Posted by: juliegriffey | April 24, 2011

Limitations of our eyes and creativity of our brains allow us to perceive motion

Demonstration of the phi phenomenon


Film, video and animation are all comprised of a sequence of still images.  However, when we view them – we don’t see the separate images; we see motion.  Why is this?

This phenomenon is commonly explained as the persistence of vision theory: the idea that when viewing an image, our retina holds on to that information for approximately 1/25th of a second.  Therefore, when images are presented at a speed of 1/30th (video) or 1/24th (film/animation) of a second we see seamless movement.

The persistence of vision theory is still taught to film and media students as the reason why quickly presented sequential images create the illusion of movement, even though the theory was debunked in 1912 by Czech Gestalt psychologist, Max Wertheimer. Wertheimer clarified that the persistence of vision theory may explain why we do not see black between the presentation of images in rapid sequence but it does not explain why we perceive motion.

Wertheimer, instead, offered a different theory as to why we perceive continuous movement when we are shown a series of images in rapid sequence: the phi phenomenon.  The phi phenomenon states that an optical illusion is created when the eye is shown a sequence of images.   In essence, the brain invents information that is not there.  While we may be cognizant of the fact that we are being shown static images, the brain can not help but to see movement.

Now you know that the next time you watch a movie, it’s not your eyes playing tricks on you that allow you to perceive the movement; it’s your brain.

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