MadSci Network: Anatomy |
Thanks for the question, Gary. The phenomenon you describe is known as retinal fatigue and its 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 rates. 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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