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How does a walk in nature impact the brain?


Increasing urbanization and migration of rural populations to cities has pushed up the number of city dwellers dramatically to over 50% of the total population in modern times. Despite the higher wages and (sometimes) better facilities for education, healthcare, and employment found in an urban setting, stress and resulting mental health conditions like anxiety disorders, depression and schizophrenia are also increased.

Study: How nature nurtures: Amygdala activity decreases as the result of a one-hour walk in nature. Image Credit: BalanceFormCreative/Shutterstock

A new study shows how exposure to nature through a one-hour walk in a natural setting can prevent mental strain and possibly reduce the incidence of mental ill-health. Carried out in the Max Planck Institute for Human Development, Max Planck Dahlem Campus of Cognition (MPDCC), Berlin, Germany, the study appears in the journal Molecular Psychiatry.

Introduction

Major mental disorders are increased by up to 56% in urban settings, which has been suggested to be the chief environmental risk factor for schizophrenia, explaining over 30% of these cases, according to some scientists. In any case, schizophrenia risk is consistently related to urban dwellings, independent of family history, drug abuse, social network support, and sociodemographic factors.

This suggests that possibly the social stress linked to an urban environment is associated with a higher incidence of schizophrenia.

Experiencing nature is known to restore the ability to focus and relieve stress because (according to the biophilia hypothesis) of the inherent human connection with nature. This has been explored with the Attention Restoration Theory (ART) and Stress Recovery Theory (SRT).

The first hinges on involuntary attention to nature, which relieves the strain on voluntary attention. The second highlights the relief of stress that comes with the calming emotions evoked by nature.

Many studies have shown that experiencing nature enhances working memory, restores focused attention, relieves fear and stress, and produces beneficial reductions in heart rate, blood pressure, and cortisol levels.

The amygdala is a brain region activated during a task that evokes social stress. The activation is greater in an urban setting, indicating the need for intervention studies to validate the causative role of an urban environment in amygdala activation.

One study showed that walking in nature for 90 minutes reduced rumination and associated brain activity, but not urban walking. This was determined using functional magnetic resonance imaging (fMRI). Using the same technology, the authors of the current paper examined brain activity in certain regions, namely, the amygdala, anterior cingulate cortex (ACC), and dorsolateral prefrontal cortex (dlPFC), before and after walking for one hour in a natural vs. urban environment.

The aim was to determine if brain regions activated by stress would be activated differently in either setting compared to baseline levels. This was measured at three points.

First, subjects were presented with 15 male and 15 female faces showing fear or a neutral expression, with continuing fMRI before and after the task. Secondly, fMRI was performed for subjects and controls after giving the participants a task designed to evoke social stress, the Montreal Imaging Stress Task (MIST).

What did the study show?

The results of the study showed that the amygdala was differentially activated in the fear vs. neutral expression viewing task depending on the environment of the walk. This was because while amygdala activity remained stable in the urban environment, it was markedly reduced after the nature walk.

This was seen when the results were pooled for Fear and Neutral expressions or separately tested. It was also seen with masked stimuli, where the faces were shown briefly and then masked for the rest of the viewing time. Most of the amygdala activation was due to activity in the right amygdala.

The same type of change was observed in the MIST participants, where amygdala activity in the stressor-exposed participants was reduced after the nature walk but not after the urban walk.

Interestingly, the participants did not report feeling stress relief, nor did they perform cognitive tasks better or show physiological indicators of stress relief. However, they perceived better restoration after the nature walk and reported greater enjoyment of the walk compared to the urban walkers.

The other brain areas examined failed to show such changes.

What are the conclusions?

The decrease in amygdala activity after a nature walk contrasts with the lack of change after an urban walk. “We interpret this as evidence showing that nature is indeed able to restore individuals from stress.”

In fact, amygdala activation was reduced in response to neutral as well as fearful faces, suggesting a general effect produced by the nature walk, increasing the activation threshold. The same effect was observed during masked as unmasked stimuli after the nature walk. This corroborates earlier results showing amygdala activation with masked stimuli, despite the lack of conscious perception.

The beneficial effect of nature exposure on stress may occur outside of our awareness.” The study also localized the activity of the amygdala to the basolateral amygdala, as well as to the right side, a new finding in fear conditioning and anxiety-related activation.

The lack of perception of change in behavioral measures was probably because the participants were stressed by the MIST performed immediately after returning from the nature walk, indicating that in future studies, behavioral measures should be interrogated immediately after the walk and before stress-inducing tests.

The absence of improved cognition with nature walks supports the SRT rather than the ART. This is the first study that shows that exposure to nature over a short period produces acute effects, unlike urban exposure, while separating the positive effects of the nature walk from the negative effects of an urban walk. “This strongly argues in favor of the salutogenic effects of nature as opposed to urban exposure causing additional stress.”

With lower amygdala activation following nature exposures, this might explain why rural dwellers have lower stress-related amygdala activity and why city dwellers living near forests have better amygdala integrity.

As urbanization is increasing rapidly, with 70% of the earth’s people predicted to move to cities over the next few decades, urban planners should include many more green spaces to maintain and improve mental health. “Spending time in urban nature (green prescription) may buffer the disadvantageous impact of the city and serve as a preventive measure against developing schizophrenia.”



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