Physical fitness may be critical for maintaining a relatively youthful and nimble brain as we age, according to a new study of brain activation patterns in older people.

For most of us, our bodies begin to lose flexibility and efficiency as we enter our 40s. Running and other movements slow down and become more awkward, and something similar seems to occur within our heads. As middle age encroaches, our thinking becomes less efficient. We don’t toggle between mental tasks as nimbly as we once did or process new information with the same aplomb and clarity.

Recently, neuroscientists have begun to quantify how those cognitive changes play out in our brains, to disquieting effect. In studies comparing brain activation in young people with that of people past 40, they have found notable differences, especially during mental tasks that require attention, problem solving, decision-making and other types of high-level thinking.

Such thinking primarily involves activation of the brain’s prefrontal cortex.

In young people, activation in the cortex during these cognitive tasks tends to be highly localized. Depending on the type of thinking, young people’s brains light up almost exclusively in either the right or left portion of the prefrontal cortex.

But in older people, studies show, brain activity during the same mental tasks requires far more brainpower. They typically display activity in both hemispheres of their prefrontal cortex.

In effect, they require more of their brains’ resources to complete the same tasks that young people do with less cognitive effort.

Neuroscientists coined an acronym for this phenomenon: Harold, for hemispheric asymmetry reduction in older adults. Most agree that it represents a general reorganization and weakening of the brain’s function with age.

But scientists have not known whether Harold is inevitable with aging or could perhaps be slowed or even prevented with lifestyle changes.

That possibility attracted the attention of Hideaki Soya, a professor of exercise and neuroendocrinology at the University of Tsukuba in Japan, who studies the effects of exercise on the brain.

For the new study, which will appear next month in NeuroImage, Dr. Soya and his colleagues recruited 60 Japanese men between the ages of 64 and 75 who showed no signs of dementia or other serious cognitive decline.

They tested each man’s aerobic fitness in the laboratory.

Then on another day, they fitted each volunteer with a series of tiny probes across their foreheads and scalps. The probes used infrared light to highlight blood flow and oxygen uptake in various parts of the brain.

With the probes in place, the volunteers completed complex, computerized tests, during which names of colors appeared in type of a different color. The word blue would appear in yellow type, for instance, and the volunteers would be expected to press keys corresponding to the name, but not the hue, of the word.

This test makes considerable demands on someone’s attention and decision-making and, in young people, has been shown to dramatically light up the left hemisphere of the prefrontal cortex.

But when the scientists examined the brain activity of these older men, they found that most also required activity in their right hemispheres. They needed more of their brains to pitch in in order to complete the task, displaying the Harold activity pattern.

However, the most aerobically fit of the volunteers did not follow this pattern. The fittest men showed little or no activation in their right hemispheres; they needed only their left hemisphere for the task.

In terms of attention and rapid decision-making, their brains worked like those of much younger people. They also were quicker and more accurate in their keystrokes, indicating that they attended and responded better than the less-fit volunteers.

Over all, Dr. Soya said, the results suggest that “higher aerobic fitness is associated with improved cognitive function through lateralized frontal activation in older adults.” Fit older people’s brains require fewer resources to complete tasks than do the brains of older people who are out of shape.

Of course, this study was observational and does not prove that fitness changed the men’s thinking, only that fit men had different brain activation patterns.

The study also did not look at exercise habits, only aerobic fitness. In general, fit older people almost certainly walk, jog or regularly engage in other moderate exercise such as swimming, Dr. Soya said. But he and his colleagues did not directly examine whether exercise affects brain activation.

Perhaps most important, this study and others looking at the Harold phenomenon do not indicate that it is necessarily a harbinger of mental decline. None of Dr. Soya’s volunteers were cognitively impaired, even if they relied heavily on Harold while thinking.

But, Dr. Soya said, the less-fit men did have brains that functionally were less sprightly than those of the fitter men and might be expected to progress earlier to more overt difficulties with memory and thinking.

The upshot of the findings, he said, is that daily mild exercise such as walking and mild jogging may affect the way the brain works, so that an older person’s brain “acts like a younger brain.”