Life

This implies that there is not a single "film roll" in the brain, but many separate streams, each recording a separate piece of information. What's more, this way of dealing with incoming information may not apply solely to motion perception. Other brain processes, such as object or sound recognition, might also be processed as discrete packets.

To investigate, VanRullen examined another neural function, called near-threshold luminance detection. He exposed his subjects to flashes of light barely bright enough to see, and found that the likelihood of them noticing the light depended on the phase of another wave in the front of the brain, which rises and falls about 7 times per second. It turned out that subjects were more likely to detect the flash when the wave was near its trough, and miss it when the wave was near its peak. The work was published in The Journal of Neuroscience earlier this year (vol 29, p 7869). "There's a succession of 'on' periods and 'off' periods of perception," VanRullen says. "Attention is collecting information through snapshots."

So it seems that each separate neural process that governs our perception might be recorded in its own stream of discrete frames. But how might all these streams fit together to give us a consistent picture of the world? Ernst Pöppel, a neuroscientist at the Ludwig Maximilian University in Munich, Germany, suggests all of the separate snapshots from the senses may feed into blocks of information in a higher processing stream. He calls these the "building blocks of consciousness" and reckons they underlie our perception of time (Philosophical Transactions of the Royal Society B, vol 364, p 1887).

It's an appealing idea, since patching together a chronological order of events hitting our senses is no mean feat. Sounds tend to be processed faster than images, so without some sort of grouping system we might, say, hear a vase smashing before we see it happen. Pöppel's building blocks of consciousness would neatly solve this problem: if two events fall into the same building block, they are perceived as simultaneous; if they fall into consecutive buildings blocks, they seem successive. "Perception cannot be continuous because of [the limits of] neural processing," says Pöppel. "A space of 30 to 50 milliseconds is necessary to bring together in one time-window the distributed activity in the neural system."

Slices of time

There's some evidence to suggest this might be what happens. In one experiment, Pöppel analysed his volunteers' reaction times by measuring how quickly their eyes moved to follow a dot jumping across a computer screen. He found that their reactions seemed to follow a 30-millisecond cycle. If the dot moved any time within this cycle, it took until the end of the interval before the volunteers would react (Naturwissenschaften, vol 73, p 267). A similar cycle has since been observed when volunteers are asked to discern whether an auditory and a visual stimulus are simultaneous or consecutive - suggesting it may be at the root of Pöppel's building blocks of consciousness.

The brain's effort to maintain its timekeeping has implications for understanding some diseases. Schizophrenia, for example, could arise from an inability to coordinate information arriving from different parts of the brain (see "Delusions on demand"). A better knowledge of the way the brain integrates the discrete packets of information might provide further insights into this.

The question of discrete versus continuous perception is not the only challenge that time presents to neuroscientists. Many, including Eagleman, are concerned with the speed at which time seems to pass in different situations. Why do we feel that some, usually frightening, experiences last longer than others, even if objectively they occurred for the same number of seconds? Eagleman experienced this apparent slowing of time as an 8-year-old when he tumbled off a roof and broke his nose.

There are two possible explanations, he says. It could be a facet of the memory, or it could be that his brain's processing speed accelerated under the stress, making outside events appear to slow down in comparison. Decades later, he decided to replicate his experience under carefully controlled conditions.

After taking half a dozen members of his lab to a nearby amusement park and finding none of the rides scary enough, Eagleman found another outfit that offered a thrill ride known as a "suspended catch air device" which drops people from a 30-metre tower into a safety net below.

To measure the speed of his plucky volunteers' perceptions, Eagleman and his team designed a wrist-worn device they call a perceptual chronometer. An LED array on the face of the device displays a flickering single-digit number alternating with the negative of its image about 20 times per second. That would normally be too quick for a human to distinguish between the two images - you would just perceive all the elements of the LED array to be shining at once - but if their perceptual clocks of the terrified subjects accelerated even a little bit, Eagleman reasoned, the number would become visible.

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