Restoring Recall: Memories may form and reform, with sleep

Registering a memory for the long haul doesn’t happen all at once, according to new studies of how people learn perceptual and motor skills. Instead, building memory is a three-pronged process that rests on sleep.

First, knowledge accrues during training and dips immediately afterward. A good night’s sleep then revives much of what was forgotten, the researchers find. Finally, recalling the learned skill the next day destabilizes the memory of it, setting the stage for an individual either to reinforce prior knowledge or lose it.

These findings, published in the Oct. 9 Nature, contrast with the long-standing psychological theory that lasting memories essentially form all at once and don’t require sleep.

“Memory seems to be a process of storage and restorage,” says neuroscientist Karim Nader of McGill University in Montreal in a commentary published with the new studies.

In this vein, one of the new investigations, directed by psychologist Daniel Margoliash of the University of Chicago, examined the first two proposed prongs of memory formation. The results indicate that sleep rescues memories that had begun to deteriorate the previous day.

The scientists trained 84 college students to identify a series of similar-sounding words produced by a synthetic-speech machine. Improvement in discerning new words depended on participants’ ability to recognize novel combinations of previously heard synthetic sounds.

In one set of experiments, participants underwent training in the morning. In subsequent tests that morning, the learners performed well, but tests later in the day showed that their word-recognition skill had declined. The next morning, after a full night’s sleep, however, the volunteers performed at their original levels.

Further testing revealed that people trained in the evening performed just as well 24 hours later as people trained in the morning did. Since they went to bed shortly after training, those in the evening group didn’t exhibit the temporary performance declines observed in the morning group, the researchers say. Studies by others indicate that brief naps may also reverse performance declines after learning perceptual skills (SN: 6/1/02, p. 341).

Another group of investigators, led by psychologist Matthew P. Walker of Harvard Medical School in Boston, probed a third, morning-after prong of memory development. This team’s work suggests that simply recalling a learned motor skill, even after sleeping on it, destabilizes the memory and sets it up either for renewed storage or replacement.

Walker’s team trained 100 adults to repeatedly press numbered keys in a specific five-stroke sequence as quickly and as accurately as possible. Volunteers remembered the sequence even if they learned a second sequence 6 hours later. Performance on both sequences improved slightly after a night’s sleep.

However, subsequently recalling a sequence made it susceptible to a memory loss. If, on day 2, people who had learned one sequence were briefly retested on it and then trained on a new sequence, their performance on the first sequence plummeted on day 3. If the first sequence wasn’t retested before learning the new sequence, volunteers performed both sequences accurately on day 3.

It’s not yet clear whether memory for real-life skills works in the same way that memory for laboratory skills does, notes neuroscientist Larry R. Squire of the VA Medical Center in San Diego. Moreover, he says, studies like Margoliash’s need to be extended to confirm sleep’s role in restoring and retaining memories.