By NATALIE ANGIER, New York Times
Published: October 27, 2008
If you want to glimpse the handiwork of one of your body’s unsung sensory heroes, try this little experiment. Hold your index finger a few inches in front of your face and sweep it back and forth at a rate of maybe once or twice a second. What do you see? A blurry finger. Now hold your finger steady and instead shake your head back and forth at the same half-second pace. This time, no blur, no Marcel Duchamp’s “Nude Descending a Staircase” effect. The finger stays in focus even as your head vigorously pantomimes its denial.