Life

THE MAN dangles on a cable hanging from an eight-storey-high tower. Suspended in a harness with his back to the ground, he sees only the face of the man above, who controls the winch that is lifting him to the top of the tower like a bundle of cargo. And then it happens. The cable suddenly unclips and he plummets towards the concrete below.

Panic sets in, but he's been given an assignment and so, fighting his fear of death, he stares at the instrument strapped to his wrist, before falling into the sweet embrace of a safety net. A team of scientists will spend weeks studying the results.

The experiment was extreme, certainly, but the neuroscientist behind the study, David Eagleman at Baylor College of Medicine in Houston, Texas, is no Dr Strangelove. When we look back at scary situations, they often seem to have occurred in slow motion. Eagleman wanted to know whether the brain's clock actually accelerates - making external events appear abnormally slow in comparison with the brain's workings - or whether the slo-mo is just an artefact of our memory.

It's just one of many mysteries concerning how we experience time that we are only now beginning to crack. "Time," says Eagleman, "is much weirder than we think it is."

By understanding the mechanisms of our brain's clock, Eagleman and others hope to learn ways of temporarily resetting its tick. This might improve our mental speed and reaction times. What's more, since time is crucial to our perception of causality, a faulty internal clock might also explain the delusions suffered by people with schizophrenia.

But first, the basics. Perhaps the most fundamental question neuroscientists are investigating is whether our perception of the world is continuous or a series of discrete snapshots like frames on a film strip. Understand this, and maybe we can explain how the healthy brain works out the chronological order of the myriad events bombarding our senses, and how this can become warped to alter our perception of time.

Spinning backwards

Some of the first hints that we may perceive the world through discrete "frames" arrived with studies of the well-known "wagon wheel illusion", in which the wheels of a forward-moving vehicle appear to slow down or even roll backwards. The illusion was first noted during the playback of old films, and it's due to the fact that the camera takes a sequence of snapshots of the wheel as it rotates. If the speed of rotation is right, it can look as if each spoke has rotated a small distance backwards with each frame, when the spokes have in fact moved forwards (see diagram, diagram).

This effect is not restricted to the movies: people also report experiencing it in real life. If these observations proved to be reproducible, it would suggest that the brain naturally slices our visual perception into a succession of snapshots.

So in 2006, Rufin VanRullen, a neuroscientist at the University of Toulouse in France, decided to recreate the illusion in his lab. Sure enough, when he span a wheel at certain speeds, all subjects reported seeing it turn the "wrong" way. "The continuity of our perception is an illusion," he concludes.

The experiment even put a number on our visual frame rate - around 13 frames per second. But what within our brain sets this particular rate? When VanRullen measured his subjects' brain waves through electroencephalogram (EEG) electrodes on the scalp, he found a specific rhythm in the right inferior parietal lobe (RPL) - which is normally associated with our perception of visual location - that rises and falls at about the right frequency. It seemed plausible that as this 13-hertz wave oscillates, the RPL's receptivity to new visual information also shifts up and down, leading to something akin to discrete visual frames.

To test this hypothesis, VanRullen used transcranial magnetic stimulation - a non-invasive technique that can interfere with activity in specific areas of the brain - to disrupt the regular brain wave in the subjects' RPLs. That inhibited the periodic sampling of visual frames that is crucial for the wagon wheel illusion, reducing the probability of seeing the illusion by 30 per cent (PLoS ONE, vol 3, p e2911). The subjects could still see the regular motion of the wheels, however, probably because other regions of the brain, which don't operate at the necessary 13 hertz, took over some of the motion perception.

The case for discrete perception is far from closed, however. When Eagleman showed subjects a pair of overlapping patterns, both moving at the same rate, they often saw one pattern reverse independently of the other. "If you were taking frames of the world, then everything would have to reverse at the same time," says Eagleman.

VanRullen has an alternative explanation. The brain processes different objects within the visual field independently of one another, even if they overlap in space, he suggests. So the RPL may well be taking the "snapshots" of the two moving patterns at separate instances - and possibly at slightly different rates - making it plausible that the illusions could happen independently for each object.

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