A brief history of time?

It's all in your mind



New research has
revealed that as we
grow older, time
appears to pass
more quickly.

How long ago did Elizabeth Hurley burst into the public consciousness with that dress? Two years ago? Perhaps three? She wore it to the premiere of Four Weddings and a Funeral in May 1994.

The chances are that you, like many people I asked, guessed 1995 or 1996. What this shows is that our perception of time distorts, as we get older. While toddlers must endure an eternity between birthdays, university acquaintances can meet a decade later without realising how many years have passed.

Where does our sense of time come from? Scientists have postulated that everyone has a cerebral pendulum, whose rhythm can change depending on brain activity. Dr Warren Meek and Matthew Matell, at Duke University in North Carolina, are working on a model of how such a clock might work. It is obvious, says Mr Matell, an experimental psychologist, that we can all keep time. "If you are playing a musical instrument, you don't need to look at your watch," he says. "You can tap out a rhythm without the prompting of an external cue."

Experiments with trained rats also show they can keep time, and their clock speeds up if the animals are given amphetamines or cocaine. "We assume it's the same for humans when they are taking amphetamines," says Mr Mattel. "If they are asked to time one minute, only a few seconds will pass before they say the time is up. That suggests that something in the brain is guiding our perception of time."

Dopamine, a brain chemical, is the most likely explanation. Increased dopamine levels cause the pendulum to swing faster. while low dopamine levels slow it down. The level of this chemical dwindles with age, so the pendulum swings slower.

Mr Matell explains: "If you ask an older person to mentally time an hour, they will all overestimate it. To them, an hour feels like 40 minutes because their clock is running slower. That's why they think time passes so quickly."

Several months ago, Dr Peter Mangan, a psychologist in Virginia, showed that age altered the perception of time. He asked three groups of people of varying ages to estimate, by counting, when three minutes had elapsed. Subjects in their twenties overshot by only three seconds. Participants aged over 60 said time was up after three minutes and 40 seconds. The scientists from Duke. who are planning to publish within the next few months, think that the clock is reset by brain activity every time we perform a task, and dopamine acts as the starting gun.

Even putting the kettle on will trigger a timing mechanism. The action of filling the kettle and switching it on causes bunches of neurons (brain cells) in the cortex, which lies just under the skull, to start firing.

This haphazard pattern of firing, says Mr Matell, is "heard" by spiny neurons in the forebrain. The spiny neurons, which are kicked into action by a rush of dopamine, collect and integrate the signals to recognise that a cup of tea is being made, and tell the mental clock to start ticking.

This silent, internal timepiece allows us to estimate, without referring to a clock, when we should go back into the kitchen to get the mugs and biscuits ready.

It also alerts us if the kettle hasn't boiled after, say, two minutes, which prompts a worried check to see if it has been switched on. We don't even have to be actively timing the kettle - we will suddenly realise that it hasn't started whistling. Another example is waiting at a traffic light. When the light goes red, the clock starts ticking. Most drivers do not wait for an amber light before engaging gear - the internal clock, honed by past experience, dictates when a driver should think about preparing to pull away. So seeing the red light starts the clock; the clock nudges us to change into gear.

The process is a loop - an action prompts the clock to be started, and then, after a time interval, we are prompted to perform another action, say checking whether the kettle is on. This loop can be seen in brain imaging experiments in both animals and people. The relevant bits of the brain, the cortex and the place where dopamine is produced, are activated when people are asked to judge time intervals.

The clock may also be involved in unusual cases where life appears to operate in slow motion. Accident victims often report this - damage to the relevant brain area, causing a rush of dopamine, may be to blame.

Patients with Parkinson's disease find it difficult to perceive time accurately - the disease is characterised by a lack of dopamine. Schizophrenia and dyslexia have also been associated with a skewed sense of time.

Human beings are not the only animals interested in keeping time. "Fish and birds can also measure time, showing it is a very ancient trait and that it must be fundamental to survival," says Dr Melissa Bateson, a colleague at Duke University.

Dr Bateson, who has studied starlings, says that being able to track time is probably important for associative learning. "They know that if they find a worm every other minute it's a good place to feed," Dr Bateson says. "That means the starlings have some way of measuring the rate at which they find food. If they find one worm every two hours, they are obviously not going to associate that particular field with food."

The clock changes when we are doing something. Dr Mangan found that every age group asked to judge a time interval while doing a task overestimated wildly. The over-sixties were the most surprised - they thought three minutes was up when, in fact, nearly five minutes had elapsed.

Mr Matell says: "When people are enjoying themselves, they dont pay attention to their internal clock." This is scientific proof that time flies when one is having fun.




19 April 2008