MadSci Network: Anatomy


Date: Tue Apr 15 17:38:21 2003
Posted By: Harry Adam, Research Associate
Area of science: Anatomy
ID: 1049084692.An

Thanks for the question, Gary. The phenomenon you describe is known as 
retinal fatigue – and it’s not as well understood as one might expect. It 
is suspected that there is a straightforward physiological component – due 
to the bleaching of rhodopsin by the intensity of the light. After 
exposure, the recovery period causes the separate cones responsible for 
colour vision to give a false sense of colour as they recover at differing 
However it is also thought that there is a component of the effect due to 
reaction in the visual cortex of the brain. Sight is a very complex eye-
brain interaction, and separating these two is not an easy matter.
That the former is involved, I have no doubt - and a simple experiment 
helps me reach that conclusion. If, for example, you stare fixedly at a 
green spot on white paper, you will cause retinal fatigue to the area of 
the retina where the spot is imaged. The fatigue will be specific to the 
green cones, and if you then stare at a blank sheet of white paper you 
will see a magenta spot similar in size to your original green spot. As 
recovery occurs the magenta spot will gradually fade. (Magenta is a 
mixture of red and blue light – which is roughly white minus green. You 
are seeing white - itself a mixture of red, green and blue, but not 
sensing the green because of the fatigue, hence you sense magenta. Give it 
a go.)
So what you see is real, but not an object!
By the way - never look directly at the sun. You might suffer more 
permament effects on your retina. You would not want the spots to be there 
for life...

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