If you want to see a rigid pencil turn to rubber, just ask an elementary-school student. In a favorite playground trick, an amateur magician picks up a pencil near the tip and lightly jiggles the whole thing up and down. When the illusion is performed correctly, the straight line turns into a wiggling wave.
So, how does the rubber pencil illusion work?
Let's start with the simple explanation: Your eyes and brain just can't keep up. When light enters your eyes, receptors called rods and cones pass a signal along nerves to your brain, which processes it. Think of each of those signals as a photograph. Your brain ties those images together so that they appear to move smoothly, just as they do in a flip-book.
"The eyes tend to sum up light over time," said Jim Pomerantz, a cognitive psychologist who studies visual perception at Rice University in Texas.
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But humans have remarkably slow visual systems, Pomerantz said. Humans can process 50 to 100 individual frames — pages in that flip-book — per second, depending on the size of what we see, according to a 2016 study published in the journal PLOS One. For context, some bird species can process 145 frames every second. There is some evidence to suggest that houseflies can process upward of 270 frames per second, and the fastest flies can process 400 frames a second.
When tracking a fast-moving object, your visual system actually doesn't sense the object moving in real time. Instead, each frame of motion leaves roughly a milliseconds-long impression on your retina, the part of the eye that senses light. That's why, if you wave your hand quickly in front of your face, you'll see a blur, and why fluorescent bulbs appear to cast a steady light. "What people don't realize is that those fluorescent tubes are flickering," Pomerantz said. If you were, say, a pigeon, you'd see a strobe light.
So, when your friend jiggles a pencil up and down, your visual system isn't actually capturing that motion in detail; it's giving you a summary, Pomerantz said. This is where things get a bit more complex. When Pomerantz published the first study on the rubber pencil illusion in 1983, he used a computer to graph out each frame of a pencil's movement in detail.
His results, published in the journal Perception and Psychophysics (opens in new tab), found that in the simulation, if a pencil is held near the tip and jiggled just so, graphs of each individual frame joined together to form a smooth curve. That's what your visual system picks up. If you were a bird or an insect, you'd see a straight line moving up and down, because those creatures can process more frames per second, Pomerantz said.
But there's more to the trick. More recent research has found that Pomerantz's theory is an important part of the story but doesn't completely answer the question of why the pencil appears to turn to rubber. Working together, teams of scientists in Germany and Ohio had participants move their eyes in specific ways while paying attention to computer simulations of jiggling lines. The idea was that the eye movement would change the "snapshots" these people captured on their retinas. If Pomerantz was completely right, it should be possible to partially "cancel" the pencil's motion, making it look more straight, by tracking it with your eyes, said Lore Thaler, a psychologist at Durham University in England.
The 2007 study, published in the Journal of Vision, found that eye movement did make the line more rigid; but not as much as it should have based on Pomerantz’s theory alone. Another experiment further supported the researchers’ suspicion that there was more to the story. A box, drawn around the outside of the line and being waved up and down in tandem also changed the perceived rubberiness of the line. The box provided context, helping the brain discern the motion of the pencil. In effect, when the box and the pencil were waved together, participants saw a straight line moving up and down.
Together, Pomerantz's theory and these results suggest that it's not just about the "snapshots" our eyes capture; it also has to do with their context and the way our brains process the snapshots.
It's unclear exactly why our brains are unable to process a straight line moving up and down, Thaler told Live Science. But scientists do know this: The human brain "just does the best it can," she said.
Originally published on Live Science.