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There are dozens of different memory systems, says Terry Sejnowski, that uses different mechanisms and different parts of the brain.
In fact, its role in making memories may explain why sleep is so popular—a necessity really—throughout the animal kingdom. Nearly all creatures great and small sleep to some extent, suggesting that evolution found it valuable for survival—despite the apparent risk of spending so much time in a defenseless and vulnerable condition.
But that doesn't mean memory was sleep's original purpose, says HHMI investigator Terrence J. Sejnowski.
"Asking what sleep is for is a little bit like asking what blood is for," he says. "Maybe blood originally was there to get oxygen to different parts of the body, but once you have this big conduit running through your body you can put all kinds of things in it." From delivering drugs to disposing of wastes, blood performs dozens of valuable tasks.
"Same thing with sleep," Sejnowski says. "It may be that it started out as some sort of way to recharge your energy supplies." But during the time that nerve cells don't have to worry about their day job, they can engage in other interesting biochemistry, free from the interference of wakeful activity.
Pursuing the secrets of that nighttime biochemistry has led Sejnowski and other scientists to a new and deeper understanding of sleep, which ultimately could benefit the many victims of sleep-related disorders.
More than 50 million Americans suffer from conditions ranging from insomnia to restless legs syndrome to narcolepsy. In some people, sleep apnea interrupts breathing and deprives the body of oxygen, fueling metabolic problems that contribute to obesity and diabetes.
"That translates into an enormous impact on quality of life," Sejnowski says.
Work in his lab, at the Salk Institute for Biological Studies in La Jolla,
California, led by graduate student Philip Low, has produced a
computer-based system for analyzing sleeptime electrical activity in the
cerebral cortex, the brain's wrinkled outer layer. That activity is recorded
via a single pair of electrodes attached to the scalp to produce
electroencephalogram (EEG) readouts.
Similar analyses for assessing sleep problems are performed at sleep clinics, where patients snooze while wearing a cap covered by a dozen or more electrodes. Human experts then study the EEG graphs to identify how long the sleeper has spent in sleep's various stages. But these analysts typically take hours or days to decipher the EEG recordings. By contrast, the computer system developed in Sejnowski's lab can report on sleep state in real time, with an accuracy rate that is as good as or better than the humans' analyses. (Sejnowski is on the scientific advisory board of a new company, NeuroVigil, that will use the system to offer sleep EEG analysis over the Internet to the nation's 2,000 or so sleep clinics.)
Photo: Bruce Weller