Lighter or darker?
Hermann grid
Look around the grid. What do you see where the white lines cross?
Gray smudges, except at the crossing you look straight at. There is nothing there.
Why? At a crossing there’s white on four sides, so the eye’s cells turn each other down more, and it looks darker. In the center of your view the cells are smaller, so the smudge disappears.
Picture: Mrmw, CC0 · Wikipedia(opens in a new tab)