View from the lab: are Sir David's wildlife documentaries all in the mind of the beholder?
(Filed: 28/01/2004)

"All is vanity, all is delusion": Tolstoy; on a battlefield rather than the BBC, but the art at which David Attenborough, the corporation's leading light, has made his name is itself filled with deceit. I do not of course suggest that he uses stuffed tigers, but that film is based on trickery, of many kinds.

Some can be forgiven. Telephoto lenses mean that sound recording can be difficult; a tiger's bone-crunching noises are hence made in a studio by the BBC's resident bone-cruncher (a busy man nowadays). Some are less easy to excuse: as the leading natural history documentarist writes below, there have been several shocking fakes perpetrated by some past Disneyfiers of nature.

Most of the dishonesty, though, is inevitable. We all know that in a film projected at 24 frames per second, a stream of single images blurs into the appearance of motion, but why? The standard explanation turns on "persistence of vision"; the notion that the next frame comes into view before the impression of the previous one has faded away.

That effect certainly exists. It explains why fireworks are fun, for without it they would be short lines of light rather than fountains of fire. However, it applies mainly in the dark, when the eye adds up photons over time to compensate for the shortage of information available (which is why night-vision shots of hyenas, which work in the same way, smear a running animal's image).

The truth is more subtle. In fact, the brain has special circuits that detect the rate and direction of motion. As an object moves across the field of view it stimulates a receptor in one place in the retina and, soon after, a second one some distance away. The first signal is delayed by the nervous system for just the time it takes the object to move between the sensors: and, if the two messages arrive at just the same time in a specialised "motion area" of the brain, that is interpreted as movement at a particular speed.

Ingenious though the mechanism is, it cannot distinguish between a spot that moves smoothly from the first receptor to the second, and one that is switched off on the journey only to return at the crucial moment - which is the real reason why a series of still images on film gives the semblance of motion. Persistence of vision is in no way involved.

However, the brain does persist in error after a film is over. As the credits stop rolling downwards, the blank screen can sometimes appear to drift slowly up. That's due to our downwards-motion detectors getting tired and firing off impulses at a slower rate when movement has been going on at the same rate and on the same track for a while (which is why a tiger creeps steadily upon its prey rather than pirouetting towards it). The brain responds by assuming a drift in the opposite direction.

Film directors know that they are peddling illusions, and try hard to get the continuity right. Dozens of websites of astounding nerdishness list their failures: "Just before Gollum steals the Ring, Frodo and Sam are sleeping side by side, but in the next shot are at right angles." They really shouldn't bother, for our visual memory is feeble unless we are concentrating on a particular object. One droll experiment had shoppers ordering goods from a man behind the counter, who stoops to get them, to be replaced by a quite different man (or even a woman) who had been hiding below. Most customers simply do not notice, which means that a televised tiger could change its stripes between shots with more or less complete impunity.

All this and much more - from the fact that Scotland Yard once searched the eyes of murder victims for an image of the killer, to the illusion that a film appears to run backwards if every other frame is changed into its negative - comes from a marvellous new book, The Space Between Our Ears, which sets out to explore how the brain sees the world.

It is a fitting winner of this year's Wellcome Trust Prize for science books.

Tonight's talk by David Attenborough comes as he, too, receives an award. The Royal Society Medal for Science Communication is named after the great original in the world of explanation, Michael Faraday. He invented a wheel which span and appeared to distort the images pointed upon it. That led to a variety of Victorian toys and, in the end, to moving pictures, Life on Earth, and tonight's ceremony. Now, film has given way to tape; but it will be a sad day when the digital tigers take over.