How does the sleeping brain distinguish a mild call or touch in the night from a life-threatening attack? Scientists aren’t sure of the mechanism, but researcher Sydney Cash thinks it produces K-complexes, electrical signals long recognized as a marker of the brain’s slide into a sleeping state.
K-complexes appear on an electroencephalograph (EEG), a readout of electrical activity in the brain, as brief high-voltage peaks that occur every minute or two as brain activity quiets just before the onset of deep sleep. Although the brain produces these distinctive waves spontaneously, they also appear on the EEG tracing when the sleeper is exposed to sounds or other potential disruptors of sleep.
“There seems to be this process that can make a determination of whether a stimulus is particularly salient and requires the subject to wake up or not,” says Cash, of Massachusetts General Hospital and Harvard Medical School. “That’s what we think is happening. We haven’t proven it, but we’ve shown that the mechanisms in play could be accounting for that.”
EEG recordings, which depend on electrodes affixed to the scalp, measure K-complex signals only from the surface of the brain. But Cash and his colleagues collected data about sleep activity from deeper in the brain by attaching sensitive microelectrodes to thin clinical electrodes already being inserted into the brains of eight people to identify the origin of their epileptic seizures. Their research was published in the May 22 issue of Science.
“We supplemented the routine electrodes with specialized microelectrodes to get an even higher level of information about neural activity,” Cash says. “These microelectrodes allowed us to sample high-frequency activity at very high resolution, as well as the activity of very small populations of neurons as they fire.”
Cash and his colleagues found that K-complexes occur throughout the brain, not just at the surface, and tend to appear when the sleeper is exposed to a low-volume sound. From this they conclude that K-complexes represent the brain’s attempt to stay asleep while it evaluates a stimulus to determine if it requires waking up.
The technique of inserting microelectrodes deep in the brain to monitor sleep activity has aroused interest from other sleep researchers.
“This work adds a new perspective,” says sleep researcher Jerome M. Siegel of the Center for Sleep Research in Los Angeles. “And work on human subjects is always a valuable addition.”
But Siegel is not so sure the evidence supports the conclusion that K-complexes represent the brain’s attempt to remain asleep because the signal appears almost simultaneously with the sound or touch that threatens to disturb the subject’s sleep. This suggests that the brain doesn’t have enough time to evaluate the stimulus. “It would be nice if the authors had additional evidence that the K-complex is occurring after the brain has decided that the stimulus is innocuous,” he said. “Five hundred milliseconds is a very short time for this determination to be made.”
Manipulating K-complex activity might lead to novel sleep aids, Cash says, “but more practically, we’re thinking about how we can use these findings to develop diagnostic measures for use with a variety of sleep disorders, and to understand better the relationship between K-complex activity and many other kinds of activity, such as what we see in epilepsy.”