HOW WE LEARN AND REMEMBER: DON'T LOSE SLEEP OVER IT!
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HOW WE LEARN AND REMEMBER: DON'T LOSE SLEEP OVER IT!
ATLANTA, October 17, 2006 - New research reveals the importance of sleep for short-term memory and the integration of things we learn over the long term, as well as the detrimental impact of interrupted sleep on learning and memory. It also gives clues to how cycles of sleep and wakefulness are regulated in the brain.
Science has long pondered the reasons behind why we sleep. Findings from new studies point to the possibility that sleep plays an active and important role in both strengthening and integrating memories laid down during the day.
In experiments conducted by Jessica Payne, PhD, and colleagues at Harvard University Medical School, two groups of people were asked to study eight lists of 12 related words, for example, thread, pin, eye, sewing, sharp, point, haystack, etc. Twelve hours later, they were asked to recall as many words as they could. Using this classic test, scientists can study the consolidation of learning and formation of associations through the subjects' false recall of a specific related word that was not on the original list -- in this case, needle -- known as a "critical lure."
In this experiment, one group studied the lists at 9 a.m. and returned for recall testing at 9 p.m. The other studied the lists at 9 p.m. and returned for testing the next morning at 9.
The researchers report three important findings: The subjects in the group that had slept between study and recall testing accurately recalled more words from the lists; they also showed more false recall, identifying as many of the critical lure words after a night of sleep as they did just 20 minutes after studying the lists. Most significantly, the scientists found that those who had slept recalled two to three times as many additional words -- ones that were neither on the original list nor considered lure words -- that were judged to be novel and creative. Many of these appeared to be crosses between two or more lists.
While sleep tempers the decay of detail memory, it completely abolishes the decline in gist memory, or memory of general ideas, normally seen across the day, says Payne. "Thus, sleep, while good for memory in general, has a special benefit for memory of gist. More intriguing, we found that sleep enhances creative associations between studied words, suggesting that sleep changes memories in a way that encourages the discovery of new and meaningful connections."
But sleep apnea, narcolepsy, insomnia, and other conditions can prevent us from getting a good night's sleep. And subsequently they can keep people from staying awake and alert during the day.
Even if one doesn't forgo an entire night of sleep, interruptions reduce the restorative benefits of sleep and interfere with normal cognition. Sleepiness increases the likelihood of accidents and can also impact decision making and memory. In an effort to learn more about this phenomenon in animals, Robert Strecker, PhD, and colleagues at Harvard Medical School developed a rat model of sleep apnea, a sleep disorder that is characterized by brief interruptions of breathing during sleep and affects more than 18 million American adults, according to the National Sleep Foundation.
In the experiments conducted by Strecker's team, the movement of an automated treadmill woke up the rats every two minutes over a period of 24 hours. Then, using a series of water maze tests, the researchers tested the rats' learning and memory of objects and where they were in their environment.
In the water maze task, a platform is placed in a tank of water under the water surface. The platform is hidden by making the water opaque, and rats are placed in the tank and allowed to swim around until they find the hidden platform. Over successive trials, the rats find the platform faster because they learn where the hidden platform is located in the tank. They use cues in the room such as furniture and posters to help them distinguish the location of the platform.
Strecker and his colleagues found that rats whose sleep had been interrupted took longer to learn the hidden platform location than controls. One day later, their memory of the location of the platform was still impaired, even though they had been allowed to sleep undisturbed after the first learning session. This suggests that sleep fragmentation affects a region of the brain known as the hippocampus, responsible for storing spatial maps in the brain, they report. In a parallel study, Strecker notes, they found that long term portentiation -- LTP, a hippocampal neural signal that is needed for memory formation -- was absent in rats exposed to sleep fragmentation.
In an additional experiment, the team tested the rats' spatial working memory, the equivalent of short term memory in humans, using a different version of the water maze. The task consisted of two pairs of trials in the water tank. In the first trial, a flag was placed on the platform so that the rat could see where it was located. After the rat climbed on the platform, it was removed from the tank. The flag was removed from the platform, hiding it, and the rat was placed back in the water tank, where it had to find the now hidden platform. Then the platform was moved to a new location in the tank, and the trails started over again. Therefore, the rat had to forget any previous platform locations and only remember the last location that was cued by a flag.
The team found that disturbed sleep prior to this task did not impair the rats' ability to perform the spatial working memory task. "At this time," Strecker notes, "it is not known why sleep fragmentation causes deficits in spatial learning and memory but not spatial working memory." Their subsequent research will attempt to identify areas of the brain and changes in the brain's chemistry that correlate with these impairments, findings that may one day lead to treatments to help people with sleep disorders recover memory function, according to Strecker.
Ongoing research conducted by Clifford Saper, MD, PhD, of Harvard Medical School, aims to build on significant advances over the past decade in our understanding of the mechanisms by which the brain controls when we are awake and when we are asleep, and how the brain relays information from an animal's environment, including the amount of daylight, to regulate the sleep-wake cycle.
Saper, who will present a special lecture at Neuroscience 2006, characterizes the brain's control of sleep and wakefulness as a switch centered in the hypothalamus, a region at the base of the forebrain that coordinates hormones, behavior, and basic functions like eating, drinking, sleeping, and sexual behavior.
Nerve cells in the arousal system use acetylcholine and monoamines, including dopamine, norepinephrine, serotonin, and histamine, as well as orexin, all neurotransmitters that help to keep the brain awake. Cells in a part of the brain known as the ventrolateral preoptic nucleus (VLPO) inhibit the activity of all these neurotransmitters. And that's what causes sleep.
"Not only does the VLPO turn off the arousal systems," says Saper, "but the monoamine arousal systems also turn off the VLPO. This kind of relationship -- where two sides of a switch are mutually inhibitory -- is called by electrical engineers a "flip-flop" switch. Because each side turns the other off, the transitions are usually very quick. Engineers therefore design a flip-flop switch into a circuit when it requires rapid and complete transitions. This is what happens with sleep, where individuals "fall asleep" in a matter of seconds or minutes, and often wake up abruptly."
But while the mechanism may be consistent for all animals, some are awake during the day and others are active at night. Saper's experiments with rats show that, though normally nocturnal, they will become daytime creatures if they are only fed when it is light. He surmises that the change from nighttime to daytime activity, which is accompanied by changes in body rhythms such as temperature and daily hormonal cycles, can be attributed to nerve cell activity in the dorsomedial nucleus of the hypothalamus. This area gets inputs related to food availability, external temperature, and other influences and regulates a creature's daily cycles by contacting the VLPO and orexin neurons, among others.