Wellcome Department of Cognitive Neurology

Nature

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THE FIRST study of what goes on in our heads during dreaming has been completed, casting light on a mystery that intrigued and baffled humans long before even the ancient Greeks suspected that dreams were messages from the gods.

The study suggests that the emotional intricacy of dreams could be caused by the revving up of circuitry deep within the brain. Meanwhile, other parts are damped down - circuitry that would normally allow people to realise they were only dreaming. This could explain why they seem so very real.

The research, reported in the journal Nature by neurologist Dr Pierre Maquet and colleagues at the Cyclotron Research Centre, University of Liège, Belgium, opens up a significant new chapter in efforts to understand what we do with a third of our lives.

"This is the first study of its kind, and the first to offer a picture of brain activity during dreaming," commented Dr Chiara Portas of the Wellcome Department of Cognitive Neurology, Institute of Neurology, London, adding that the scans support the idea that dreams are formed by drawing on a well of emotional memories.

Scientists have scant idea of why we sleep, and find dreaming an even bigger mystery. Theories range from the brain clearing its memory of junk to the liberation of suppressed subconscious urges.

The reason for their ignorance lies in the astonishing design of the brain. The most complex known object in the universe, it contains as many nerve cells - neurons - as there are stars in the Milky Way; about 100 billion of them. Each communicates with thousands of its neighbours, generating an unimaginable chatter.

Before the discovery of rapid-eye-movement sleep in 1953, people believed this chatter ceased during sleep, like an office at the end of the working day. Scientists now know that activity declines by up to 40 per cent during the deepest ("slow wave") sleep, but for four or five relatively lengthy periods every night the brain is as active as it is while we are awake.

He allowed 30 young men to try to sleep for two trial nights, a difficult feat given they were pinned in place by a special face mask

Even when the sleeping brain is fully switched on, however, the body is switched off during so-called rapid-eye-movement sleep - REM; the skeletal muscles are paralysed and nothing moves except the eyeballs, which swivel and rotate behind their closed lids. So instead of getting up and moving around as the brain instructs, the sleeper can dream.

The challenge was to persuade his subjects to sleep in the scanner. First Dr Maquet used a relatively quiet type of scanner technique, called positron emission tomography (PET). Then he allowed 30 young men to try to sleep for two trial nights, a difficult feat given they were pinned in place by a special face mask, and had one arm strapped down so that a radioactive isotope could be introduced through a tube.

Nineteen were selected for the study. For the scan, the subjects were injected with an isotope of oxygen. An array of detectors around the head picked up gamma rays emitted by the isotope and a computer transformed the information into a colour-coded image. In this way, and thanks to a statistical analysis method developed at the Institute of Neurology, he could highlight which parts of the brain were the most hungry for oxygen and thus the most active during sleep.

Dr Maquet could confirm whether they were dreaming by monitoring the electrical activity of the brain via electrodes placed on the scalp. The output traces of deep sleep and dreaming sleep are quite different. On awakening the subjects, they were also asked if they had been dreaming. All had. The PET scans revealed regions that were more active, more so than during wakefulness. Further research will disentangle which areas of brain activity were caused by REM sleep and which by dreaming.

We may even look back and be appalled that we could have dreamt ridiculous, bizarre or disturbing things

However, intriguing results have already emerged. There was, for example, activity at the top of the spine in a region called the brainstem. This complements studies of humans and higher mammals which found that cells in the brainstem fire intensely during REM sleep and are quiet during deep sleep. There was also activity in the left thalamus, a region that receives the sleep signal from the brainstem. For Dr Maquet, the most fascinating find is that the men showed an activation in two structures called the amygdalas, almond-shaped regions that play a role in the formation and consolidation of memories of emotional experiences. Thus it seems that REM sleep contributes to memory processing, perhaps to consolidate recollections of events.

"That was completely unexpected," he said. "The amygdala has received less attention from sleep researchers than regions such as the brainstem."

The study also underlines one of the puzzles of dreaming. It is only when we wake up that we realise we have been deceived by our minds. We may even look back and be appalled that we could have dreamt ridiculous, bizarre or disturbing things: "Why didn't I realise that the hobgoblin was a fiction?"; "But I can't walk on water"; or, "'I should have known it was a dream."'

Among the hubbub within the sleeping brain, Dr Maquet found reduced activity in an important region called the prefrontal cortex. Occupying almost a third of the entire cortex, it has evolved in humans to greater complexity and size than in any other organism, underlying its key role in consciousness.

The prefrontal cortex lies near the front of the brain and is where the planning of behaviour and self-awareness reside. By dampening activity in this region, a person might not realise that impossible or bizarre events in a dream are unreal. This may also account for distortions in the dreamer's perception of time, the inability to reflect on their plight, and the forgetfulness that often follows waking.

To sleep, perchance to experience amygdalocortical activation and prefrontal deactivation? This work may spoil some of the poetry of dreams but it does at last allow us to peer inside the twilight world of the mind.

31 August 1996: Dream On
11 April 1996: How to tell if you have really had a brainwave

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