For today’s Imagine It segment, I thought it would be fun to talk about how your brain processes the information that your eyes send it. The Trizonal Space Warper is a pretty cool tool to explore this because it illustrates how the neurons in your brain can become fatigued when you send them a continuous input.
When you look at something, your brain has to process all sorts of information coming from your eyes such as color, contrast, depth and motion. Lets take a specific example such as motion. When you look at something such as a waterfall, there are nerve cells in your brain that fire off a signal that represents, “downward motion.” There are also some cells (a small number) that might randomly fire off the wrong signal – “upward motion.” Your brain receives both signals and does a sort of strength comparison. It determines that it is receiving more “motion down” signals and so we perceive the water as moving “down.”
Keep in mind that your brain is a big chemical machine. All these nerve cells generate their electrical signals from a chemical reaction. If you keep asking them to send the same signal for a long time, they will run out of their local supply of chemicals. They become tired or fatigued and this is the interesting part. If you keep watching the waterfall for say 30 seconds then look at the trees nearby an interesting thing happens. The nerve cells for downward motion are firing slow – we have tired them out by staring. There are still a few cells that might fire off randomly an upward motion signal. These may out number the downward signal for a few seconds. The end result is your brain “sees” the trees as moving slowly upward even though they are not really moving!
Animated version of the Tri-zonal Space Warper. Click for a much larger and rotating version.
In the early 19th century psychologists experimented with spinning spirals and the effects they had on apparent motion. In 1977, Jerry Andrus, a well known magician, improved on the simple spiral with what he called the Tri-Zonal Warper. His improvement stacks three spirals with alternating expanding and contracting zones. These multiple zones create a much stronger motion aftereffect than a single spiral.
To the right is an animated version of Jerry’s Tri-Zonal Warper. Click on the small image to open up a much larger and moving image. To experience the effect the best, stare at the center of the rotating spiral for at least 20 seconds then look at something that is not moving.
Now that you know what the effect looks like you are ready to conduct a real experiment. When this motion aftereffect was first noticed (first mention comes from the ancient Greeks) it was not understood if the effect originated in the retina of the eye or in the brain. You can do a simple experiment to find out. Fire up the spinning spiral image, but this time cover your right eye. Stare for 30 seconds with your left eye, then quickly cover your left eye. We are trying to see if the effect will transfer from one eye to the other. If you see the effect after swapping eyes, then the origin has to occur not in the retina but further along the visual processing system.
As with all good experiments, don’t limit yourself to just collecting data from one person – like yourself. Give this a try with your friends and family and see what happens. Let us know in the comments below what you find.