Cognitive Benefits From Living Near a Forest

Research on brain plasticity supports the notion that our environment can shape brain structure as well as function. Enriched environments elicit brain plasticity in animals. In humans it is unclear which environment is enriching. Living in a city has been associated with increased amygdala activity in a stress paradigm, and being brought up in a city with increased pregenual anterior cingulate cortex (pACC) activity. Increased pACC activity has been implicated in human anxiety disorders and depression (Amemori and Graybiel, 2012; Ernst et al., 2016).

Image from blueforestconservation.com

Researchers set out to identify geographical characteristics that constitute an enriched environment affecting the human brain. A total of 341 older adults participated to establish three latent brain factors (amygdala, pACC and dorsolateral prefrontal cortex (DLPFC)) to test the effects of forest, urban green, water and wasteland around the home address. The results reveal a significant positive association between the coverage of forest and amygdala integrity. This evidence suggests that forests may have health-promoting effects on the integrity of the amygdala. Since cross-sectional data does not allow causal inference it could also be that individuals with high structural integrity choose to live closer to forest (Kühn et al., 2017).

Background

In experiments conducted on rodents, researchers have observed experience-dependent alterations of the brain in the form of adult neurogenesis and synaptic plasticity using so-called “enriched environments”. In such an environment, rats could roam freely with those held in cages with access to food, water and bedding. That is, a complex environment that offers a higher degree of stimulation has been observed to facilitate brain plasticity as revealed by studies measuring neurons. Still the evidence acquired in studies on enriched environments in animals provides important information to infer what an enriched environment may look like in humans.

At first sight one may conclude that city dwellers experience more complexity and novelty in their environment compared to people living in more rural regions. However, in contrast to this, a growing body of research suggests that urban environments encompasses a set of adverse psychosocial influences that facilitate chronic stress. This is in line with epidemiological evidence showing that mental health problems are more frequent in urban as compared to rural areas. This has been shown for mood and anxiety disorders as well as schizophrenia, with up to 56% higher prevalence rates when comparing most to least urbanized regions. Reasons for this may lie in the repeated infringement of personal space in cities that may trigger the brains’ threat system and in particular the repeated exposure to strangers may facilitate chronic engagement of the amygdala.

At the same time a growing body of research has shown that living close to natural landscapes has beneficial effects on mental health as well as well-being, mood, cognition, but also longevity and mortality. Researchers have observed that moving to greener urban areas is associated with improvements in mental health. Moreover, it has been shown that more green space in deprived urban neighbourhoods is associated with less perceived stress and healthier diurnal cortisol responses. In Japan a so-called practice of “forest bathing” has been established under the term “Shinrin-yoku”. Although the empirical evidence base on Shinrin-yoku is small, first studies have demonstrated beneficial effects of passive viewing and active exploration of forest landscapes onto stress markers such as concentrations of cortisol, pulse rate, blood pressure, parasympathetic and sympathetic nerve activity.

Green landscapes and forests may be viewed as environmental enrichment factors in humans. What has been shown in the neuroscientific literature is that urban upbringing and city living might be detrimental by affecting stress processing in humans. More concretely, current city living has been associated with increases in amygdala activity in comparison to living in more rural areas, whereas being brought up in an urban environment in the first 15 years of life increased stress related functional brain activity in the perigenual anterior cingular cortex (pACC).

Researchers set out to apply established characteristics of geographical features within cities in more depth and associate these cross-sectionally with brain structural integrity. The purpose of the study was to investigate what may constitute an enriched environment for human beings on the micro level within the city of Berlin, complementing previous studies that discriminated brain differences of inhabitants of cities, towns and rural regions on the macro level (Kühn et al., 2017).

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References

Amemori, K. and Graybiel, A. (2012). Localized microstimulation of primate pregenual cingulate cortex induces negative decision-making. Nature Neuroscience, 15(5), pp.776-785. https://doi.org/10.1038/nn.3088

Ernst, J., Hock, A., Henning, A., Seifritz, E., Boeker, H. and Grimm, S. (2016). Increased pregenual anterior cingulate glucose and lactate concentrations in major depressive disorder. Molecular Psychiatry, 22(1), pp.113-119. https://doi.org/10.1038/mp.2016.73

Kühn, S., Düzel, S., Eibich, P., Krekel, C., Wüstemann, H., Kolbe, J., Martensson, J., Goebel, J., Gallinat, J., Wagner, G. and Lindenberger, U. (2017). In search of features that constitute an “enriched environment” in humans: Associations between geographical properties and brain structure. Scientific Reports, 7(1). https://doi.org/10.1038/s41598-017-12046-7