The black lines taken away: the blue is only lines, and the white between them is plain white.
Shapes that aren’t there
Neon color spreading
Is there a pale blue disc in the middle?
No: only the lines are blue. The rest is white.
Why? Your brain guesses there’s a see-through blue shape and spreads the blue across the white spaces between the lines.
Picture: Drawn by Science Playground, after blebspot · Wikipedia(opens in a new tab)