Ambiguous Cube

1. Cut the chenille stems so you have eight pieces, each 6 inches long.
2. Bend chenille stems to form three-legged corner supports, as shown in the photo below.
   
3. Build an open cube by inserting the corner supports into the ends of the straws (see photo below). Bend the corner supports as necessary to form the angles required.
   

Place the cube on the floor and stand two to three feet away. Adjust your position so you’re looking down at one of the cube’s vertical edges, not at one of its flat faces (see photo below).

Adjust your distance so the top corner nearest you lines up (approximately) with the center of the bottom of the cube, as shown in the photo below.

Close one eye and stare at the cube. Try to look “past” the cube—or concentrate your gaze on the bottom back corner and “will” it to move toward you. Try to avoid focusing your attention on places where the straws cross. If all works well, the cube should suddenly appear to be standing along an edge, or possibly on a corner, rather than on a flat face.

If you experience this effect, hold the focus of your gaze and gently lean slightly to the left, and then to the right. You should see the whole cube rotate in the direction you move.

Opening your other eye will normally destroy the illusion.

Note: In the field of perception, it’s unfortunately the case that some people will not experience the desired effect. If that happens to you, we offer our condolences!

This three-dimensional cube creates an ambiguous figure because it can be perceived in two ways: one is the actual physical cube; the other—the “rotating” cube—is an illusion.

If you were successful in perceiving the illusion, the bottom back corner of the cube seemed closer to you than the top front corner. This happens because closing one eye hinders your depth perception, making it easy for your brain to ignore the cues provided by the places where the straws cross. Normally, your two-eyed stereovision gives you enough information to interpret the cube correctly, which is why the illusion disappears when you open your other eye.

Why does the reversed cube appear to rotate as you change your viewing position? Try this: Close one eye, hold one of your index fingers at arm’s length, and line it up with some faraway reference point, such as a picture on a distant wall. Move your head to the right and notice that your finger appears to move to the left relative to the distant object; move your head to the left, and your finger seems to move to the right. Relative to the distant object, your finger (which is the closer object), moves in a direction opposite to that of your head.

When you see the cube in its “rotating mode” and you move your head to one side, your brain expects the parts of the cube closest to you to move to the other side. But the parts of the cube that your brain perceives as being closest are actually the parts that are more distant—so they move to the right instead. Trying to make sense of the situation (as it always does), your brain interprets the visual input as best it can: It “sees” the object rotating to follow your gaze.

Visual illusions are fun to play with, but they are also the subject of serious study by neuroscientists. The “mistakes” our brains make provide insights about our process of perception.

Additional 3-D Ambiguous Figures

You can experience a similar illusion with a variety of other three-dimensional figures. A tetrahedron, such as the one shown below, is just one of the possibilities.

Try making your own ambiguous figures from additional straws and chenille-stem corners and see what illusions you can generate. Try figures of different sizes. Try putting them on a table rather than on the floor, or try suspending them with string. Try different colors. Use your creativity—and your curiosity!

Necker's 2-D Cube

The ambiguous two-dimensional figure below is called a Necker cube. This classic optical illusion was first described in 1832 by Swiss geographer Louis Albert Necker. The square at the lower left of the drawing may at one instant seem like the outside surface of a cube that slants downward and to the left, and then an instant later it may seem to be the inside surface of a cube that slants upward and to the right.

(The image below is essentially the same cube, but with the square at the lower left shaded. If you have trouble perceiving the illusion, this enhancement may help.)

Check out this episode of Build Your Own Exporatorium to see other ways to create this Snack.