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You snooze, you learn? | (UPI) -- Studies suggesting rats dream about tasks they have learned could shed light on how humans form memories, researchers reported Wednesday.

The investigators from the Massachusetts Institute of Technology in Cambridge found rodents can retain and recall long stretches of trained behavior in complex dreams throughout the sleep cycle. The conclusions come from tests monitoring the electrical activity of nerve cells, or neurons, in the brain's learning and memory center. The neurons' activation pattern corresponds to the activity being measured, in this case, a rat running laps around a track to get a food reward.

The scientists then saw an edited version of the signature pattern matching the tour around the race course replayed by the rodent's brain while the animal was asleep. This suggested memory processing mechanisms were at work.

At least one researcher cautioned against reading too much into the results, however.

"Although I believe the authors present a true phenomenon, it is unclear whether this has truly something to do with memory consolidation," Paul Franken, of the Department of Biological Sciences at Stanford University in Palo Alto, Calif., told United Press International. "Important questions on sleep function are being addressed, and strong new techniques are developed, but for now we have to be patient for more definitive answers."

Drawing parallels between the mechanisms underlying both species' brain activity during sleep, learning and memory, the study authors expressed confidence the results of the rat experiments could have human implications.

"Sleep disorders, disturbance and deprivation are commonplace in today's society," said Matthew Wilson, associate professor of brain and cognitive sciences at MIT's Picower Center for Learning and Memory, who led the research. "We're only beginning to understand the impact that sleep has on normal memory and cognitive function and how it might interact with other neurological and psychological conditions."

The findings might offer clues to the role of slumber in the normal acquisition of knowledge and memory, insights necessary for understanding the consequences of sleep disruption, the investigators told UPI.

"This work provides direct insight into the way in which memory is processed during different stages of sleep and therefore provides a model for the study of the complex relationship between sleep and awake cognition and memory," Wilson said.

The investigators found during a predominant stage of sleep -- which in humans accounts for some six of the eight hours of shut-eye required by the average person -- the rodent brain replayed an edited recording of the electrical neuron response to the rat race or other activity.

The research follows up on Wilson and company's landmark findings last year that animals can remember extended sequences of events during rapid eye movement sleep. This fifth and last stage of slumber, in the hours just before awakening, is marked by extensive physiological changes, including accelerated respiration and increased brain activity, eye movement and muscle relaxation, as well as dreaming. The discovery in 1953 of this sleep state, in which the brain buzzes with activity, awakened avid interest among researchers who previously had yawned at the topic.

In the work reported in the Dec. 19 issue of the journal Neuron the scientists focused on slow-wave sleep, also called "non-REM" in scientific parlance or, in misleading vernacular, "dreamless." Comprised of four stages, non-REM slumber accounts for some 75 percent of the repeating adult sleep cycle. It encompasses initial drowsiness as activity plummets and eyelids droop, a period of light snoozing as the heart rate slows and body temperature drops in preparation for deep dormancy and the onset of REM sleep.

REM and non-REM dreams differ markedly, the latter consisting of brief, fragmentary impressions, devoid of visual images -- dreams that frequently are forgotten.

"(The findings show) that slow-wave sleep (or deep sleep in humans) may be important in consolidating memory," Dr. Clete Kushida, of Stanford's Sleep Disorders Clinic and Research Center, told UPI.

In the experiments, Wilson, along with brain and cognitive sciences graduate student Albert Lee, monitored nerve cell action, first as rats ran laps around a track and later as they slept. They found the neurons fire according to the animal's activity. As the rodents napped immediately after the drill, Wilson and Lee observed an edited version of the recorded pattern of firings was replayed during non-REM sleep.

"We found that brief segments of awake sequential experience were replayed in the hippocampus (the brain center for learning and memory) at high speed during slow-wave sleep, following awake behavior revealing the early processing of sequential event memory during this sleep period," Wilson explained. "This may relate to work in humans that suggests the amount of slow-wave sleep early in the night -- as well as the amount of REM sleep later in the night -- is correlated with subsequent enhancement of performance on learned tasks."

The experiments followed up on work that had shown replays of maze running or other activities during REM sleep.

"This time, we were looking at slow-wave sleep and asking whether memory patterns related to rats running back and forth on tracks were also reactivated during this other sleep period," Wilson said.

In contrast to REM sleep, during which events were replayed in "real" time, non-REM sleep produced a fast-forwarded version of the actual activity, the researchers pointed out.

"We found that brief memory sequences corresponding to running single laps on the track were replayed in short bursts at high speed. A 4-second lap on the track replayed in 100 to 200 milliseconds (during non-REM sleep)," Wilson explained. "This was unlike the earlier REM replay, which lasted several minutes and was played back in approximately real time."

They noted another difference in timing.

"Unlike the REM sleep replay, the slow-wave sleep replay only seemed to occur during the period of sleep immediately following the behavior and was not detectable 24 hours later," Wilson noted. "(This suggests) that it was part of the initial storage or processing of (recent) memory during sleep, while REM memory reactivation, which was robust even after 24 hours, might represent the more gradual re-evaluation of older memories."

Humans appear to have the same setup, the scientists said.

"Rodents and humans, as well as virtually all mammals, have very similar sleep structure and physiology," Wilson noted. "They both have REM and SWS (slow-wave sleep). Additionally, the structure of the brain region involved in memory formation that we have been studying -- the hippocampus -- is similar between rodents and humans."

Wilson said no one knows how memory and dreams are reflected in the activity of neurons in the human brain because the technology is not yet available to measure the activity in non-invasive ways. However, he told UPI, "it is quite likely that there is significant (similarity) of function (between the two species)."

The investigators think both REM and non-REM are important for the optimal sleep experience.

"Simply put, SWS might be more critically involved in memory storage, with REM more involved in analysis of experience," Wilson said, adding both would be important for learning.

"By opening this window into the mind of the sleeping rodent," he concluded, "we are well on our way to understanding the role of sleep in memory and learning through the combined efforts of human and animal research."

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