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London

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 immobile 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 hellbent 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 skill 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 perception 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—or
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 Wearden. Chemical or physical
influences that speed up the ticks of this clock—or slow them
down—lead us to think that more—or 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.

Squeezing balls

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 dopamine-producing cells. Though they were 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 intervals— the 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 “clock-watcher” 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 mainstream thinking about human perception, which emphasises instead
how our abilities to recognise and judge events depend on the processing of
information flowing 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.

Slow motion

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 believed 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.