Awake and Learning: Memory storage begins before bedtime

Learning isn’t a task that just happens overnight. While research has suggested that a good night’s sleep aids in memory storage, some memory is processed while a person is still awake, a new study finds.

Previous research in both people and animals has found that the parts of the brain engaged in learning a task reactivate during sleep, perhaps transferring a memory from short-term to long-term storage (SN: 10/11/03, p. 228: Restoring Recall: Memories may form and reform, with sleep).

But sleep may account for only a few steps in the transfer process, says Philippe Peigneux, a neuroscientist at the University of Liège in Belgium. Rather than passively holding on to memories until bedtime, the wakeful brain may get a head start on memory consolidation.

To learn what happens to these memories during waking hours, Peigneux and his colleagues imaged the brains of 15 volunteers to determine how quickly they learned lessons from several tasks.

In the study, each participant spent 30 minutes learning either a spatial or a procedural task. In the spatial task, the subjects studied and then navigated routes through a virtual town to locate an object. In the procedural task, participants learned to press a button corresponding to each of four positions of a dot that appeared in a repeating sequence on a screen.

The researchers scanned the participants’ brains with functional magnetic resonance imaging (fMRI) immediately before and after each learning episode. During the scan, a subject performed a separate, distracting task that didn’t require any learning.

After a 30-minute break, the team scanned the subjects’ brains a third time. After the participants performed each task one more time, a final, fourth scan revealed which parts of the brain were active while the subjects performed the task.

Activity in specific learning areas of the brain, as determined by the fourth scan, was higher after the initial task than before it. For the spatial task, activity increased immediately in the hippocampus and other regions corresponding to navigation. After the procedural task, increased activity didn’t appear right away in some of the relevant brain regions. But activity did show up in all those areas by the third fMRI scan. For both tasks, brain activity indicative of memory processing persisted through the third scan, 1 hour after the task had been completed.

These findings, published in the April PLoS Biology, suggest that the brain can begin to process some memories almost immediately, even while subjects are performing unrelated cognitive tasks, the researchers report.

While previous neuroimaging studies had looked at what happens in the brain during a learning task, this is the first imaging study to track what happens “off-line” after a learning session, says neurologist Ilana Hairston of the University of California, Berkeley. That strategy eliminates any potential effects of concurrent learning in studies of memory consolidation.

“It’s a unique experiment and remarkable work,” Hairston says. “On the one hand, it supports the idea of off-line [memory] consolidation. On the other hand, it also suggests that sleep may not be necessary for the process.”