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flow into the head from a stable world.

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    Because vision appears so effortless, we are like fish challenged to understand water: since the fish has never experienced anything else, it is almost impossible for it to see or conceive of the water. But a bubble rising past the inquisitive fish can offer a critical clue. Like bubbles,visualillusions can call our attention to what we normally take for granted—and in this way they are critical tools for understanding the mechanisms running behind the scenes in the brain.

     
    You’ve doubtless seen a drawing of a cube like the one to the right. This cube is an example of a “multistable” stimulus—that is, an image that flips back and forth between different perceptions. Pick what you perceive as the “front” face of the cube. Staring at the picturefor a moment, you’ll notice that sometimes the front face appears to become the back face, and the orientation of the cube changes. If you keep watching, it will switch back again, alternating between these two perceptions of the cube’s orientation. There’s a striking point here:
nothing has changed on the page, so the change has to be taking place in your brain
. Vision is active, not passive. There is more than one way for the visual system to interpret the stimulus, and so it flips back and forth between the possibilities. The same manner of reversals can be seen in theface–vase illusion below: sometimes you perceive the faces, and sometimes the vase, even though nothing has changed on the page. You simply can’t see both at once.

     
    There are even more striking demonstrations of this principle of active vision. Perceptual switching happens if we present one image to your left eye (say, a cow) and a different image to your right eye (say, an airplane). You don’t see both at the same time, nor do you see a fusion of the two images—instead, you see one, then the other, then back again. 12 Your visual system is arbitrating a battle between the conflicting information, and you see not what is really out there, but instead only a moment-by-moment version of which perception is winning over the other. Even though the outside world has not changed, your brain dynamically presents different interpretations.
    More than actively interpreting what is out there, the brain often goes beyond the call of duty to make things up. Consider the exampleof the retina, the specialized sheet of photoreceptor cells at the back of the eye. In 1668, the French philosopher and mathematician Edme Mariotte stumbled on something quite unexpected: there is a sizable patch in the retina where the photoreceptors are missing. 13 This missing patch surprised Mariotte because the visual field appears continuous: there is no corresponding gaping hole of vision where the photoreceptors are missing.

     
    Or isn’t there? As Mariotte delved more deeply into this issue, he realized that there
is
a hole in our vision—what has come to be known as the “blind spot” in each eye. To demonstrate this to yourself, close your left eye and keep your right eye fixed on the plus sign.
    Slowly move the page closer to and farther from your face until the black dot disappears (probably when the page is about twelve inches away). You can no longer see the dot because it is sitting in your blind spot.
    Don’t assume that your blind spot is small. It’s huge. Imagine the diameter of the moon in the night sky. You can fit seventeen moons into your blind spot.
    So why hadn’t anyone noticed this hole in vision before Mariotte? How could brilliant minds like Michelangelo, Shakespeare, and Galileo have lived and died without ever detecting this basic fact of vision? One reason is because there are two eyes and the blind spots are in different, nonoverlappinglocations; this means that with both eyes open you have full coverage of the scene. But more significantly, no one had noticed because the brain “fills in” the missing information