The top row shows the two circles from the longer duration part of the movie; the bottom row shows the two from the shorter part. I overlap them so that you can see more clearly whether they are the same (or not).
Probably everyone reading this has seen the demonstration where staring at, for example, a green and black striped flag with a yellow field and black stars will cause you to see an afterimage that looks like the red, white, and blue flag of the United States. That is a result of one form of adaptation in your visual system. There are many forms of adaptation that occur in different anatomical locations in your nervous system and for different physiological reasons. This demonstration takes advantage of a faster form of adaptation than that responsible for the flag/afterimage effect. Both forms of adaptation probably take place largely in your photoreceptors, but the mechanisms are different. Anyways, watch the animation below. For best effect, you should try to see it on a CRT monitor because LCD monitors have problems that make some of what I’ll describe below partially untrue. However, the effect will work on an LCD monitor, so if that’s all you have...
An Effect of Adaptation (probably in your cones)
You can look pretty much anywhere, but ‒ especially if you’re using an LCD monitor ‒ you should start by looking in the center between the two circles. You should see an orange-ish red circle on the left and a green circle on the right for two seconds, then they are replaced by another pair of circles for half a second. How do those circles compare to each other? You probably think the one on the left is greenish, and the other looks more orange. In fact, those two circles are identical. Not only that, they are both uniform. If you do not move your eyes during the movie, they will not look the same. However, if you look directly at one of the circles and then look at the other one just after they switch from the longer duration presentation to the shorter, you might start to believe me that the two circles shown during the shorter presentation are, in fact, physically the same. If you’re looking at an LCD monitor, it may be harder to convince yourself because they may not be quite the same depending on where you’re looking and how far you are from the monitor. Moving farther away will help.
To really convince yourself that the two circles shown during the shorter presentation are the same, you can stop the animation. They should look the same after a couple of seconds of viewing. To stop the animation, right click (Windows) or hold down the control key and click (Macintosh) and click "Loop" to uncheck it. Right (or control) click on the image again and select "Play" to get the animation to the end. Right (control) click on it again and select "Back" until you get the two circles that should now appear yellow. In case you have trouble doing that, the following image shows all of the physical stimuli I used to generate the movie.
The top row shows the two circles from the longer duration part of the movie; the bottom row shows the two from the shorter part. I overlap them so that you can see more clearly whether they are the same (or not).
This page was last edited on August 25, 2004
© Mickey P. Rowe, 2004