i dont think it has anything to do with your brain filtering information (although that is definately a fact that a normal functioning brain does filter out, what it considers, usless information). It has to do with the color of pink and the green. Its like looking at the sun too long and it leaves those color spots for a few seconds. Notice that if you shift your eye just a little bit, the pink comes back. The green color cancels out the pink after a certain amount of time, which is why you have to stare at a non-moving object on the pic.