An experiment reveals the effects of an hour spent in the forest on the brain

An experiment reveals the effects of an hour spent in the forest on the brain

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Several studies have shown a link between urban living and an increased risk of mental disorders such as anxiety disorders, depression and schizophrenia. It is therefore essential to understand how exposure to urban and natural environments affects mental health and the brain. A team from the Max Planck Institute for Human Development looked into the matter: in a new study, it shows how an hour spent in nature can be enough to reduce stress.

The human brain has always been shaped by its environment. Today, more than half of the world’s population lives in cities, and this number is expected to approach 70% by 2050. Living in cities has been shown to be a risk factor for mental health: anxiety, mood disorders, severe depression and schizophrenia are up to 56% more common in urban areas than in rural areas.

In contrast, a growing body of research has demonstrated the cognitive and affective benefits of exposure to natural environments. In particular, being in nature has been shown to improve working memory capacity, promote attention recovery and help relieve stress. Evidence of these beneficial effects was observed through psychological assessments and physiological measures (heart rate, cortisol levels, etc.).

Proven effect on areas of the brain involved in stress management

It has been proven that the amygdala—a brain nucleus involved in stress management and serving as a kind of “warning system”—is more active during a social stress task in urban residents than in rural residents. However, no studies have yet investigated the causal effects of natural and urban environments on stress-related brain mechanisms. How do you know that the stress relief after being in nature is the result of exposure to the natural environment itself or simply the absence of adverse urban influences?

To find out more, the researchers conducted a new study looking at brain activity – specifically in brain areas related to stress – before and after one hour of exposure to natural or urban environments. Areas of interest included the amygdala, anterior cingulate cortex and dorsolateral prefrontal cortex, researchers say Molecular psychiatry.

The team recruited 63 healthy participants, including 29 women; their brain activity was examined by functional magnetic resonance imaging (fMRI) before the walk (taken in the Grunewald forest or on a busy street in the center of Berlin). They also had to fill out questionnaires, some of which were specifically designed to induce social stress. Please note that the participants were not informed about the purpose of the research.

a) Amygdala activity when viewing fearful faces (a) and neutral faces (b) significantly decreased after a walk in the forest. (BOLD = Depends on blood and oxygen levels) c) Location of the amygdala. Bottom: example of fearful or neutral faces used in the experiment. © S. Sudimac et al.

Equipped with wristbands measuring various physiological parameters, they had to follow a specific route without deviating from the route or using their mobile phone to avoid potential distractions. After the walk, each participant took another fMRI and then performed another stress-inducing task. The results of the study showed that amygdala activity decreased after a walk in nature, while it remained stable after a walk in an urban environment. Nature therefore has beneficial effects on areas of the brain associated with stress.

Useful results for urban planning policy

This is the first study to demonstrate a causal link between nature and mental health. ” Interestingly, brain activity in these regions remained stable and did not increase after a city walk, contradicting the widely accepted idea that being in a city causes additional stress. », emphasizes Simone Kühnhead of the Lise Meitner Group for Environmental Neuroscience at the Max Planck Institute for Human Development.

If a short walk in nature is enough to positively affect areas of the brain involved in stress management, this approach may be an effective preventative measure against mental stress and potentially disease. It could also buffer the potentially damaging impact of city life on the brain.

Simone Kühn and her collaborators have already shown, in study published in 2017 in Scientific reportsthat city dwellers living within a kilometer of a forest had a physiologically healthier amygdala structure and were therefore likely to be more resistant to stress. A Japanese study also showed that walking in the woods, as well as sitting and meditating, lead to a decrease in prefrontal hemoglobin concentration, an observation that was interpreted as a sign of relaxation.

In order to study the beneficial effects of nature in different populations and different age groups, Kühn and his team are currently working on another study to determine the impact of an hour-long walk in natural and urban environments on the stress of mothers and their babies. In anticipation of these results, this study reaffirms the importance of creating more accessible green spaces in cities for urban planning policies to improve the mental health and well-being of citizens.

source: S. Sudimac et al., Molecular Psychiatry

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