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Essentially the short-term, scratch-pad system we use to manage relevant information, solve real-time problems and regulate our current emotional state, working memory is roughly equivalent to random access memory in a computer and functions on a daily basis like money in a bank account: Use it, and it depletes until it can be replenished.

Heavy cognitive tasks, such as scanning an alley for armed insurgents, require working memory. So do emotional challenges, like dealing with the stress of leaving one’s family for an overseas deployment.

According to Ms. Jha, depleted working memory has been linked to emotional impulsivity, prejudiced behavior, domestic violence and alcoholism.

“It’s the core resource for regulating your own behavior,” she said. “It’s not like your psychological state or mood is separate.”

In the M-Fit study, troops who meditated regularly increased their working memory capacity; moreover, they were more aware of their physical responses to combat stress.

In a fight-or-flight situation — for instance, a firefight — the pupils dilate to take in more information. Blood flows away from the stomach and into the muscles, producing the familiar “butterfly” sensation. Heart and breathing rates rise. Stress hormones course through the body.

More importantly, blood flow in the brain is redirected away from the areas that control rational thought and toward the areas associated with instinct and survival.

“It’s really hard to access rational thought during high-intensity stress situations,” said Jared Smyser, 28, a former Marine who lives in Richmond, Va., and is training to become an M-Fit instructor. “All this stuff happens in your body because we’ve evolved to get away from predators. But it’s not really relevant in today’s warfare. You need to be calm, collected, making better decisions.”

According to Ms. Stanley, meditative training can help troops do so by increasing efficiency in the insular cortex, which allows people to rapidly switch between thinking and unthinking states of mind.

“It can be exercised when we are attending to sensations in the body,” she said. “So a whole lot of our course is teaching the ability to track those sensations. People come into the course thinking it will ruin their ability to respond fast in combat, but actually, we’re enhancing their ability.”

In the future, Ms. Stanley said, meditation may become as standard in the military as rifle practice, another way of making troops more effective and resilient. Next year, the Marines will incorporate M-Fit classes into an infantry school at Camp Pendleton, making the program a tentative part of its regular training cycle.

Mr. Smyser, who served in Iraq in 2005, said military mental training is overdue.

“It absolutely would have beneficial to me [in Iraq],” he said. “I was very skeptical at first, but I’ve seen benefits in my own life. I’m interested in working with veterans with PTSD. And if we teach this upfront, we might be able to prevent some of the problems we have to fix afterwards.”