Science | Science Experiment of the Week 3 | 240 - Waterfall Effect

Optical illusions caused by the brain's motion detectors.

This Week's Experiment - #240 Waterfall Effect

These experiments are from Robert Krampf - The Happy Scientist

This week's experiment is one that I have wanted to share with you for a long time. The problem has been finding a way for you to see it. To really get the biggest impact, you need a large waterfall. Realizing that most of you do not have one of these in your back yard, I have been searching for other ways for you to see this wonderful optical effect. You will need:

One of the following:
a waterfall
a television
access to websites

If you happen to have a waterfall nearby, you are all set. You need to be dow nstream of the waterfall, looking up at it. Stare at the falling water for about 30 seconds. Then look at some nearby trees or a building. What you see should impress you. A vertical strip of the trees will seem to flow upwards! I first saw this at Fall Creek Falls in Tennessee. At the time, I had never heard of the waterfall effect and it gave me quite a challenge chasing down an amazing mystery in those days long before the ease of Internet research.

If you don't have a waterfall, you can also use your television. Many of the news channels now have scrolling bands of text at the bottom of the screen. As the words move across the screen, they are a reasonable substitute for the waterfall. Stare at a point in the middle of this band of moving words for about 30 seconds and then look at a picture or something with a nice pattern. You will see a horizontal band of the pattern seem to flow sideways. Not quite as impressive as a waterfall, but still pretty neat.

If you have access to websites, you are in for a wonderful treat. There are quite a few websites which have rotating spirals which use the waterfall effect to produce some spectacular illusions. One of the sites is:

dogfeathers.com

After staring at the spiral for about 15 seconds, look at a picture of something with a pattern. I found woodgrain on the panels of my wall did very well. For a special treat, stare at the spiral and then look at the back of your hand! Be prepared to be startled.

Why do you see this? The reason is related to the color reversals that we saw in last week's experiment. In looking at the strangely colored flag, you "fatigued" the color receptors in your eyes. Staring at something that is yellow desensitizes the cells in your eye to yellow. When you then look at something that is white (a mix of all the colors), the signals that report the yellow part of the light are weaker than the others. This causes the white paper to look blue, as you see the spectrum with the yellow taken out.

A similar thing is happening as you stare at the motion. The big difference here is that the "fatigue" happens in your brain, not in your eyes. The part of your brain that deciphers the impulses from your eyes into sight also detects motion. These "motion detectors" are direction specific. If the signal from the upwards motion detectors are stronger, your brain recognizes it as upwards motion. If the right motion sensors are stronger, you see movement to the right. If all of the signals are the same strength, you don't see any motion.

If one part of your vision continually sees motion downwards, while the rest of your vision sees stationary objects, the part that sees the motion becomes fatigued. As the nerves fire continuously, the signal weakens. If you then look at a stationary pattern, the signals would normally return to all the same strength, but instead, the downwards sensors are tired and their signal is weaker. The normal signal from the upwards sensors is stronger and tells your brain that you are seeing things move upwards.

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