Read more: “About time: Adventures in the fourth dimension“
CAN you count off 5 seconds in your head? Chances are that if you did, and then checked your estimate with a stopwatch, you would be pretty close. But have you ever stopped and wondered how your brain achieves this amazing feat?
Time perception is one of the enduring mysteries of the brain. While we have a fairly good grasp on the millisecond timing involved in fine motor tasks and the circadian rhythms of the 24-hour cycle (see “The body’s metronome”), how we consciously perceive the passage of seconds and minutes – so-called interval timing – remains decidedly murky.
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For a start, there is no dedicated sensory organ for time perception, as there are for perceiving the physical and chemical nature of our environment through touch, taste and smell. Time is also unusual in that there is no clinical condition that can be defined purely as a lack of time perception, which makes it difficult to study. “What we really want is someone who is as bad at timing as amnesiacs are at memory,” says of Keele University in the UK. “But there are no such people.”
Some believe there’s a reason for this. of Duke University in Durham, North Carolina, thinks timing is so fundamental to cognition that our brains have developed several back-up systems that can kick in if the main clock is damaged, which is why it is so difficult to find anyone who cannot perceive time.
As for what the biological basis of such clocks might be, nobody really knows. The closest anyone has got to an explanation of interval timing is the “pacemaker-accumulator model“, which proposes that the brain has some kind of pacemaker that emits regular pulses and that these are stored in an accumulator, where they can be counted to estimate how much time has elapsed.
The problem with this model is that while it fits neatly with observations of time perception, it is short on specifics. It doesn’t say what the pacemaker is, where it is located, what the pulses are, where they are stored or how they are counted. Various ideas have been put forward but the model remains largely theoretical.
A complete theory of time perception will also have to explain why it is so flexible. Cocaine, amphetamine and nicotine have all been shown to speed up time perception, while some antipsychotic drugs slow it down. All interfere with the neurotransmitter dopamine. People with disorders in the dopamine system, such as those with Parkinson’s disease or schizophrenia, also suffer distortions in their perception of time.
Time can be stretched and shrunk in other ways. It seems to slow down when you are frightened and flies when you are having fun. Time seems to pass more quickly as you get older.
The key to these puzzles may be how we think about how we perceive time. “We’re under the illusion that time is just one thing, but we can take various aspects of time and manipulate them separately from the others,” says of the Baylor College of Medicine in Houston, Texas. That might mean our perception of it also has several components.
Perhaps the best illustration of the different ways we can perceive time is an experiment in which Eagleman persuaded study participants to be dropped backwards off a tall tower into a safety net 30 metres below. As they fell, they were asked to look at a wrist-worn LED display showing a number that alternated with its negative image 20 times a second. Usually this is too fast to be perceived, but if “brain time” really does slow down in scary situations they should have been able to read the number.
Although the participants reported that the fall seemed to last about 35 per cent longer than the 2.5 seconds it actually took, none of them could read the number ().
The reason time seems to slow down when you are falling, Eagleman suggests, is that intense or novel situations command our attention and cause our brains to soak up more detail. Eagleman has shown that when the brain is exposed to the same image over and over, and is then confronted with a different image, the new one seems to last longer, even though it is displayed for the same period of time. The brain also uses more energy when exposed to the new image. “The length of time something seems to have lasted appears to correlate with the amount of energy the brain uses to record an event,” Eagleman says.
“Time seems to slow down when you’re frightened and flies when you’re having fun. And it seems to pass more quickly as you get older”
This observation may also help explain why time speeds up as we get older. For children everything is new and the brain is processing vast amounts of information about the world. As we age and the brain has learned the rules of the world, it ceases to record as much information. “It is as if at the end of a summer you look back and you don’t have that much footage, so it seems to have gone by more quickly,” says Eagleman.
This might suggest that we can stretch our lifetimes by trying to pack as many different and exhilarating experiences into it as possible. The trouble is just finding the time.
Read previous article: “The body’s metronome“
Read next: “Everyday time warps“
