This is one of those psychological stuff rite...If you
concentrate on a certaion group of white dots, your eyes will
notice black dots forming on the white dots somewhere, but when you
concentrate on where you thought you saw the black dots, they
disappeared...I think it got to do with the eyes and the way it
visualize combinations of coulours...i'm not sure...read an article
abt that somewhere b4....last time i think the experiment was on
white, yellow and red colours....
But pls do correct me if i'm wrong... 
