You have probably enjoyed many high-octane film scenes involving Johnny Depp and other action stars in which you were actually watching their stunt doubles without even noticing. Why couldn’t you tell the difference? According to a new paper in Cell Biology (PDF) from researchers at the University of California - Berkeley and MIT, the answer’s a bit more complicated than tricks involving camera angles — and it involves the same perceptual mechanisms that help prevent our world from slipping into a confusing chaos.

To the study’s press release:

Researchers have pinpointed the brain mechanism by which we latch on to a particular face even when it changes. While it may seem as though our brain is tricking us into morphing, say, an actor with his stunt double, this “perceptual pull” is actually a survival mechanism, giving us a sense of stability, familiarity and continuity in what would otherwise be a visually chaotic world, researchers point out.

“If we didn’t have this bias of seeing a face as the same from one moment to the next, our perception of people would be very confusing. For example, a friend or relative would look like a completely different person with each turn of the head or change in light and shade,” said Alina Liberman, a doctoral student in neuroscience at UC Berkeley and lead author of the study[.]

In searching for an exact match to a “target” face on a computer screen, study participants consistently identified a face that was not the target face, but a composite of the faces they had seen over the past few seconds. Moreover, participants judged the match to be more similar to the target face than it really was. The results help explain how humans process visual information from moment to moment to stabilize their environment.

“Our visual system loses sensitivity to stunt doubles in movies, but that’s a small price to pay for perceiving our spouse’s identity as stable,” said David Whitney, associate professor of psychology at UC Berkeley and senior author of the study.