Sweet Dreams: Rats, Sleep and Memories

Sleeping may have more benefits than just health: it may actually improve your grades.

A new study by a MIT professor published last week in the journal Neuron suggests that sleeping rats exhibit brain activity that may resemble human dreams. This discovery may shed light on the link in humans between dreams and memory.

Although scientific studies in the past have shown a link between dream-filled sleep and memory formation, this is the first time scientists have caught a living brain in the act of reviewing the day's events.

Scientists say this study is the first step in the quest to find out why we dream and how important sleep is for the formation of stable memories.

"One of the possibilities is that in these sleep and dream states, we are reevaluating past experience" says MIT Associate Professor of Brain and Cognitive Sciences Matthew A. Wilson, who was the lead author of the study.

"The larger research program is looking at how different parts of the brain participate in memory," he says. "Sleep is part of that process."

Rat Race

Wilson and his team implanted sensors in the hippocampus of rats, the part of the brain that is responsible for short-term memory in both rats and humans. The team tracked the activity of a dozen to a few dozen of the neurons in the hippocampus--only a tiny fraction of the cells in that part of the brain.

The team monitored the activity of the neurons while the rats were learning to find their way through a maze, and again while the rats were sleeping. The patterns of activity matched so closely that the team could tell were the rat would be in the maze if it were awake.

Although Wilson cautions there were no experiments to determine if the rats' performance improved after these "dreams," he says the results may mean the rats are "learning to be more efficient to perform a task they were familiar with."

But even if the rats are honing their maze-running skills by re-living earlier experiences in their hippocampi, it doesn't mean they are dreaming in the way humans understand it.

"We cannot look at any neuron's activity and say that the person or animal is conscious or aware of that activity," says Robert A. Stickgold, assistant professor of psychiatry at Harvard Medical School.

Stickgold said the observed activity in the rat hippocampus "probably have to do with consolidation of memories. Whether there is anyone home to perceive these memories is another question."

Although the technical functions of the hippocampus may be the same in both species, each animal probably perceives things very differently. It is unclear whether or not rats are self-aware at all.

"The structure of the regions such as the hippocampus is very similar [in rats and humans]" Wilson says. "We always like to believe we have special higher order faculties, and that's certainly true, but we need to identify the similarities before we can determine what the differences are."

According to Wilson, whose research has focused on the human brain, the similarities between the rat and human brain mean they probably work in the same way.

"I would be surprised if we could do similar experiments in humans and not get the same results," Stickgold says.

Sleep and Memories

In a study published in an October issue of Science, Stickgold had a group of people with normal memory function and a group of amnesiacs, who have damage to their hippocampi, play Tetris over a period of days.

Although the amnesiacs could not remember having played the game later that day, both groups reported thinking of falling blocks as they fell asleep at night. These results suggest that some forms of dreaming may not rely solely on the hippocampus.

According to Stickgold, the neocortex, a part of the brain that is normally associated with the high intelligence of primates, takes part in the dreams of amnesiacs.

During rapid eye movement (REM) sleep, the part of the sleep cycle most closely associated with dreaming, Stickgold says "there is a lot of communication between the neocortex and the hippocampus."

The precise nature of that communication and whether it is necessary for dreaming is still unknown.

Amnesiacs may have been able to later remember the Tetris game because their neocortexes, which analyze short-term memory and establish longer-term memory, were undamaged.

No All Nighters

Staying up late studying for a test may not hurt because memories can still form in the hippocampus, but pulling all-nighters may ultimately hurt your performance on final exams.

"In terms of forming those memories of facts you are trying to cram, that happens in the hippocampus," Stickgold says. "The first recording in the hippocampus is very fast and reliable. But if you don't get sleep afterward, you may not get the memories into the neocortex."

Diarra K. Lamar '01, a cognitive neuroscience concentrator in Lowell House, say he knows lack of sleep can get in the way of memory, but he's not overly concerned.

"I know that an all-nighter hurts recall," he says. "But I don't feel hindered. There were times when I had a lot of information, so I slept to help retrieval."

Lamar says he has averaged between four and five hours of sleep per night during the term.

David S. Rosenthal '59, director of University Health Services, says that he recommends at least six hours of sleep per night and that lack of sleep is a problem for undergraduates, but is especially a problem for graduate students.

"We see depression, difficulty with grades, difficulty with completing work, and disorientation," he says.

One of the treatments for people who are stressed and having trouble sleeping, according to Rosenthal, is to let them walk through a room-sized maze. One such maze is set up at the Dana-Farber Cancer Institute in Boston.

Unlike the tasrat mazes, however, the human mazes have paths that lead to the other side.

"A labyrinth is a good way to relax," Rosenthal concludes.

--Staff writer Jonathan H. Esensten can be reached at esensten@fas.harvard.edu.