D: Here's a simple demonstration you can do with cool implications. Find a large object that is brightly colored--let's make it a green door. Stand with that door to your side but don't look directly at it; you want it to be in your peripheral vision. Now, without shifting your gaze, examine the door in the edge of your field of vision. What color is it? Answer? It's still green.
Y: It's still green? What kind of experiment is that? I thought you were going to say something happens to the color.
D: That's exactly right! The color stays green, which means something has happened--because nothing in your peripheral vision should have any color at all.
Y: Nothing in your peripheral vision should have any color? Why not?
D: Because color is what we perceive when light of a particular frequency meets cones--special cells in your retina. But the light being reflected off the door is only landing on the outside edge of your retina. And there are almost no cones on the edges. Everything we see roughly seventy-five degrees away from the point we're fixed on should be black and white.
Y: That's fascinating! But why is the door still green?D: Presumably the door remains green because vision isn't a simple matter of retinal cells. All the data your eyes send is interpreted by the brain. Your brain knows that the door is green, so it supplies information that isn't really there to keep the image stable. This is just one of the many ways your brain saves time and energy by making reasonably safe assumptions about the world around you.