Horror films take advantage of the difference between what is visible and what we actually notice. Our visual system cannot process every part of a scene in detail at once. In Hereditary, for example, viewers may follow Peter through the house without noticing his mother, Annie, on the ceiling. Psychologists call this inattentional blindness: missing something visible because attention is elsewhere. A related effect, change blindness, explains why viewers may miss the girl becoming an old man in the background of The Conjuring 2.
Directors also guide attention through a character’s gaze. We tend to look in the same direction as someone nearby, so a film can show a character looking towards danger before revealing it. Alternatively, a wide view without a clear guide makes viewers search for a possible threat. The opening of It Follows uses this approach to build tension.
Another technique relies on peripheral vision, which detects movement at the edges of our sight quickly but provides less detail. Our eyes then turn towards the movement to identify it. The delay can create unease. Horror also uses our tendency to recognise faces in ordinary patterns, such as curtains. This effect, called pareidolia, helps film-makers turn uncertain shapes into disturbing images.
