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For decades scientists have known that the ability to remember newly learned information declines with age, but it was not clear why. A new study may provide part of the answer.

The report, posted online on Sunday by the journal Nature Neuroscience, suggests that structural brain changes occurring naturally over time interfere with sleep quality, which in turn blunts the ability to store memories for the long term.

Previous research had found that the prefrontal cortex, the brain region behind the forehead, tends to lose volume with age, and that part of this region helps sustain quality sleep, which is critical to consolidating new memories. But the new experiment is the first to link structural changes directly with sleep-related memory problems.

The findings suggest that one way to slow memory decline in aging adults is to improve sleep, specifically the so-called slow-wave phase, which constitutes about a quarter of a normal night's slumber.

Doctors cannot reverse structural changes that occur with age any more than they can turn back time. But at least two groups are experimenting with electrical stimulation as a way to improve deep sleep in older people. By placing electrodes on the scalp, scientists can run a low current across the prefrontal area, essentially mimicking the shape of clean, high-quality slow waves.

The result: improved memory, at least in some studies. "There are also a number of other ways you can improve sleep, including exercise," said Ken Paller, a professor of psychology and director of the cognitive neuroscience program at Northwestern.

Dr. Paller said that a whole array of changes occurred across the brain during aging and that sleep was only one factor affecting memory function.

But Dr. Paller said that the study told "a convincing story, I think: that atrophy is related to slow-wave sleep, which we know is related to memory performance. So it's a contributing factor."

In the study, a research team in California took brain images from 19 people of retirement age and 18 in their early 20s. It found that a brain area called the medial prefrontal cortex, roughly behind the middle of the forehead, was about a third smaller on average in the older group than in the younger one — a difference due to natural atrophy over time, previous research suggests.

Before bedtime, the team had the two groups study a long list of words paired with nonsense syllables, like "action-siblis" and "arm-reconver." The team used such nonwords because one type of memory that declines with age is for new, previously unseen information.

After training on the pairs for half an hour or so, the participants took a test on some of them. The young group outscored the older group by about 25 percent.

Then everyone went to bed — and bigger differences emerged. For one, the older group got only about a quarter of the amount of high-quality slow-wave sleep that the younger group did, as measured by the shape and consistency of electrical waves on an electroencephalogram machine, or EEG. It is thought that the brain moves memories from temporary to longer-term storage during this deep sleep.

On a second test, given in the morning, the younger group outscored the older group by about 55 percent. The estimated amount of atrophy in each person roughly predicted the difference between his or her morning and evening scores, the study found. Even seniors who were very sharp at night showed declines after sleeping.

"The analysis showed that the differences were due not to changes in capacity for memories, but to differences in sleep quality," said Bryce A. Mander, a postdoctoral fellow at the University of California, Berkeley, and the lead author of the study. His co-authors included researchers from the California Pacific Medical Center in San Francisco; the University of California, San Diego; and the Lawrence Berkeley National Laboratory.

The findings do not imply that medial prefrontal atrophy is the only age-related change causing memory problems, said Matthew P. Walker, a professor of psychology and neuroscience at the University of California, Berkeley, and a co-author of the study.

"But these things are interrelated," Dr. Walker said. "Essentially, with time, the less and less tissue you have in this prefrontal area, the less and less quality deep wave sleep you get, the less and less you remember of content that you just learned."