Arryn Robbins at University of Richmond Magazine:
Horror movie fans have pointed out numerous examples of spooky figures and other unnoticed details in the background of scenes.
These blind spots exist because your visual system can’t process everything in a scene at once. You extract detailed information only from wherever your attention is directed. Anything outside the spotlight of your attention may go unnoticed, no matter how strange.
Inattentional blindness is the failure to notice something in plain view because your attention is engaged elsewhere; it’s the phenomenon behind the infamous invisible gorilla experiment, in which about half of participants busy counting basketball passes failed to notice a person in a gorilla suit strolling through the middle of the game.
Horror leans on inattentional blindness frequently, from the ghost boy in Insidious to the background ghosts scattered through Netflix’s The Haunting of Hill House. Something is plainly visible, though you often don’t notice it until a rewatch, and this delayed discovery can make the scene even creepier.
More here.