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The key to the brain's ability to make such good use of nighttime downtime is something it shares with your computer: the capacity to run multiple programs at once--to go on wrestling with a problem even when you've turned your conscious attention elsewhere. The aha moment you experience when you've been trying to remember the name of a song and three hours later it hits you is no accident. "Conscious awareness is able to focus on only one thing at a time," says Barrett, "but problems go on getting processed under the radar."
Sleeping doubles down on this phenomenon. The traffic-cop role the prefrontal cortex performs when we're awake does more than just keep the brain focused on a particular conscious task. It also screens out thoughts that it decides you oughtn't think at all. The forbidden concepts aren't just things that are socially inappropriate--though those are on the list--but also those deemed rationally inappropriate. In sleep, that brake on your imagination comes off, which is what happened in the German mathematical study.
At the same time the prefrontal censor is dialing itself down, the brain's visual centers, in the occipital lobe at the back of the head, are dialing up. The hallucinogenic quality of dreams is a result of the visual centers' mixing images at will. That's usually just chaff, but not always. One night in 1816, Mary Shelley dreamed of a man assembled from bits beyond the grave--and went on to write Frankenstein.
Just as important as which regions are working is how they communicate. We think of the left hemisphere as the rational, mathematical region and the right as the creative, more bohemian one, and that's a fair division. But a new study conducted by neuroscientist Lisa Aziz-Zadeh of the University of Southern California found that the brain is not quite so bifurcated.
When architecture students undergoing functional magnetic resonance imaging (fMRI) brain scans were asked to perform a visual-spatial task--arranging geometric shapes in their heads to see if they could be assembled into a square or a triangle--the right, artistic hemisphere carried the load. When they were given a slightly more creative task--arranging a circle, a C and an 8 in various ways to form a face--the right hemisphere called on the assistance of the left. "The specific regions that are active during the creative process largely depend on the kind of task the person is engaged in," says Aziz-Zadeh.
A 2008 study by the University of Rome found that something similar goes on during sleep. With the help of EEGs, the investigators tracked communication between hemispheres when subjects were awake, in NREM sleep and in REM. In the waking and NREM states, information traveled mainly from left to right, consistent with the idea that the left brain controls and constrains the right. During REM sleep, however, there is no preferred direction. The creative right can thus come out of the shadow of the literal left.