Do not underestimate the power of a walk in the woods: A new study suggests that even a 90- minute stroll in a natural environment can lead to measurable changes in the brain, and may help combat depression.
Previous research has shown that just a 50-minute walk in nature can improve your mood, decrease your anxiety and even improve your memory. But for the new study, published this week in PNAS, the research team wanted to see if they could understand what the mechanisms for these positive effects might be.
To help them figure it out, they decided to focus specifically on what psychologists call "rumination," which has been shown to predict depressive episodes.
"Ruminative thought means something very specific in psychology," said Gregory Bratman, a PhD candidate in environmental science at Stanford University and the lead author of the study. "It is repetitive thought that is focused on negative aspects of the self."
Examples of rumination include spending a lot of time thinking back over embarrassing or disappointing moments, or rehashing recent things you've said or done.
To see how a walk in nature affects ruminative thought, the researchers randomly assigned 38 volunteers with no history of mental illness to take a 90-minute walk in an urban green space near Palo Alto or a loud, busy street with three to four lanes of traffic in each direction.
Just before embarking on their walks, all participants were brought into the lab, where they completed a 12-question rumination questionnaire, responding to statements like "I often reflect on episodes in my life that I should no longer concern myself with" or "Sometimes it is hard for me to shut off thoughts about myself" on a five-point scale from strongly disagree to strongly agree.
They also had their brains scanned with a neural imaging technique that allowed the researchers to measure how much blood was flowing through the subgenual prefontal cortex -- an area of the brain that lights up when a person engages in rumination.
Participants were then driven either to the nature walk or the urban walk. The researchers also gave the volunteers smartphones and instructed them to take photos along the way. The photos were used to verify that the participants actually went on the walks.
Both walks were just a 15-minute drive from the lab. When the 90 minutes were up, the volunteers were brought back to the lab, where they completed the rumination questionnaire again and had another brain scan.
The researchers found that those who went on the nature walk showed reductions in both self-reported rumination and in the profusion of blood flow to the subgenual prefontal cortex. They observed no significant changes in the urban walkers.
"It was quite remarkable to us," Bratman said. "Especially because we weren't asking people 'How do you feel right now?' We were asking, 'How do you tend to think?' To change anything about how one describes how they think is quite compelling."
But he added that the work to quantify how nature affects our psychology has only just begun.
For example, he'd like to determine what it is about the natural environment that causes a decrease in rumination. He'd also like to figure out how long an encounter with nature would have to be to have desirable effects, and whether it would differ by landscape.
"We are trying to bring everything into the lab and tease apart the factors that do and don't have an impact," he said.
Ultimately, Bratman hopes that his research will help inform urban planning.
"Urbanization is increasing at an unprecedented rate, and we're also seeing an uptick in the rates of anxiety disorders and depression in cities," he said. "We want to figure out how do we bring more nature to people."