Though evidence of such changes already has been shown in Buddhist monks, a new study presented last week at the annual meeting of the Society for Neuroscience found that areas of the cerebral cortex, the outer layer of the brain, were also thicker in people who practice the Eastern discipline of meditation the Western way.

Such a thickening could explain why meditation can reduce stress and improve health measures such as blood pressure. But a heftier brain could also help keep some aspects of aging, such as memory loss, at bay.

The findings encouraged neuroscientists who know full well that most Americans, even those who meditate, don't live like monks. Buddhist monks, after all, meditate for hours every day. They devote their lives to it, and it's part of an overall religious philosophy.

But Americans who meditate — perhaps 5% of society, estimates the Meditation Society of America — have families. They have jobs. They juggle car pools, soccer games and social obligations.

Even ardent American meditators usually carve out only 45 minutes or an hour a day to mindfully breathe, rid their heads of external chatter and, with luck, find some inner serenity. That's the group studied by researchers at Massachusetts General Hospital.

"Studies have shown people who meditate are more relaxed, and skeptics will say, 'Of course they're more relaxed. They're just sitting there,' " said Sara Lazar, lead author of the study. "But sitting and relaxing in front of the TV doesn't make your brain grow."

The researchers studied 20 people with extensive training in Buddhist insight meditation and who had been doing it for an average of nine years. During those years, they meditated for about 45 minutes, six days a week. Researchers compared structural magnetic resonance images of their brains with those of a control group of 15 non-meditators.

Meditation changed gray matter. Those who regularly meditated had increased thickness in a region called the insula, central to integrating thoughts and emotions. That might help explain how meditation relieves stress. Years of practicing meditation also affected areas controlling heart rate and breathing.

Most of the increased thickening was in the right hemisphere, in the prefrontal cortex, which sustains attention and regulates memory. Those areas generally thin as people age, so one hypothesis is that meditation might slow age-related brain loss. Three of the 20 meditators practiced yoga in addition to meditation and had even greater increases in brain thickness.

It could be that people drawn to meditation already have thicker brain matter. But the finding fits with recent evidence that the brain is capable of changing structure and function — and that used circuits get stronger, while those ignored shrink and weaken. People who speak two languages, for example, have thicker areas of the brain that control language, and musicians' brains change after years of practice.

But speaking or playing the piano require interaction with the outside world. Now science is beginning to show that the brain may also be capable of changing in response to purely internal mental exercise.