WHERE exactly do dreams come from? Judging by the intriguing case of a stroke victim who lost her ability to dream, they originate deep in the back of the brain, in a region responsible for vision. The study, which is the first to look in detail at a non-dreamer, raises intriguing questions about the connection between dreams, sleep and our mental abilities.
Soon after her stroke, the 73-year-old woman reported an incredibly vivid hallucination, and then had no dreams at all for three months, recovering only very limited dreaming after that. In brain scans, Claudio Bassetti and Matthias Bischof, neurologists at the University Hospital Zurich in Switzerland, found damage to both sides of a small part of the visual, or occipital, cortex (Annals of Neurology, vol 56, p 1).
But being dreamless did not seem to hamper their patient. Two weeks after the stroke, the woman showed no signs of problems with memory, attention or any other mental abilities. To Bassetti, the fact that these other functions were unaffected suggests that dreams may not have any major purpose in adulthood. 鈥淚t supports the hypothesis that dreams reflect mental activity in the brain, but don鈥檛 have a specific function of their own,鈥� he says.
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Dreams are often linked to rapid eye movement (REM) sleep. But when the researchers monitored the woman鈥檚 sleep for six weeks after the stroke, her REM sleep patterns appeared normal. This backs the idea that REM sleep and dreaming have different origins in the brain, says Bassetti.
Mark Solms, a sleep expert at the University of Cape Town in South Africa, agrees. 鈥淲e are in the embarrassing situation where we don鈥檛 yet know what the functions of dreams or REM sleep are,鈥� he says. 鈥淏ut the evidence is pointing to functional differences and different mechanisms.鈥�
He does not agree that dreaming has no function, though. He points out that although the woman鈥檚 sleep pattern was mostly normal, she did wake up more often than usual soon after periods of REM sleep. Solms speculates that this supports one of the oldest theories of dreaming, that we dream to protect our sleep. For instance, people often incorporate a ringing alarm clock into their dream, possibly deluding themselves that the alarm was not real. The woman may now lack this sleep-protection mechanism.
Bassetti thinks that the woman鈥檚 initial intense hallucination could have been caused by the stroke-affected region becoming overactive before it died off 鈥� an effect sometimes seen during damage to brain tissue. If that鈥檚 the case, it suggests that the same part of the brain is responsible for hallucinations and dreams.
Other cases in which people lose their ability to dream suggest frontal brain regions, important in motivation and emotion, are vital for dreaming. But Solms thinks the occipital region is also important in dream representation. 鈥淭hese structures light up when we produce mental images,鈥� he says.