This is a myth we need to get rid off. As @ said, the human eye receives light continuously and the human brain can handle a lot more than 21 FPS. But how much a brain can handle varies from one person to another. A trained person can handle over 200 fps, a normal one around 60. But in generell you can say that young persons can handle more than older ones. You maybe thought about the amount of frames a human brain needs to see a series of frames as a video, which is around .