What I love about todays XKCD is that even if I cross my eyes, to visually overlay a copy of dark-bg over light-bg... the dark-bg dress is still perceived by my brain to be considerably lighter.
sometimes, I feel "ghost cats". I can feel them reaching up at the back of my chair, ready to scratching-post it. I whirl around to tell them off... both cats are asleep in the catbed.