A new study explores how a single bout of moderate exercise affects brain activity and mental performance in younger and older adults.
A single brisk walk can change what happens in the brain, but those changes are not always easy to understand.
Brain activity can also shift because a person repeats the same mental task and becomes more familiar with it.
That creates a problem for exercise experiments. If brain activity changes after a workout, researchers need to know whether exercise caused the change or whether practice also played a role.
A new experiment from the National Institute of Advanced Industrial Science and Technology (AIST) separated these effects by comparing walking with sitting in younger and older adults.
Practice can mimic exercise
The team focused on cognitive flexibility, which means being able to switch from one rule to another when a situation changes.
This is part of executive function, the mental control needed to manage attention and carry out goals.
Repeating a task can make people faster and can change how much brain activity is needed.
If this practice effect is ignored, part of the change may be wrongly linked to exercise.
Walking compared with sitting
The analysis included 16 young adults with an average age of 25 and 19 older adults with an average age of 62. The older adults were generally healthy and had been screened for several major health conditions.
Each person completed both an exercise session and a sitting session on different days. The visits happened at the same time of day and were at least 48 hours apart.
During exercise, participants completed a five-minute warm-up followed by 30 minutes of treadmill walking.
Speed and slope were adjusted so heart rate stayed near 50 to 70 percent of each person’s age-predicted maximum.
Young adults walked faster, but both age groups exercised at a similar level compared with their predicted maximum heart rate.
Switching rules in MRI
Before and after walking or sitting, participants completed a mental task inside a functional magnetic resonance imaging scanner, or fMRI.
This scan tracks changes in blood oxygen that are linked with brain activity.
Participants looked at colored shapes. Sometimes they judged the shape, sometimes the color, and sometimes they had to keep switching between those two rules.
Each person had three scans during each visit: before the session, about 15 minutes afterward, and about 45 minutes afterward.
Across both visits, each person completed six 16-minute task scans.
Participants also practiced the task for at least 10 minutes before the first scan each day. Even after that practice, their brain responses kept changing as they repeated the task.
Older brains worked differently
Older adults showed more widespread brain activity than younger adults while switching between rules.
The extra activity appeared in several areas involved in attention, control, movement, and information processing.
Older adults were somewhat slower or less accurate on the basic tasks.
However, the extra reaction time caused specifically by switching rules was not clearly different between the age groups.
The authors suggest that older adults may recruit more brain areas to help maintain performance. Still, more brain activity does not always mean better brain function.
Practice changed the brain
As participants repeated the task, brain activity fell in several regions.
These included part of the left frontal area, part of the left visual area toward the back of the brain, and part of the right cerebellum.
No brain region showed a clear rise in activity with repetition. At the same time, older adults became faster at switching rules, while younger adults did not show the same clear improvement.
In older adults, faster responses were linked with lower brain activity in several regions. This suggests that practice may have helped the brain handle the task more efficiently.
Some of this effect also appeared to last between visits.
Brain activity was lower when participants returned to the task on the second experimental day.
Walking maintains brain activity
Walking produced a different pattern.
In the right dorsolateral prefrontal cortex, a frontal brain area involved in mental control and switching rules, activity fell after walking in young adults but stayed almost unchanged in older adults.
A whole-brain analysis also found that activity in the right postcentral gyrus and both superior parietal lobules stayed more stable after walking.
These areas are involved in sensory processing and shifting attention during a task.
After sitting, activity in these regions fell as the task was repeated. Walking therefore appeared to reduce that drop in a few specific brain areas.
However, this did not lead to a clear improvement in performance. Walking did not significantly improve reaction time or error rates compared with sitting in either age group.
More activity isn’t better
The results suggest that one bout of moderate walking can change the pattern of brain activity during a demanding mental task.
They do not show that one walk immediately improves executive function.
The fMRI signal needs careful interpretation because it measures changes in blood oxygen, not the electrical firing of brain cells directly.
Exercise also changes heart activity, breathing, blood flow, and oxygen delivery.
Because of this, some changes seen after walking may reflect blood flow as well as changes in neural activity. The paper could not separate those effects.
This shows why higher brain activity should not automatically be called better. Lower activity sometimes went with faster performance, while walking preserved activity without improving task scores.
Limitations of the study
The experiment was small and tested only one type of mental task. It also tested only one form of exercise, moderate treadmill walking.
The 16-minute task sessions created strong practice effects, which may have hidden smaller exercise effects. Exercise intensity was estimated from age instead of being measured with a full exercise test for each person.
The team did not measure regular physical activity or sleep, and both could have influenced the results. The healthy older group does not represent all older adults.
The clearest message is that repeating a mental task can strongly change both brain activity and performance.
Exercise can also affect brain activity, but here those changes were limited to certain regions and did not clearly improve performance.
A single walk may change how the brain responds to a task before any behavioral benefits appear, but larger studies are needed to understand what those changes mean.
The study is published in the Journal of Magnetic Resonance Imaging.
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