If you would like 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 entire thing up and down. When the illusion is performed correctly, straight line turns into a wiggling wave.
Therefore, how does the rubber pencil illusion work?
Let’s start with the simple explanation: Your eyes & brain just cannot keep-up. When light enters your eyes, receptors called rods & cones pass a signal along nerves to your brain, which processes it.
Consider each of these signals as a photograph. Your brain ties those images together in order that they seem 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 the Rice University in Texas.
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, consistent with a 2016 study published in the Journal PLOS One.
For context, several bird species can process 145 frames every second. There’s 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 does not 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 is why, if you wave your hand quickly in front of your face, you will see a blur and why fluorescent bulbs appear to cast a steady–light.
“What people do not realize is that those fluorescent tubes are flickering,” Pomerantz said. If you were, say, a pigeon, you would see a strobe light.
So, when your friend jiggles a pencil up & down, your visual system is not actually capturing that motion in detail. It’s giving-you a summary, Pomerantz said.
This is where things get a little 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 & Psychophysics, found that in the simulation, if a pencil is held near the tip & jiggled just so, graphs of every individual frame joined together to form a smooth curve. That is what your visual system picks-up.
If you were a bird, or an insect, you would see a straight line moving up & down, because those creatures can process more frames per second, Pomerantz said.
But there is more to the trick. More recent research has found, Pomerantz’s theory is a crucial part of the story but does not completely answer the question of why the pencil appears to turn to rubber.
Working together, teams of scientists in Germany & 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, by making it look more-straight, by tracking it with your eyes, said Lore Thaler, a psychologist at the 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, there was more to the story. A box, drawn around the outside of the line and being waved up & down in tandem also changed the perceived rubberiness of the line.
The box provided context, helping 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 & down.
Together, Pomerantz’s theory and these results suggest that it isn’t just about the “snapshots” our eyes capture, it also has to do with their context and the way our brains process the snapshots.
It is unclear exactly, why our brains are unable to process a straight line moving up & down, Thaler said. But scientists do know this: “The human brain just does the best it can,” she said.
