April 27, 2007 — Declarative memories — memories for facts and events in time — become more resistant to interference during sleep, according to a study that will presented at the 59th Annual Meeting of the American Academy of Neurology (AAN) in Boston, Massachusetts.

"We know that sleep helps boost memory for procedural tasks, such as learning a new piano sequence. But we're not sure, even though it's been debated for over a hundred years, whether sleep impacts declarative memory," said lead author Jeffrey Ellenbogen, MD, a clinical fellow in medicine at Harvard Medical School, in Boston.

To test whether sleep strengthens declarative memory in the face of interference, a research team led by Dr. Ellenbogen conducted a study whereby 48 people between the ages of 18 and 30 were divided evenly into 4 groups: a wake group without interference, a wake group with interference, a sleep group without interference, and a sleep group with interference.

All groups were taught the same 20 pairs of words in the initial training session. The wake groups were taught the word pairings at 9 AM and then tested on them at 9 PM, after 12 hours of being awake. The sleep groups were taught the word pairs at 9 PM and tested on them at 9 AM, after a night of sleep.

Just before testing, the interference groups were given a second list of word pairs to remember. The first word in each pair was the same on both lists, but the second word was different, testing the brain's ability to handle interference. The interference groups were then tested on both lists.

The investigators found that subjects in the sleep groups had superior recall, relative to those in the wake groups. The difference between the sleep and wake groups was greatest when the subjects were tested after interference (76% vs 32% of words recalled correctly in the sleep group vs the wake group; P < .0001).

"These results mean that sleep does in fact lead to a benefit for declarative memory consolidation," Dr. Ellenbogen told Medscape. "We were surprised by the magnitude of the effect," he said. "The benefit of sleep [for memory consolidation] was even larger than we were anticipating."

The authors think the results may help in understanding the neurobiology of memory consolidation and could have important applications for patients with dementia and sleep disorders.

"The real strength of the research by Ellenbogen's team is in the way they measured memory," said Matthew Walker, PhD, director of the sleep and neuroimaging lab, department of psychiatry, Harvard Medical School, who was not involved in the study.

"What Ellenbogen and colleagues have done very cleverly was not simply just train subjects on a list of words and then test them after wake or after sleep," Dr. Walker told Medscape. "They trained subjects on a list of words, and then just before they tested them after wake or sleep, they quickly had them learn a new set of words. And what they found is that when you do that across sleep, sleep provides a remarkable benefit in the face of interference for those old memories."

This is a novel finding, according to Dr. Walker, because it reveals something new about what sleep does to memory. "It also tells us that the biggest effect of sleep can be seen only when you test whether or not that memory holds up against interference from additional new learning."

American Academy of Neurology 59th Annual Meeting: Abstract S39.003. April 28 – May 5, 2007.

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