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In the past several months, a bevy of studies have added to a growing literature on the mental and physical benefits of spending time outdoors. That includes recent research showing that short micro-breaks spent looking at a nature scene have a rejuvenating effect on the brain — boosting levels of attention — and also that kids who attend schools featuring more greenery fare better on cognitive tests.

And Monday, yet another addition to the literature arrived — but this time with an added twist. It’s a cognitive neuroscience study, meaning not only that benefits from a nature experience were captured in an experiment, but also that their apparent neural signature was observed through brain scans.

The paper, by Stanford’s Gregory Bratman and several colleagues from the United States and Sweden, was published Monday in the Proceedings of the National Academy of Sciences. In it, 38 individuals who lived in urban areas, and who had “no history of mental disorder,” were divided into two groups — and asked to take a walk.

Half walked for 90 minutes through a natural area near the Stanford campus. The other half walked along a very busy road in downtown Palo Alto, Calif. (along El Camino Real, for those who know the area). Before and also after the walk, the participants answered a questionnaire designed to measure their tendency toward “rumination,” a pattern of often negative, inward-directed thinking and questioning that has been tied to an increased risk of depression, and that is assessed with questionnaire items like “My attention is often focused on aspects of myself I wish I’d stop thinking about,” and “I spend a great deal of time thinking back over my embarrassing or disappointing moments.”

Finally, both before and after the walk, the participants had their brains scanned. In particular, the researchers examined a brain region called the subgenual prefrontal cortex — which the study calls “an area that has been shown to be particularly active during the type of maladaptive, self-reflective thought and behavioral withdrawal that occurs during rumination.”

The result was that individuals who took the 90-minute nature walk showed a decrease in rumination — they actually answered the questionnaire differently, just a short period of time later. And their brain activity also showed a change consistent with this result. In particular, the scans showed decreased activity in the subgenual prefrontal cortex, the region of interest.

“This provides robust results for us that nature experience, even of a short duration, can decrease this pattern of thinking that is associated with the onset, in some cases, of mental illnesses like depression,” says Gregory Bratman, the lead author of the study.

What’s particularly valuable is that the brain scans allowed for the examination of a potential cognitive mechanism by which nature experiences help our mental states. Without such evidence, psychological research can in effect only speculate on occurrences within actual regions of the brain. “That’s why we wanted to push and get at neural correlates of what’s happening,” said Bratman.

In other words, the new research provides a new kind of evidence that is not only consistent with — but also strengthens — the growing body of research on the benefits of nature exposure.

Granted, brain scan research can be controversial – and it’s not as if conditions like depression have a single, simple cause. So as with all research, this work will need to be extended and verified by future studies.

The researchers set their study in the context of modern trends toward ever larger numbers of people living in cities — and an already demonstrated link between urbanization and mental health problems, such as depression and anxiety.

“We just passed the halfway point recently where 50 percent of humanity lives in urban areas,” said Bratman. “Along with this trend comes a decrease in nature and nature experience.” And the urbanized percentage of humanity is projected to be 70 percent by the year 2050, the study said.

But a key question raised by this is, precisely how would an urban environment worsen — or at least, fail to protect against — a mental behavior like rumination?

The idea seems to be that living in an urban area “is associated with many kinds of stressors, whether it be noise, increased social interactions, traffic,” said Bratman, which in turn increases rumination and anxiety — though he admits that this link in the study’s chain of logic needs further demonstration.

Still, it makes sense. Just think of waking up to the sound of a garbage truck in the morning outside your window — and how the accumulation of things like this can lead to negative repercussions on our psyches. Meanwhile, the authors speculate, nature environments allow for “positive distractions” that block or counteract these negative mental processes. Rumination is “this inward focused, maladaptive choice of where you direct your attention,” said Bratman, and nature gives an alternative opportunity for attentional focus.

The researchers also tie their results to a large literature on so-called “ecosystem services” — valuable benefits, such as carbon sequestration or water purification, provided by natural environments. The work suggests that on top of these benefits, there may also be “psychological ecosystem services” as well.

That’s a mouthful — but the underlying thought that it captures is pretty simple. Spending time outdoors, in nature, is good for you. The new study just adds — in a new way — to a growing body of evidence that demonstrates that.

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