Sleepers yield memorable brain images.

Plunge into slumber and you may catch some memories as well as some Z's. The sleep phase known as rapid-eye-movement (REM) sleep appears to fortify memories of recently learned visual and spatial skills, a team of neuroscientists finds.

"[Our] results support the hypothesis that memory traces are processed during REM sleep," the team reports in the August NATURE NEUROSCIENCE.

Other researchers reject any connection between REM sleep and memory.

Pierre Maquet of University College London and his colleagues used positron emission tomography scanners to record rises and falls in blood flow in the brains of healthy adults. The technique provides an indirect measure of brain-cell activity.

Seven volunteers underwent scans while awake and resting and then again as they worked on a laboratory task for 4 hours in the late afternoon. This task consisted of watching for any of six markers to appear at specific points on a computer screen. The individuals had to push as quickly as possible one of six keys that corresponded to the marker's position.

The researchers studied another six adults as they performed the same task and while they slept that night. Five more were scanned at rest and while they slept.

Participants in the first two groups performed the task faster and more accurately the following day, demonstrating that learning occurred, Maquet's group reports.

The researchers also calculated average brain activity from the scans for each of the three groups. They identified four brain areas that became more active both during task training and during the REM sleep of volunteers who worked on the task. No other sleep stages exhibited any brain-activation patterns in common with the lab task.

Earlier research has shown that the brain regions activated during training and again during REM sleep contribute to perception and motor control. Though the new findings support contested evidence that REM sleep bolsters memory, how it does so remains unclear, the scientists say.

Maquet's team notes that concentrations of the neurotransmitter acetylcholine surge in the brain during REM sleep. Acetylcholine aids memory formation and reactivates brain areas necessary for performing a newly learned task, the researchers theorize.

"It's an open question whether [the new study] shows memory consolidation during REM sleep, but I have a hard time coming up with alternative hypotheses for what's going on," remarks neuroscientist Robert Stickgold of Harvard Medical School in Boston.

REM sleep clearly facilitates memory for well-learned activities, Stickgold argues. The brain may also activate weakly associated material during REM sleep, contributing to the bizarre quality of dreams, he says.

In a paper scheduled for an upcoming BEHAVIORAL AND BRAIN SCIENCES, Robert P. Vertes of Florida Atlantic University in Boca Raton and Kathleen E. Eastman of Northern Arizona University in Flagstaff present evidence to challenge any role for REM sleep in memory. For instance, they point out, disruptions of REM sleep induced by antidepressant medications don't interfere with memory.

Vertes calls the new data "provocative" but inconclusive. The findings, he asserts, may point to comparable states of vigilance during training and subsequent REM sleep, reflecting REM's primary responsibility for preparing the brain to wake up.