How does experiencing nature reduce levels of mental illness?

This is a knowledge translation piece based on a study conducted by Gregory Bratman, Paul Hamilton, Kevin Hahn, Gretchen Daily, and James Gross (1).

Background: Rapid urbanization is responsible for decreased nature exposure in today’s society. In turn, decreased exposure to nature is linked to increased levels of mental illness, including anxiety disorders and depression (2).

Purpose of the Study: One potential mechanism by which nature promotes mental wellness is through decreasing rumination, which can be defined as repetitive negative self-reflection. Rumination is associated with heightened risk of mental illnesses, such as depression, as well as greater activation of an area of the brain called the subgenual prefrontal cortex (sgPFC). The sgPFC displays increased activity during sadness, behavioural withdrawal, and rumination in both healthy and depressed individuals.

The researchers hypothesized that nature is able to improve mental health through decreasing rumination and sgPFC activity by shifting attention away from negative, self-relational emotions.

Method of the Study: 38 participants without a history of mental disorder were recruited for the study. All participants lived in an urban environment and consequently had elevated levels of rumination associated with deprivation of regular contact with nature. The 38 participants were split into two equal groups (n=19) and were randomly assigned to take either a 90-minute walk in an urban or natural setting.

To assess the effects of the 90-minute walk, three measures were collected before and after the walk:

1. Self-reported levels of rumination through a questionnaire
2. Blood perfusion in the sgPFC via an ASL scan which images blood flow (greater blood flow indicates greater activation)
3. Heart rate and respiration rate

Results and Discussion: Analysis of self-reported rumination levels revealed that there was a decrease in rumination for the nature group, but not for the urban groups (P<0.05). Furthermore, ASL scans taken pre- and post walk found decreased blood perfusion in the sgPFC in the nature group only, indicating reduced activation of that area (P<0.0001). Given that changes in heart rate and respiration for both nature and city groups were similar, researchers were able to ascertain that the decrease sgPFC activation and rumination were not attributed to differences in walk-related physiological activation.

These results show that nature is able to improve mental well-being by lowering rumination and sgPFC activation. However, as these effects are achieved through shifting attention away from negative, self-relational emotions, any environment can potentially achieve the same results. Nevertheless, psychologically beneficial environments tend to be locations in nature, as nature has the tendency to be aesthetically pleasing and noise-reduced (3–4).

Sources

1. Bratman G, Hamilton J, Hahn K, Daily G, Gross J. Nature experience reduces rumination and subgenual prefrontal cortex activation. Proceedings of the National Academy of Sciences. 2015;112(28):8567–8572.

2. Lederbogen F, et al. (2011) City living and urban upbringing affect neural social stress processing in humans. Nature 474(7352):498–501.

3. Herzog TR (2003) Assessing the restorative components of environments. J Environ Psychol 23(2):159–170.

4. Korpela KM, Hartig T, Kaiser FG, Fuhrer U (2001) Restorative experience and self-regulation in favorite places. Environ Behav 33(4):572–589.