A simple optical illusion, called the motion quartet, reveals impressive things about how the brain works in general, and the particular link ups of individual brains. Find out whether your cerebral hemispheres are firmly linked!

The motion quartet is a fairly basic optical illusion. It's so basic that it barely qualifies as an illusion at all. It's more like willful deception. All it requires is a diagonal pattern that switches positions once, and then hops back again. Sometimes this pattern is a four-square checkerboard, with light and dark squares. Sometimes it's just two dots, arranged diagonally like the 'two' side of a die. Whatever it is, it changes off. If the checkerboard has the black squares on top on the left side and on bottom on the right side, it flips to an image in which the black square is on top on the right side and on bottom on the left. Then it flips back. If the die has the right dot higher than the left, it flips so the left dot is higher than the right, and then flips back.

When people see this, their minds infer some motion, the same way successive images in a flip book give the impression of motion in the mind of the flipper. Because the motion quartet is so simple, and because it repeats between two images, there are two possible impressions that the viewer can get. The first is that, in the case of the checkerboard, the black squares are jumping up and down. The black square on the left starts at the top, jumps down to the bottom, and jumps back up again. The bottom one on the right jumps up and falls back down. They're moving vertically.

The second visual impression that the viewer gets is that the black squares are jumping back and forth. The black square on the top left jumps to the right. Then it jumps back. The black square on the bottom right jumps to the left, then jumps back. They're moving horizontally.

Neither impression is wrong or right, but one impression is much more common than the other. When the squares are equal distance apart, and the viewer is told to focus at the center of the illusion, overwhelmingly people get the impression of the squares jumping up and down - moving vertically. It's only if the left and the right squares are pushed closer together, and the top and bottom ones separated, that viewers tip towards the impression that the squares are moving back and forward horizontally.

The impression of vertical movement is carried out inside each hemisphere of the brain. The left and right side don't have to communicate to see the squares jumping up and down. Since this is the first impression that people get, it might be the lasting one. To understand horizontal motion, the two sides of the brain have to communicate with each other. This makes it harder to see than vertical motion.

Researchers found that some people have an easier time seeing than others. The larger the fiber between the two sides of the brain, and the better the conduction of the nerve impulses, the farther away the left and right sides of the illusion could be when the illusion was perceived as horizontal jumping rather than vertical jumping.

Perception is not just a matter of willpower or mental acuity. Sometimes it's just plain physiology.