Why can't people figure out if the actor that's doing a fight scene is the stunt double or the actor himself?

Researchers from UC Berkeley wanted to solve this conundrum by trying to discover the brain mechanism that caused this effect.

They found out this effect is part of the "perceptual pull" that is a part of the human survival mechanism creating stability, familiarity and continuity in a chaotic world.

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Alina Liberman, one of the researchers, said if people don't have a bias of continuously seeing   the same faces in a movie, the movie becomes very confusing to people.

In the real world, this could mean friends and relatives will be completely different persons with each turn of their head and changes in light and shade, she added.

David Whitney, one of the researchers, said a person's visual system loses its sensitivity when looking at stunt doubles in movies. That this result is normal, he added.

He admitted it would be a real problem if people can't recognize faces. Thankfully, this "face blindness" is rare.

It's common to have changes in viewpoint, noise, blur and lighting that causes a person to look different from time to time.

Researchers said the results suggest visual systems tend to be biased against wavering perception and biased towards continuity.

To confirm their findings, participants in the study looked at dozens of dissimilar faces. After six seconds, researchers flashed a target face in the computer screen for less than a second then flashed almost similar faces on the screen.

Participants were asked to determine the closest image similar to the target face. Researchers affirm that regardless of the number of faces they saw, the participants could still figure out facial identities similar to the faces they saw seconds ago.

They explained that people tend to look at a "continuity field" that helps visually mold similar objects they saw within a 15 second time frame.