Nov. 29, 2011 at 8:22 AM ET
Ever since the dawn of humanity, people have wondered about the purpose of dreams. We’ve imbued these mental meanderings with all sorts of powers, from forecasting the future to providing a window into the soul.
But scientists say they now know what dreams are for: they sooth the sting out of troubling memories. And when dreams don’t do their job, horrific memories can take over a person’s life, as they do with PTSD, a new study suggests.
Matthew Walker and colleagues at the University of California, Berkeley, found that the brain uses dreams to strip the emotional content from memories of painful events.
Here’s how the researchers think it works. During dream, or REM, sleep, our brain chemistry changes, leaving us with lower levels of stress hormones. While we’re in this quieter state, the brain mulls over what happened and then files away the memory – but with less emotion attached.
So, when everything works right, when we later recall these events we’ll remember what happened, but less of the pain associated with them.
Walker and his colleagues tested their theory in an intriguing, but simple, experiment.
The researchers asked 35 healthy volunteers to lie in a brain scanner while looking through a series of 150 images, which ranged from bland to emotionally jarring. One image might show a tea kettle, for example, while another might show the aftermath of a horrific car accident. As they were looking at the images, the volunteers were asked to rate the emotional intensity of what they were viewing.
Half the volunteers looked at the images in the morning, while the other half looked at the images just before bedtime. Twelve hours later the volunteers were asked once again to look at and to rate the images while being scanned. This meant that half the volunteers had a night’s sleep in between scans.
What the second set of scans and ratings showed was telling. Volunteers who had slept through the night rated the horrific photos as far less emotionally charged – and their brain scans showed a much lower level of activity in the amygdala, a brain region central to emotional processing.
It’s like dreams become the brain’s psychotherapist, Walker explains. Just as we can benefit from reviewing disturbing events in the safety of a therapist’s office, our brain benefits from processing these same types of events in the quieter dream state.
Walker suspects that the system short-circuits in PTSD sufferers because their brains are constantly charged up even during dreams. And research in veterans with PTSD appears to bear this out, Walker says. When veterans with PTSD are given medications that knock back a neurotransmitter that keeps the brain in an excited state, sleep appears to improve symptoms of the disorder.
“We’re hoping to provide the mechanism by which that drug has its effect,” Walker says.
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