MadSci Network: Neuroscience

Re: Why do you see the complementary colors after you stare?

Date: Wed Mar 8 14:48:41 2000
Posted By: Cynthia L. Park, Grad student, Cognitive Psychology, Florida Atlantic Universty
Area of science: Neuroscience
ID: 952390220.Ns

The afterimage effect occurs when you stare at a color for a while and then 
look at something white - like a blank piece of paper or a wall. You will 
then see colors that are "opponents" to the original colors in the image.  
Green's opponent color is red. Yellow's opponent color is blue.

The reason for this is that some neurons in the retina and thalamus are 
turned "on" by red but turned "off" by green.  Others are turned "on" by 
green and turned "off" by red.  (The same thing goes for the blue/yellow 
combo.)  (If you detect these colors at a particular point on the retina, 
you cannot simultaenously detect the opposing color at the same point; you 
therefore cannot see a greenish red.)  The afterimage comes into play when 
you "tire out", say, your green response by staring at green.  When you 
then stare at white (which contains all colors, including red), only the 
red part of the green/red pairing will fire normally.

Put another way -
1. Stare at green and the green/red neurons (neurons that are turned on by 
green and off by red) will fire until they get tired.
2.  Stare at white and the red wavelengths in the white light will turn on 
the red/green neurons (those that are turned on by red and off by green).  
The green wavelengths in the white light won't be able to turn on your 
green/red neuronsm because those guys are tired.

Hope this helps.

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