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In the human brain, cells talk to one another through the routine exchange of electrical signals. But when people fall into a deep sleep, the higher regions of the brain - regions that during waking hours are a bustling grid of neural dialogue - apparently lose their ability to communicate effectively, causing consciousness to fade.

Writing today (Sept. 30) in the journal Science, a team of researchers led by University of Wisconsin-Madison professor of psychiatry Giulio Tononi reports that the fading of consciousness during dreamless sleep seems to occur as the different regions of the cerebral cortex that mediate perception, thought and action become functionally disconnected.

Tononi and his team observed the disconnect when brief, magnetically generated pulses of electricity were directed to specific regions of the brain. The pulses stimulated an electrochemical response from the targeted cells, which, when the subject was awake, rippled across the brain, traveling along networks of nerve fibers to different cerebral destinations. But when the subject was in deep sleep, the same response was quickly extinguished and did not travel beyond the stimulated cells.

When consciousness fades, according to Tononi, "the brain breaks down into little islands that can't talk to one another."

The new findings are important because they provide the first direct clues about how the brain alters the state of consciousness during sleep. Consciousness is a scientifically murky realm as little research has been conducted on how the brain sustains and alters the various states of mind. Tononi, one of the few scientists exploring the frontiers of consciousness, has theorized that conscious thought depends on the brain's ability to integrate information.

"Sleep is the most familiar alteration of consciousness," he says. "It happens every night to all of us. Every night, when you fall into deep sleep, your consciousness usually fades."

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Source:Vanderbilt University Medical Center

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