Lighter or darker?
Asahi illusion
Is the white in the middle brighter than the white page around it?
No: it is the same white, though it seems to glow.
Why? Your brain reads the shading as light coming from the middle. In experiments, the pupils of people looking at it even get smaller, as if it really were brighter.
Picture: Vectorization: Mrmw, CC BY 4.0 · Wikipedia(opens in a new tab)