation display. The illusion breaks down if you stand several feet away from the monitor, and watch the illusion (a long mouse cable or a friend is necessary to do this.)

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Joe Palca is a science correspondent for NPR. Since joining NPR in 1992, Palca has covered a range of science topics — everything from biomedical research to astronomy. He is currently focused on the eponymous series, "Joe's Big Idea." Stories in the series explore the minds and motivations of scientists and inventors. Palca is also the founder of NPR Scicommers – A science communication collective.