|MadSci Network: Neuroscience|
Erica, The reason we can't see colors at night is pretty interesting. It all comes down to the fact that humans really have two different visual systems at work in our eyes. Seems a little crowded inside our eyeballs, doesn't it? Our two different visual systems are built to do two different things. One is built to work in the daytime or else when it's bright out at night. The other is built to work at night, or else when the lights get very dim. First I'll talk about how our dark system works, then I'll move to how the bright system works (and why the bright system sees colors but the dark doesn't). The night time system is called the 'scotopic' system. I think 'scotopic' is Latin for 'It's really dark in here' (just joking). There are cells called rods in the back of your eye that make this system work. The rods catch light and tell your brain that you saw something. Anyway, the rods aren't built to see colors. As an aside, rods are also not built to see very well. Try reading a letter chart at night. Your vision will be a lot worse than 20/20, that's for sure. But our night time system is built to make us really sensitive to dim lights, because our rods are very sensitive to small amounts of light. The reason we can see colors in the daytime is because our daylight, or 'photopic' system, is at work. The cells in your eye that detect light for your photopic system are called cones. They are built to see really fine details, so that's why you see 20/20 in the daytime. There are three different kinds of cones. One kind sees red light, one sees blue, and one kind sees green. All colors are made up of a combination of these three colors. So, when you see a color, your three kinds of cones get together and decide how much of red, blue, or green is in it, and then your brain puts that information together, and you see colors. So going back to the night system, we only have one kind of rod. Rods don't care what color something is, they only care how dim or bright it is. So they don't tell your brain anything about color. Hope that this is helpful. If you need any more info, feel free to ask! Tom
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