Take a look at this picture, and tell me where the brightest parts and dimmest parts are:
Well, obviously, just to the left of the color gradient, it gets darker than anywhere else. Also, just to the right, it gets lighter than anywhere else. And that's the way it is, isn't it?
Or is this an optical illusion; a trick of your eyes? Let's chop the image up into three parts and find out:
Well damn, there isn't a "darker band" or a "lighter band" there at all, this is just your inability to see properly! This effect of your eyes is known as a Mach band. (Maybe that will be the new title of the next Playstation hit?) Why does this happen? Because all brightness is relative when it comes to your eyes. If a bright thing is next to a dark thing, it appears brighter than if it's next to another bright thing; and vice versa for dark things. When I take the context away, you don't see the illusion. Neat, huh?