Nearly every film has one: a climactic scene in which everything around the hero slows down. There might be slow running, slow falling, or even the squint of slow recognition, each meant to signal a moment in which something is about to change. In The Matrix, which features one of the most famous slow-mo scenes in movie history, Keanu Reeves’ Neo manipulates time to dodge a spray of bullets coming at him. Directors use this technique to heighten the emotion of what we’re watching—to highlight what’s important in no uncertain terms. Turns out, our brains do it, too.

In reality, when our brains sense danger—whether we’re being shot at, darting after a child running into traffic, or falling off a cliff—time does not slow down, nor do we access a latent superpower to see it as...