Mental health problems are more common in urban areas than rural areas. Exposure to nature has been shown to improve memory, attention, stress, and reduce negative emotions, but the underlying neural mechanism for these effects is unknown.

In this randomized controlled trial, 63 participants (average age of 27) went for a one-hour walk in either an “urban” (e.g., busy street in a city center) or “natural” (e.g., urban forest) setting. The participants were brought by taxi to the start point and given their straight-line route, and they carried a mobile phone that logged their global positioning data during the walk. They also wore sensors for physiological indicators of stress.

Before and after the walk, the participants completed various questionnaires and tasks. The Fearful Faces Task (FFT) and the Montreal Imaging Stress Task (MIST), as well as a questionnaire to evaluate rumination (i.e., continuous deep thoughts about the same thing), were completed during a functional magnetic resonance imaging (fMRI) scanning session. Both the FFT and the MIST are used to induce social stress and evaluate amygdala expression. After the walk, restorative attention was also evaluated with a questionnaire.

A one-hour walk in an urban forest reduced amygdala activity and increased perceived restorativeness when compared to a one-hour walk on a busy street (which didn’t change any outcomes). Interestingly, in the FFT, amygdala activity decreased while watching fearful and neutral faces in the “natural” group, compared to the “urban” group. The differences in amygdala activity for the MIST trended toward significance for the “natural” group, while it remained stable in the “urban” group. No changes in self-reported measures, cognitive tasks, or physiological indicators of stress were reported.

The nature of this research is difficult to control, given the broad scope of environmental conditions, as other activities related to nature may be involved (e.g., exercise, social interactions, less noise and/or movement), especially since there may be differences depending on preference, experience, or personality.

Many studies have reported benefits from exposure to nature and green spaces. A 2020 review of 14 studies reported reductions in blood pressure, heart rate, physiological and psychological stress, and cardiometabolic health markers in association with forest walking or therapy programs.^[12] A 2022 review reported that moderate exercise performed in forests or green spaces stimulated the immune system and may limit the severity of COVID-19.^[13] A 2022 systematic review of 10 studies in China reported that exposure to green spaces was inversely associated with BMI, overweight, and obesity, and air quality was found to mediate the relationship between greenness and obesity.^[14] The research tends to focus more on how exposure to green spaces promotes health, rather than how it directly treats specific diseases (i.e., salutogenesis),^[15] but the benefits may come from a combination of associated factors, such as increased exercise and reduced levels of sensory stimulation. That said, it’s not clear what aspects of nature are driving this relationship and the underlying mechanisms are far from understood.

A 2015 study that compared a 90-minute walk in a natural environment to the same walk in an urban environment reported decreases in rumination and reduced activity in the subgenual prefrontal cortex, another brain region involved in emotional regulation.^[16] This brain region is close to and linked to the amygdala, but changes in amygdala activation were not reported in this study.^[17][18] Not only is this unclear support for the study under review, but the same group of authors from the study under review reported (in another publication) that the reductions in amygdala activity were sex specific and only occurred in women and not men. Women also performed better on the arithmetic tasks after the walk in nature, while men performed better after the walk in the urban environment.^[19] This suggests that there are many more variables to unpack in the relationship between exposure to green space and specifics of well-being, such as social/emotional stress, as well as how researchers can better control, study, and measure the variables involved.

The amygdala has been shown to be more active during social stress in rural dwellers than urban dwellers.^[20] Greater urbanicity, altered amygdala activation, and less amygdala connectivity have even been associated with schizophrenia.^[21][22] Thus, it has been suggested that exposure to green spaces may serve as a preventive measure against developing schizophrenia. Although there are associations with activity in certain brain regions and certain outcomes, causal connections are not clear. It has been suggested that amygdala activation in response to cognitive tasks (e.g., FFT) can vary depending on individual differences in cognitive traits, such as what an individual deems important,^[23] while baseline amygdala activation (i.e., the persistence of amygdala activity) appears to vary between individuals and may depend on daily affect (i.e., mood/feelings/attitudes) and well-being.^[24]

Overall, it appears that the amygdala keeps track of the positive and negative stressful events that people experience. If someone experiences a calming event, such as a stroll in an urban forest, the activity in the amygdala decreases and the person becomes less irritable or “on edge”. Again, this fits into the concept of salutogenesis that treats stress capacity as a resource for well-being that can be used up and replenished.^[15] Differences in more or less persistent amygdala activity following stressful events may partially explain one’s ability to cope with stress and elucidate mental disorders (e.g., adaptive vs. pathological anxiety or a spectrum of disorder based on baseline brain activity). If the amygdala is very active, and its role is to interrupt ongoing mental activity to redirect attention, it could bring one to focus on objectively harmless stimuli. For example, hyperexcitability of the amygdala has been observed in people with anxiety.^[24][6][25] Amygdala activity may go further and influence inflammatory responses:^[26] a 2015 study reported that greater amygdala activity in response to a stressor was associated with greater increases in inflammatory activity.^[27] In fact, emotional regulation has been shown to modulate the association between cumulative stress and epigenetic aging, adrenal sensitivity, and insulin resistance (read more in this Editor’s Pick).^[28] But, again, it’s rather early to make quick conclusions based on associations, especially when using unspecific, qualitative measurements such as blood flow in specific areas of the brain that are incompletely understood.^[4] These relationships are still speculative and many questions remain unanswered.