readings> our consciousness of time

IN HIS cult book Awakenings, the neurologist Oliver Sacks tells the story of a group of patients for whom time stopped. Suffering the aftereffects of encephalitis lethargica, a brain infection that swept the world in the 1920s, they remained immo bile and impassive in their wheelchairs for many decades until Sacks began treating them with a new drug, L-dopa. Once roused, many of the patients revealed that they had been conscious all along, but it was a frozen consciousness.

One woman described it as like living in a still pond forever reflecting itself. Her awareness of the world was bright, fixed and hard-edged-like the picture in a stained-glass window, fantastically pure but empty of possibilities. Catatonic schizophrenics and people under the influence of LSD may also feel that the flow of time has ground to a halt or become distorted. And when recalling moments of extreme alarm, such as the split second before a car crash or a bungee jump, people often talk about time "freezing' or things happening in slow motion.

Clearly our ability to sense time is the work of brain processes that sometimes go awry. Most of us take the sense for granted-but not the handful of researchers around the world who are hell bent on discovering where it comes from in the brain. 'Now it's obvious that a sense of time is something the brain must actively construct, there are questions to answer," says Russell Church, a neuroscientist at Brown University in Rhode Island.

Here's the biggest question: is our ability to keep track of time governed by a specialised clock mechanism inside the brain, or is it a by-product of more general faculties, such as memory and visual awareness?

The debate goes back years but is acquiring a new lease of life thanks to some provocative new brain-imaging studies and experiments in which researchers manipulate the brains of lab animals to speed up or slow down their sense of time. But first some basics. Putting subjects into the kind of drugged or terrified states that lead to gross distortions in time perception would be unethical as well as impractical. Instead, psychologists must rely on less extreme experiments, such as studying subtler time-keeping distortions that we don't even notice.

It's clear too that the brain has many timing skills. At one end of the spectrum is the kind of millisecond timing that the brain uses to coordinate muscle movements. Researchers are divided over whether the nerve circuits that handle this skill have a role in constructing a conscious awareness of the passing of time, but John Wearden, a psychologist at the University of Manchester, is sceptical. He thinks the same is true of our ability to discriminate between very short flashes of light or sound of different durations, a skffl which some researchers claim is accurate to a phenomenal 1 part in 10 000.

"What the brain is probably reacting to here are differences in the energy of stimulus, rather than the duration," says Wearden At the other extreme is our percepfion of the passing of hours, days, weeks and years. Here again, argues Wearden, there is no direct monitoring of time by the brain. Instead, we seem to rely on our memory of how many events filled a period. Distortions are commonplace. Psychological studies show that our memories tend to retrospectively shrink empty minutes, hours and days while magnifying action-packed ones. In other words, time might seem to drag when we're bored, but our memories record just the opposite impression.

Wearden believes this helps to explain why landmark events such as Christmas seem to come round faster each year. "As you get older, you do less, things seem more routine-so you find yourself thinking that it's only event 17 500 of the year and yet already it's Christmas. Normally, I should be on event 25 000 by now."

But the real controversy rages over the in-between range-from about a tenth of a second up to a few minutes. Over this range the brain seems to measure the passage of time directly and with astonishing accuracy. In a typical test at Wearden's laboratory, subjects would be asked to press a key after half a second had elapsed, or 0.7 seconds or some other short interval. After a little training, during which subjects would receive feedback on their performance, their average estimates were accurate to within a few per cent. What's more, this high level of accuracy is sustained whether we are estimating mere fractions of a second or tens of seconds.

This consistency is strong evidence that subjects are relying on a single internal reference clock, say experts like WeardenChemical or physical influences that speed up the ticks of this clock-or slow them down-lead us to think that moreor less-time has passed than really has.

Certainly, the idea has been responsible for some pretty wild experiments. In the 1930s, an American physiologist called Hudson Hoagland was looking after his wife, who was in bed with the flu and a high temperature. Hoagland noticed that when he left her alone for a few moments, she complained that he had been gone a long time. So being a scientist, he asked his wife to count to 60 while he measured her temperature and kept an eye on his watch. The hotter she was, the faster she would count. Hoagland suspected that the heat made her internal clock run faster. This, in turn, would make it seem as though he had been out of the room longer because, for her, more time would have ticked away.

