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The Senses

Help Link : Persistence of vision
History : History of Motion Pictures


In the 20th century, movies and television have contributed substantially to the transmission of social and cultural values, because of the power that these media have to evoke reality. For a long time, scientists thought that the phenomenon of retinal persistence explained why we experience the succession of still images in a film as if they were scenes that are actually moving. Retinal persistence lets us store a visual impression in memory for a few hundred milliseconds after the visual stimulus has disappeared, so scientists inferred that this phenomenon might fill in the intervals of darkness between the still images that are projected rapidly when we view a film. In this way, it was thought, each new image was imprinted on the retina before the impression of the preceding one had faded completely, so that the one dissolved into the other, creating the illusion of continuous movement.

Eventually, however, psychologists rejected this explanation, for several reasons. For one thing, we continue to have the illusion of movement even when still images are presented to us at speeds of 10 per second or even slower.

One of the other problems pointed out was that if retinal persistence actually played such a significant role in creating the illusion of movement, it would do so by piling new images on top of old ones that were still discernible. The differences in the positions of the objects in the images would therefore create trails like those you see in time-lapse photographs that show all the components of a series of movements simultaneously.

A yet greater problem for this theory was that retinal persistence appears only about 50 milliseconds after the image ceases. But when a film is projected at normal speed, the viewer sees at least two still images during this interval. Consequently, the first image would not start to "persist" until the second one was already being projected—a serious contradiction to the theory that the illusion of movement is created when the first persistent image blends into the second.

The illusion of movement in motion pictures is now believed to produced by a different phenomenon, known as the beta effect. This effect occurs when two images whose elements are in slightly different positions from each other are presented one after the other in quick succession. The brain then automatically perceives movement, because of the way that the receiving fields of the retinal cells and the various areas in the visual cortex integrate visual information to detect movement and determine its direction.

So in a sense, we are victims of the beta effect whenever a series of still images passes in front of our eyes rapidly, whether we are watching a dramatic film, or a documentary, or a cartoon, or even looking at one of those little books where the images seem to move when you flip the pages quickly (see box below).

There is another theory that retinal persistence does play a role in the way we perceive motion pictures, by reducing the amount of flicker that we notice as the projector shutter opens and closes 48 times per second. But even this theory has been called into question.

Retinal persistence actually involves two distinct phenomena. The first, positive afterimage, is the one that was long cited to explain how motion pictures work. One good example of a positive afterimage is the image of a flash that persists in your vision for a fraction of a second after someone takes your photo with a flash camera.

The second phenomenon, negative afterimage, can last for several seconds after the stimulus stops, so it is easier to perceive. The reason that it is called a negative afterimage is that its elements have the complementary colours of those in the original image, and the opposite luminance. For example, if you stare at a green object for a while, you will see a red image of this same object for a few seconds if you quickly look away from it to a white surface.

Experience : Afterimage Experience : La persistance rétinienne Experience : Persistence of Vision Experience : negative afterimage

Flipbooks are a very simple form of animated film that illustrates the beta effect. In these small books, each page contains a drawing that is slightly different from the one before it. When you flips the pages rapidly, your eye sees each image for a brief instant and interprets the series of changes from one image to the next as movement.

Link : The FLIP BOOK Experience : Activity: on the move Experience : Flip Book:snowman

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