To wake or not to wake?
New evidence provides clues about the role of a key sleep-related brain activity pattern in the brain: this waveform may help keep the mind asleep through nonthreatening disturbances, rather than wake it up as previous studies have suggested, a paper in this week's Science reports.
Image: Wikipedia The brain pattern in question, called the K-complex (KC), is the largest characterized neurological event in the healthy human brain. Common throughout certain sleep stages, the KC is defined by a br
New evidence provides clues about the role of a key sleep-related brain activity pattern in the brain: this waveform may help keep the mind asleep through nonthreatening disturbances, rather than wake it up as previous studies have suggested, a paper in this week's Science
The brain pattern in question, called the K-complex (KC), is the largest characterized neurological event in the healthy human brain. Common throughout certain sleep stages, the KC is defined by a brief, high-amplitude waveform followed by a longer, voltage-negative peak. KCs were first described in the1930s, less than 20 years after the first human studies employing electroencephalography (EEG), a method for recording the electrical activity of the brain along the scalp. KCs, researchers found, could be elicited by a soft noise, such as a knock on the door of a sleeping subject. This raised the question of whether KCs participate in rousing the sleeping brain or maintaining sleep through nonthreatening stimuli.
"There's been a lot of debate about what [KCs] mean," said linkurl:Ian M. Colrain,;http://www.sri.com/policy/healthsci/humansleep/colrain.html director of SRI International's Human Sleep Research Laboratory in Menlo Park, CA, who was not involved with the research. This study, he said, suggests KCs are "more consistent with a sleep preservation mechanism than an arousal mechanism."
"Because they've been able to indicate in some more detail what's happening in the brain during these [KCs], we can now delve into the mechanism underlying the phenomenon," he added.
The researchers, led by neurophysiologist linkurl:Sydney Cash;http://www.massgeneral.org/children/research/researchers/sydney_cash.aspx of Massachusetts General Hospital, investigated KCs by measuring brain activity in epilepsy patients who were undergoing surgery. Cash and his colleagues used electrodes placed on the brain's surface as well as microelectrode arrays that penetrated the cortex to obtain recordings of neuronal activity that were "much more spatially precise and temporally precise than you can do with an extracranial scalp EEG," said Cash.
While the subjects were asleep, the researchers elicited KCs by playing a recording of a short, soft beep. They observed KCs in all regions of the patients' cortex, and found that they represented localized areas of decreased brain activity. This suggests that KCs are isolated cortical "down-states," periods of neuronal silence well-characterized in animals, said Cash.
"This raises the possibility that it is a mechanism by the brain to try to stay asleep," he said. Exactly how the brain processes a stimulus and declares it irrelevant is still unclear, he explained, but he speculates that KCs might be the mind's indication that there is no need to wake up.
One hypothesis about why animals sleep is that the down-states that occur during sleep aid in consolidating memories and stabilizing neural connections. In light of this idea, the fact that KCs are states of reduced brain activity suggests that they are involved in this consolidation process, Colrain said.
KCs occur predominantly during stage 2 sleep -- a lighter state of sleep than REM sleep -- which takes up about 50% of the night. Until now, this stage "has essentially been ignored in the functional significance of sleep," Colrain said. The finding that KCs are indeed down-states and occur throughout the cortex may "explain why we have so much of it."
Researchers are just "starting to piece together some of the puzzle about what's happening in the cortex during this event," Cash said. "This is a stepping stone toward better understanding of sleep itself."
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[ April 2009]*linkurl: The Gears of the Sleep Clock;http://www.the-scientist.com/2009/4/1/42/1/
[April 2009]*linkurl: Unlocking the clock;http://www.the-scientist.com/article/display/54665/