Well, while I have never played Rock Band, I can relate to what you are describing. I don't know how Rock Band's screen looks, is it bright note streams on a darker background? If so, then that explains a lot. It's just like if you look at the sun for too long, you will get the sunspot on your vision, and when you close your eyes, it will show up like a negative. Any source of light, magnified by a dark background, will leave some impression on your sight if looked at long enough. It happens all the time with me too, but with other things (lights, the sun, Guitar Hero, etc.).