Brief sessions of brain exercise can have long-lasting benefits for elderly people, helping them stay mentally fit for at least five years, one of the most rigorous tests of the "use-it-or-lose-it" theory suggests.

For people age 73 on average, just 10 sessions -- less time than it takes to stay physically fit -- helped keep their brains sharp.

The brain training involved hour-long classes and included exercises done on a computer. While it is uncertain whether similar results would occur with mental exercise done at home, other research has shown that intellectual tasks such as crossword puzzles and reading can help keep the brain sharp as people grow old.

The study is "the toughest test of these hypotheses to date," said Jeff Elias, chief of cognitive aging at the behavioral science research branch of the National Institute on Aging, which helped pay for the $15 million study.

The study appears in Wednesday's Journal of the American Medical Association. It was led by Sherry Willis, a human-development professor at Penn State University.

Age-related mental decline is expected to affect 84 million people worldwide by 2040, according to an accompanying editorial.

Nearly 3,000 men and women in six cities -- Baltimore, Birmingham, Alabama; Boston, Massachusetts; Detroit, Michigan; Indianapolis, Indiana; and State College, Pennsylvania -- participated in the study. Most were white; about one-fifth were black.

They were randomly assigned to six-week training sessions in either memory, reasoning or speedy mental processing, and were tested before and after. A comparison group received no training but was also tested.

About 700 of the 1,877 people who completed all five years also got short refresher sessions one year and three years after their initial training.

The memory training included organizing a 15-item grocery list into categories such as dairy, vegetables and meat to make it easier to remember and locate items.

The reasoning training taught participants how to see patterns in everyday tasks such as bus schedules and taking medicines at different doses and times.

The speed training had participants quickly identify flashing objects on a computer screen. Those are some of the same reaction skills used while driving.

Nearly 90 percent of the speed training group, 74 percent of the reasoning group and 26 percent of the memory group showed almost immediate improvements in scores on tests of the mental functions they were trained in. The improvements in most cases lasted throughout the five years of the study and were most notable in people who got refresher sessions.

The comparison group participants also showed some improvement -- perhaps just from the stimulation of being tested -- but it was not as great.

After five years, the participants assessed their ability to perform everyday tasks such as shopping, driving and managing their finances. And the researchers rated the participants in their everyday functioning.

Only the group that received reasoning training reported substantially less decline than the comparison group. And only one group actually performed better, in the researchers' opinion -- those who got refresher sessions in speed training.

Willis said bigger differences probably were not seen because the participants were all pretty healthy throughout the study.

Sheryl Zimmerman, a University of North Carolina at Chapel Hill researcher on aging, said the study is important even if it doesn't show that mental training is a cure-all.

"The fact that a modest amount of cognitive training had ANY results five years later ... is notable," said Zimmerman, who was not involved in the research.