The Pulfrich Illusion TB

To travel throughout English speaking countries to help people to experience the Pulfrich Illusion!

This traveling science lab TB will give most people an opportunity to experience the strange perceptual optical effect called the “Pulfrich Illusion.”

The way to see this effect is to do any of several things. You can have someone pivot a stick in a tic-toc fashion using a table top or the corner of a wall to guide the motion to keep it moving straight. You can also hang a pendulum of sorts and swing it in a straight line. One more way is to have a helper run a pencil back and forth along the straight edge of a table with you looking at it from the other side. Place the dark lens over one eye but look through both eyes. Now watch the movement of the object closely.

You should perceive the straight moving object as moving in an elliptical pattern and NOT in a straight line! A pencil moving along the table toward the side of the face with the darkened eye will appear to approach you. Yet that is impossible because the pencil is touching the table! A swinging pendulum, if closely approached, will look as though it passes through the person when the pendulum swings by moving one of the two ways.

Long ago when I read about this phenomenon in the wonderful book called “Eye and Brain, the Psychology of Seeing” I wondered if things moving across a TV screen would appear to be in front of the screen when moving one way and behind the screen when moving the other way. Sure enough, it worked! That makes for an interesting football game. Not more than a year after I made this discovery the Pulfrich illusion was incorporated into the half time of a Super Bowl game. In preparation for the big event people could get the required special glasses at Quick Trip for free. The advertisement showed true 3D depth by continuously circling around any objects that were being featured at the time.

The way the illusion works is this. To see the third dimension each eye needs to see an object from a different viewpoint, just as a person’s two eyes do normally. The greater the difference in what we call perspective that is evident in an object, the closer it must be, or so our brains have learned to tell us. Objects that are very close are perceived quite differently by each eye. They are also shifted significantly to the right or the left with respect to the background; another depth cue for the brain. Objects that are too far off to see in 3D appear the same to each eye and are not shifted noticeably. So how does darkening one eye cause the perception of 3D? The trick is this: the signal processing by the darkened eye is slowed down. When it sends a signal to your brain of the image it saw, the moving object was in a slightly different place for that eye than for the other one. The brain quickly merges the two images, one of which was “taken out of time." Because of the delay, the dark eye saw the object from a slightly different angle (perspective) than the other eye. Two different left and right perspectives provide the brain with information that we have learned to perceive as relative distance. The problem is, this time the difference in the two images was not correct for the true distance of the object! The degree to which the differences in perspective is wrong is most pronounced at the points where the moving object is going the fastest across your field of view; hence the elliptical pattern.

Hold a pair of sunglasses held vertically or use this TB to try this at home. Watch TV and look for a slight illusion of depth in anything that moves left or right. You can even do this with a home video. The ideal method is to take a video looking straight out a side window while moving down the road. If you are driving to the left then darken the right eye or the other eye if you are going the other direction. You should see the things that are closer look closer.

One more thing bizarre thing you can do with the one-eye-dark trick is to turn the TV to an all-snow station, like channel 1. Notice with both eyes bright there is no apparent directional movement in the snow. Now darken one eye and stare at the screen. You should soon see the snow appear to be moving in an ellipse! It’s weird!! Darken the other eye and it’ll reverse direction! I can do all these things to some degree by simply rolling up one index finger to produce a small hole to look through. That then is the darkener for one eye. Even squinting one eye works a little!

The Pulfrich Illusion is demonstrated online at: http://dogfeathers.com/java/pulfrich.html

I hope you enjoyed my traveling science lab!