Over the next few decades, the observation inspired a series of bizarre experiments in which volunteers sat in sweat rooms kept at a sweltering 65 'C or had heating helmets placed over their heads. "Some of the subjects collapsed," says Wearden. "You'd never be allowed to do these experiments today." But you can draw on the results. Wearden recently pooled all the data and analysed them. His conclusion: raising the brain's temperature can alter a person's sense of time by up to 20 per cent.

Whatever the detailed explanation for this effect, Wearden is convinced of the implications: "How on Earth would you predict this result if there wasn't some kind of a chemical or physical process in the brain counting time?" But the real boost for the clock theory has come from more recent experiments, which show that heating the brain is only one way to alter someone's sense of time. Wearden and others have found they can make people overestimate time intervals by first exposing them to a train of clicks. Meanwhile, at Haverford College in Pennsylvania, Marilyn Boltz and her team have achieved something similar with recordings of car horns. The researchers believe the horns work by raising subjects' stress levels.

Certain drugs, too, distort the brain's ability to keep track of time. At Brown University, Church and his colleagues have found that stimulants such as methamphetamine make rats overestimate how much time has passed. Animals trained to press a bar at fixed time intervals begin to press it about 10 per cent sooner. By contrast, a drug called haloperidol-a tranquilliser used to treat schizophrenics-delays their response. Elsewhere, researchers have found that marijuana makes monkeys underestimate the passing of time by up to 20 per cent.

Of course, such experiments reveal nothing about what the animals are actually experiencing. But it seems likely that these time distortions go unnoticed. The animals respond as if their time sense were still accurate. Church believes these observations support the idea that the brain has a mechanism for measuring short time intervals in the seconds to minutes range. The race is now on to discover what this mechanism is.

For the moment, the evidence is far from conclusive, leading some researchers to deny such a mechanism exists. But not Warren Meck. He and Sean Hinton at Duke University in North Carolina have been using functional magnetic resonance imaging to scan the brains of people as they attempt to keep track of time without counting. In one set of experiments, volunteers were asked to squeeze a ball at 11-second intervals (having earlier practised). Of course, squeezing a ball will activate parts of the brain involved in motor control and touch. But when the researchers subtracted these from the images, they found a patch of intense activity in the brain's basal ganglia-a large grey mass of nerve cells deep within the cerebral hemispheres. Is this the location of the brain's clock?

The basal ganglia are a collection of small structures linked to the brain's cortex by loop-like nerve circuits (see figure). Increasingly, neuroscientists believe these loops have a key role in coordinating the way information flows around the decision-making centres of the frontal cortex. Based on his brain scans, Meck believes that some of the loops also physically mark time for the brain. In effect, each tick of the clock is determined by the time it takes an electrical signal to flow round the loops.

Other researchers are reserving judgment until the results are published in full. But Meck himself is confident he is on the right track, mainly because the loops are connected to another structure in the midbrain, known as the substantia nigra, which seems to have a role in setting the pace of the internal clock. One of its jobs is to pump out a chemical called dopamine. A crude analogy is to think of dopamine as a lubricant that smoothes communication between nerve cells.

In the frontal cortex, dopamine seems to help us move seamlessly from one line of thought to another, or to convert our intentions to walk, sit down or reach for a cup of coffee into a smoothly executed sequence of actions. Some schizophrenics seem to be overly responsive to dopamine and have minds that slip and slide from thought to thought. By contrast, patients with Parkinson's disease, which progressively destroys the dopamine-producing cells in the basal ganglia, move jerkily and have a tendency to "freeze". And researchers now know that the sleeping sickness patients described by Sacks were suffering from a similar lack of dopamine-hence the beneficial effects of L-dopa, a chemical mimic of dopamine.

The signs are that dopamine also greases the cogs of the brain mechanism we use to measure short time intervals. Working with Church a few years ago, Meck trained rats to press a lever after a specified time in order to receive a food reward. Having learnt the correct interval, the rats were given drugs that selectively kill dopamineproducing cells. Though thev were e still able to press the lever, the rats could no longer time the interval. But the skill returned when the researchers gave the rats L-dopa.

