Wednesday, 2 December 2009

It moves!

Following up on yesterday's post on Eadweard Muybridge, when I was writing the book I discovered that I had totally the wrong idea of how we see a film as moving images. I was all prepared to write about 'persistence of vision' - in fact I did so in the first version of the manuscript. I was not alone in this mistake. You can still find plenty of websites and books that talk about persistence of vision - How Stuff Works, for instance, says 'Movies work because of persistence of vision, the fact that a human eye retains an image for about one-twentieth of a second after seeing it.' But if I'd left it in, I would have been writing a load of rubbish.

The idea of persistence of vision, first put forward around the same time as the emergence of the movie industry, depended on the assumption that some sort of after-image remained in the brain long enough to overcome the blank gap while the picture was changing to the next one, and that the two slightly different images then merged together to form the effect of motion. Unfortunately, more recent research makes it clear that after-images don’t form until around 50 milliseconds after the image has ceased to be projected, which isn’t quick enough to bridge the gap between frames.

Practical experience from the early days of cinematography showed that you had to change the pictures around 50 times a second to fool the eye. Early silent movies were shot at around 16 frames per second, with each frame shown three times, while sound movies run at 24 frames per second, showing each frame twice. The images are on screen for too short a time for persistence to account for the lack of visible flicker. And persistence of vision was never an adequate explanation for the second effect, apparent motion, as persistence would result in multiple images building on top of each other, not in the appearance of movement.

This illusion is a reflection of the brain’s ability to interpolate and substitute what it thinks is the right thing to see for the actual visual signal it is receiving from the optic nerves. The concept of persistence of vision relied on an outdated idea that the eye was like a camera obscura, projecting images onto the “screen” of the brain. In fact the brain contains a range of different visual sensory “modules” dealing with requirements like motion detection, object and pattern recognition, detail selection, and so forth. (These modules are conceptual rather than physical; they don’t uniquely occupy a single set of brain cells.)

These different modules don’t handle a single picture, but rather many different elements. The retina of the eye contains around 130 million light-sensitive receptors. When a photon penetrates to the back of the retina (the photoreceptors are back-to-front with the sensitive part at the rear, a clumsy arrangement that may well be an accident of evolution), it triggers a photochemical reaction. This reaction sends a signal back toward the surface of the retina, where input from different receptors is combined before feeding the information through the optic nerve to the brain. This nerve has a lot fewer nerve fibers than there are receptors in the eye, so the signal has already been processed before reaching the brain.

The combined image we “see” is much more an illusion than it appears, being a reaction to these complex inputs and a combination of the response of the brain modules that cope with motion, pattern, detail, and so forth. The suppression of the flicker between frames of a movie and the merging of still pictures into motion is not due to simple persistence; it is a side effect of the way the various complex systems involved in processing the optical data work together. Fascinating stuff.

Image from Wikipedia

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