well since i have my screen set low it doesn't have the option to fluctuate - can't go below zero without an overlay. so as a test i turned up the brightness and check out some white backgrounds and then quickly introduce a dark object and it's definitely there. it's very subtle (i only went to 40% to test) but it's there. this is a feature that should NOT be in a device made people who like to draw. fluctuating screen brightness can totally screw things up for a very particular artist.
i hate that you brought this up b/c now i'm gonna start using it at a brighter setting just so i can see the effect more often, lol.