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Thursday, Nov. 1, 2001
These dreams are made of . . . what?
By ROWAN HOOPER
Ever had a sleepless night before an exam, cramming in the things you didn't learn in time? Even after 40 hours without sleep, it is still possible to disgorge crammed information. But remember those facts a week later? Forget it.
Ever spent so long playing a video game that residual images of the game permeate your dreams?
Ever heard your cat mew in its sleep and wondered if it is having a troubled sleep? Are animals intelligent? Do sheep, for example, dream? Probably not much: Sheep have only half an hour of REM sleep per night.
These gems are from this week's issue of Science, which has a special section on sleep and dreams. It's a safe bet that as soon as humans evolved consciousness, they started wondering about dreams. They seem to reflect memories and things associated with memories, and are often warped in bizarre ways (and we all know how the brain is untroubled by inconsistency or impossibility during a dream). They are also highly emotional, and unpredictable. But we don't have much idea of why we dream.
According to the three reviews in Science, there is still no consensus on the function of sleep, and dreaming is even harder to understand.
The most widely accepted theory attempting to explain sleep and dreaming is that they are the brain's way of reprocessing information "off-line." The idea is that when a person is awake ("on-line"), the brain is busy processing sensory stimuli; we need downtime to consolidate and to file memories.
But there have been few scientifically rigorous studies on dreaming. Much research, says Jerome Siegel, of the Brain Research Institute, University of California, is compromised by poor experimental design. In one type of experiment, for example, aimed at testing whether deep sleep is needed for learning, a rat is confined to a small platform surrounded by water.
When it falls completely asleep, the rat also falls off the platform and into the water. Rats that are sleep-deprived like this are not so good at remembering tasks learned the day before, as rats that are allowed to have deep sleep. But obviously the physical stress of the experience might affect the rat in ways other than the tiredness caused by sleep deprivation.
Robert Stickgold and colleagues at the Laboratory of Neurophysiology at Harvard Medical School, Boston, review the evidence that changes in brain activity during sleep account for the weird nature of dreams. The big breakthrough -- and the start of modern research into sleep -- came in 1953, when rapid-eye-movement sleep was discovered.
It is during REM sleep, toward the end of the sleep phase, that the most intense dreams occur. It's why the residue of dreams is sometimes hard to shed in the morning.
Stickgold and researchers found that during REM sleep, the parts of the brain that are associated with attention-holding and "episodic" memories (i.e., memories of specific events) become less active. The hippocampus, which controls the part of the brain concerned with episodic function, is effectively off-line, requiring dreams to be constructed from what's left on-line: the limbic forebrain and the amygdala -- regions important in mediating emotion. Thus, say Stickgold and coworkers, dreams are formed using weak memory traces from regions of the brain associated with emotion. Hence the bizarre and emotional content of dreams.
The team have also attempted to trace the source of dreams -- by having people play the videogame Tetris and the downhill ski simulator Alpine Racer II.
Test subjects were then woken at different stages of sleep. At the beginning of sleep, when "hypnagogic" dreams occur, images of Tetris or of skiing were reported in 89 percent of subjects. Better than rolling off a ledge into water, but it is not as straightforward as the figure suggests: Sometimes subjects reported images from previous (obviously memorable) games of Tetris; Alpine Racers reported images from actual skiing.
In the second paper in Science, Pierre Maquet of the Wellcome Department of Cognitive Neurology, University College London, finds that neurons that are activated during learning are reactivated during sleep, suggesting that the brain is processing newly learned information (the attention-holding part of the brain is apparently not needed for this). By this rationale, as more is learned, REM sleep should increase.
However, Siegel reports in the third paper that there is no relation between the amount of REM sleep and learned information. His approach is to compare the amount of REM sleep across mammals.
"Contrary to what might be expected, humans do not exhibit unusually high amounts of REM sleep," said Siegel. The platypus -- an egg-laying mammal -- is not considered the most intelligent of mammals, yet of its 14 hour total sleep time, eight hours are REM sleep. This compares to two hours REM out of a total of eight hours sleep for humans. Some whales and dolphins have almost no REM sleep, while the hedgehog spends 3 1/2 hours per sleep cycle in dream land.
The British polymath David Hartley wrote, in 1791, that dreams function to strengthen memories. Although initially popular, Hartley's ideas are now largely unknown. Compare this to Freud, who has had a huge influence on psychology. Despite writing before Darwin was born, Hartley was trying to answer a functional, evolutionary question: Why do we dream?
The question touches upon the biggest unsolved mystery in biology, perhaps in all science: consciousness. There is still a long way to go before the mystery is solved. The brain is so complex that perhaps consciousness will only be understood by mimicking the brain in a computer. The next question will then be, as Philip K. Dick famously asked, "Do Androids Dream of Electric Sheep?"
E-mail Rowan Hooper at rowan@ japantimes.co.jp