I find it fascinating that the brain can ‘add’ information to what we perceive. This is likely to be the case so we can make better sense of our visual world.
Recently, I have been reading about the watercolour illusion, an optical illusion where colour can appear to ‘spread’. In the first image below, the enclosed spaces appear to be coloured a pale orange/ yellow tint. In fact, the enclosed areas are white, and are identical to the background. This spreading effect is due to the combination of the purple/ orange contours which form the border. Within this image, there is also a figure/ground effect with the enclosed shapes appearing to be solid objects on a ground. Interestingly, if the purple and orange contours are reversed, as in the second image below, the enclosed spaces appear to be holes, rather than solid. A recent paper discusses the effect in more detail: http://ipe.sagepub.com/content/3/5/257.full.pdf