Dream a Little Dream of Recall

As the sleeping brain builds memories it generates dreams about recently learned material

By Bruce Bower, Science News

People who have nap-time dreams about a task that they’ve just practiced get a big memory boost on the task upon awakening, Harvard researchers report.

Those who dream about anything else have no such enhanced recall, the team reports in a paper published online April 22 in Current Biology. Neither do those who stay awake, even if they think about the task.

“I was startled by this finding,” says study coauthor Robert Stickgold, a cognitive neuroscientist at Harvard Medical School. “Task-related dreams may get triggered by the sleeping brain’s attempt to consolidate challenging new information and to figure out how to use it.”

His new findings elaborate on research suggesting that sleep generally enhances memory and learning (SN: 4/28/07, p. 260).

Dreaming about a demanding undertaking doesn’t cause enhanced memories for that experience, Stickgold emphasizes. Rather, memory-fortifying brain processes during sleep cause the dreams, he proposes. During slumber, Stickgold posits, a structure called the hippocampus integrates recently learned information, such as how to navigate a virtual maze, while other brain regions apply this information to related but broader situations, such as how to navigate a maze of job application forms.

That’s a “tempting speculation,” remarks physiological psychologist Jan Born of the University of Lübeck in Germany. Stickgold’s idea has much potential for fostering advances in dream research, Born says.

Stickgold’s group focused on dreams that occur during non–rapid eye movement, or NREM, sleep. Previous studies found links between chemical and electrical activity in the brain during NREM sleep and better learning by rats and people. Neural activity sparked by recent learning has not been observed during rapid eye movement, or REM, sleep, which often includes especially vivid and bizarre dream elements.

In the new investigation, 99 college students age 18 to 30 spent an hour practicing a virtual maze task on a computer. In a series of trials, volunteers navigated through a complex, three-dimensional maze, starting from a different spot each time. They were instructed to remember the location of a particular tree in the maze.

For the first 90 minutes of a five-hour break from practicing, students were assigned either to take a nap or to engage in quiet activities such as watching videos.

Nappers’ electrical brain activity was monitored with scalp sensors. Experimenters questioned the students about their dreams just before they fell asleep, after one minute of continuous NREM sleep and at the end of the nap period. Volunteers who stayed awake recounted their thoughts at the start, middle and end of the 90-minute session.

After lunch and a period of quiet activity, participants reentered the virtual maze at random spots and were asked to find the tree that they had previously tried to remember.

Those who had dreamed of the experimental task — four of 50 nappers — found the tree much faster than they had in initial trials. These individuals described dreams such as seeing people at particular locations in a maze or hearing music that had played in the lab during testing.

All of the volunteers who dreamed about the maze had performed relatively poorly during pre-nap training, Stickgold notes. Memory processes invoked by the sleeping brain may respond most strongly to challenges perceived as difficult and important to solve, he suggests.

Stickgold’s group is now designing a more exciting maze task intended to elicit task-related dreams in a larger proportion of volunteers.

The researchers also plan to examine whether people who have REM dreams about a maze task during a full night’s slumber navigate that maze better the next day.

Labels: advice, breakthrough, dream, dreams, harvard, learn, maze, mazes, medical, nappers, REM, research, science, sleep, solve, study

Elephants dream of munching sweet grass under a starry savannah sky. Dogs, paws aquiver, tails thumping faintly in slumber, chase squirrels in the park. And cats, of course, dream of mice.

Or so humans, prone to anthropomorphic conjecture about the four-legged world, have long suspected.

Yet what animals dream about -- or indeed, whether they dream at all -- has remained resistant to scientific scrutiny, if only because animals cannot describe their closed-eye experiences in words.

Now, however, two researchers studying memory have offered compelling evidence that the brains of sleeping animals are at work in a way irresistably suggestive of dreaming. And the animals in question -- four pink-eared, black-and-white laboratory rats -- appeared to be dreaming about something very specific: the maze they were learning to run.

The researchers, who reported their findings in today's issue of the journal Neuron, found that patterns of brain activity identified when the rats ran a circular maze -- receiving a reward of chocolate-flavored sprinkles -- were exactly duplicated when the rats were sleeping.

In particular, the patterns, detected in the firing of clusters of cells in the hippocampus, an area involved with memory formation and storage, were reproduced during phases of sleep that in humans are strongly linked to dreaming. And they were so precise the scientists could tell where in the maze the rat would be if it were awake, and whether it would be moving or standing still.

''The animal is certainly recalling memories of those events as they occurred during the awake state, and it is doing so during dream sleep,'' said Dr. Matthew Wilson, the senior author of the report and an associate professor of brain and cognitive sciences at the Massachusetts Institute of Technology.

Dr. Wilson added that the research was not proof, in the purest sense, that animals dream, because the dreaming experience is subjective and, ''our ability to ask the animal to report the content of these states is limited.''

But the findings, he said, ''are the strongest evidence we have to date that animals have something close to human dreaming,'' adding, ''Call it whatever you want.''

Though only four rats were studied, Dr. Wilson and other scientists said the number was sufficient to attain statistical significance. Also, they said, the elaborate nature of the controls used in the study made it unlikely that the results were spurious, though more research is needed to replicate and extend the findings.

''The likelihood that this would occur by chance is exceedingly small,'' Dr. Wilson said.

Scientists familiar with the work said the research was important not only for the glimpse it offered of the sleeping animal brain, but also because it lent support to the idea that sleep played a critical role in the encoding and storage of memories. The study demonstrates, for the first time, that complex, episodic memories are replayed or ''rehearsed'' in the hippocampus during sleep, perhaps representing a process by which memory is gradually consolidated and passed to other parts of the brain, a model championed by several researchers.

''I am delighted,'' said Dr. John Allan Hobson, a professor of psychiatry at Harvard and the director of the Laboratory of Neurophysiology at the Massachusetts Mental Health Center in Boston, ''because it suggests that, as we have long suspected, sleep has a lot of functional significance for learning and memory.''

The relationship between sleep and memory is still debated within the field, but studies by Dr. Robert Stickgold, of Dr. Hobson's laboratory, and others indicate that when people learn new skills, their performance is dependent on how much they get of two types of sleep: Nondreaming or slow-wave sleep early in the night, and so-called rapid eye movement sleep, or R.E.M., later in the night. In humans, R.E.M. sleep is when most dreaming occurs.

But Dr. Howard Eichenbaum, university professor of psychology of Boston University, said that even with the new study, the case for sleep as memory consolidator was far from proved.

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Labels: dream, maze, mazes, rats, REM