How come if I stare long enough I see a reverse image?
After staring steadily out the window for a few minutes and then shifting my gaze to the blank wall across the room, I can still see the ghost image of the window and some objects around it. What's the straight dope on the cause of the negative afterimage?
Low-quality drugs were usually the problem when I was a kid, L., but I'm sure this is not a concern for young folks today. Basically, there are two kinds of afterimages--negative afterimages (the kind you noticed), and complementary afterimages (i.e., red becomes green, blue becomes orange, etc.). The latter are the kind you find in intro-to-psych textbooks, where you stare at, say, a green figure for a while, then shift your eyeballs to a blank page, whereupon you see a red ghost image that last a few seconds.
Related to this are aftereffects, which usually have to do with perception of motion, orientation, and whatnot. For example, next time you're chugging down the highway in the back of somebody's station wagon, stare fixedly at the lane stripes receding in the distance. When the car stops, it'll look as though the stripes are heading toward you--i.e., as if the car were backing up. Similarly, if you stare for several minutes at a set of lines that is tilted out of the vertical, a set of lines that actually is vertical will appear to be tilted in the opposite direction.
Psychologists get paid millions to dig up tidbits like this, but they're not so hot when it comes to explaining what causes them. The best guess is that afterimages are related somehow to nerve fatigue. It's known that if you stare at a brightly colored figure long enough, after a while the color seems much less intense. It's also known that at various points in the eye-to-brain nerve linkage there are things called "opponent process" cells, which fire faster than normal in response to a given color, but slower than normal in response to that color's complement.
One scenario has it that if you stare at, say, a red patch long enough, your red-sensitive opponent-process cells get all tuckered out. Then, when you avert your eyes to a neutral color, the fact that these cells are ticking along more slowly than usual is interpreted by the brain to mean you're seeing green.
This sounds plausible until you realize that afterimage is actually an amazingly complex phenomenon. For instance, sometimes (usually after a very brief stimulus) you'll see a positive afterimage. Then we have the formidable task of explaining motion aftereffects. If I were you I'd just keep my eyes closed until the whole thing went away.