So when we stare at a black dot surrounded by a pattern of gray lines against a white background, our mind assumes what the intersection of the gray lines will look like without adding a black dot ...

That was before I read Jude Stewart's Patternalia: An Unconventional History of Polka Dots, Stripes, Plaid, Camouflage and Other Graphic Patterns ($25, Bloomsbury), which was published this month.