Since then, Meck has selectively severed nerves in rats in a bid to pinpoint the parts of the brain involved in timing short intervals. In his current model, the dopamine-producing cells of the substantia nigra send out a steady stream of chemical and electrical pulses. These travel round the nerve loops marking out time, and the more dopamine there is, the faster the pulses go. Other facts seem to fit the picture. Meck believes it is no coincidence that stimulants that speed the internal clock, like methamphetamine, also boost dopamine levels. Or that Parkinson's patients who lack dopamine have problems timing short intervals. Or that dopamine is one of many chemicals pumped out by brain cells when we are frightened or alarmed, or simply asked to focus on something strange (like a train of clicks in a psychology lab).

However, Wearden thinks there are still too many gaps in our knowledge to say just how central the dopamine system is to the internal clock. Sure, things like fear, mental concentration and methamphetamine boost dopamine in the brain, but they also boost levels of other chemicals that stimulate brain cells, such as noradrenaline. Perhaps these chemicals can also influence the internal clock. What's more, rival researchers believe there is another brain structure that helps us to keep track of short intervalsthe cerebellum, best known for controlling fine movements.

Others think there is no need to invoke a localised brain clock at all. The idea of nerve loops measuring time is clunky and outdated, argues Dean Buonomano, a neuroscientist at the University of California in San Francisco. It treats time as something the brain adds after the fact rather than something already present in the basic information it receives about the world through its senses. Buonomano points out that brain cells involved in processing auditory and visual information don't just fire in response to the spatial qualities of patterns of light and sound-they also respond to temporal changes in those patterns. In the brain's visual centres, for example, many cells will only fire if an object is moving or changing in some way.

"It's silly to see temporal processing as a distinct problem to be solved by a particular brain centre, because temporal information is being represented everywhere in the brain," says Buonomano. But if this is right, why does interfering with specific parts of the brain-like the basal ganglia-alter its ability to keep track of time?

Donald Woodward, a neuroscientist at the Bowman Gray School of Medicine in Winston-Salem, North Carolina, believes the answer could be that the dopamine system is the brain's "clockwatcher" rather than the clock itself. Information needed to make time judgments is processed in many parts of the brain, but the basal ganglia enable us to draw on this information.

According to this model, the basal ganglia help the brain to focus on whatever happens to be the most critical aspect of a task. This might be the size of the gap through which it is safe to squeeze the car or the exact shade of a colour to match a set of curtains. Or it might be timing. This would ceratinly be a more orthodox explanation than the clock model of Meck and Wearden. If the brain has an internal clock, the implication is that it is capable of constructing a sense of time with no outside help. The raw information is generated within by its own "ticking" circuits. And this is at odds with main stream thinking about human perception, which emphasises instead how our abilities to recog nise and judge events depend on the processing of information flow ing into the brain. Yet this still leaves the mystery of the vivid time distortions encountered during car crashes, LSD trips or catatonic states. Here researchers can only speculate. Perhaps it is simply a question of degree.

If a person's internal clock runs 5 to 10 per cent faster or slower they probably won't feel any difference. But if the internal clock runs excessively fast, perhaps we eventually become aware of a problem and begin to feel that things are happening to us in slow motion.

Or perhaps this type of awareness has more to do with the brain losing the ability to control thoughts and translate them into actions. In normal consciousness, every moment is filled with the opportunity to focus on something. But maybe this vanishes when the frontal cortex has no dopamine to lubricate it (as is beheved to happen in a catatonic trance), or when the brain is subjected to a paralysing flood of sensation (as in a car crash or LSD trip). And when the potential to act vanishes, perhaps we simply cease to feel actively involved in each moment.

Scientists may be getting closer to knowing how the brain measures the passing of time. But they are a long way from understanding how - or even whether - the same mechanisms can explain subjective distortions in the perception of time. Not that experts like Church find that too dismaying. He is just thankful that after many years of neglect, the field of time perception seems at last to be taking off: "The study of time was considered a little dull - no one could really see what the problem was." They can now.

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