The study addresses the well-established link between urban living and increased risk of mental disorders like anxiety, depression, and schizophrenia. Previous research indicates heightened amygdala activation during stress tasks in urban dwellers compared to rural dwellers. However, no studies had directly examined the causal impact of environmental exposure on stress-related brain mechanisms. This study aimed to fill this gap by conducting an intervention study comparing the effects of a one-hour walk in a natural environment (forest) versus an urban environment (busy street) on brain activity measured using fMRI. The biophilia hypothesis and theories like Attention Restoration Theory (ART) and Stress Recovery Theory (SRT) provide frameworks for understanding the potential restorative effects of nature. While previous research showed cognitive and affective benefits of nature exposure, including improved working memory and stress reduction, the neural underpinnings remained unclear. This study hypothesized that stress-related brain regions (amygdala, anterior cingulate cortex (ACC), dorsolateral prefrontal cortex (dlPFC)) would show less activation after exposure to nature compared to the urban environment.

A substantial body of literature links urban environments to poorer mental health outcomes, with a higher prevalence of anxiety, mood disorders, depression, and schizophrenia in urban compared to rural areas. Urban upbringing is a significant predictor for schizophrenia. Conversely, exposure to nature offers stress relief and restoration, supported by the biophilia hypothesis and theories like ART and SRT. ART highlights nature's role in facilitating attention restoration, while SRT emphasizes nature's stress-reducing effects through positive emotions and decreased arousal. Existing research demonstrates the cognitive and affective benefits of nature exposure, including improved working memory, attention restoration, and stress reduction, reflected in physiological markers like decreased heart rate and cortisol levels. However, the neural mechanisms underlying these effects remained largely unexplored before this study, with only limited fMRI intervention studies examining the impact of nature walks on specific brain regions. This study aimed to build upon existing research by investigating the direct causal effect of nature and urban environments on stress-related brain areas using a robust fMRI intervention design.

Sixty-three participants (29 female, mean age 27.21 years) were recruited and randomly assigned to either a natural or urban walk condition. The study procedure involved pre-walk fMRI scans (including a Fearful Faces Task (FFT) and Montreal Imaging Stress Task (MIST)), a 60-minute walk in the assigned environment (GPS tracked), and post-walk fMRI scans (including FFT, MIST, and a Social-Evaluative Threat task (SET)). Physiological data (EDA, HRV, heart rate) were collected during the walk using Empatica E4 wristbands. The FFT measured amygdala activity in response to fearful and neutral faces. The MIST induced social stress through mental arithmetic tasks with time constraints and feedback designed to induce social comparison. fMRI data were preprocessed using SPM12, and ROI analysis focused on the amygdala, ACC, and dlPFC. Statistical analysis involved two-way mixed ANOVAs to examine the interaction between environment and time (pre- and post-walk) on brain activity in different conditions and subregions of the amygdala. Additional analyses explored the impact of the intervention on behavioural and physiological data.

The study's key finding was a significant environment-by-time interaction in amygdala activity during both the FFT and MIST. Specifically, amygdala activation decreased significantly after the nature walk but remained stable after the urban walk. This effect was observed for both fearful and neutral faces in the FFT, and was consistent even when considering masked stimuli (which participants were not consciously aware of). The interaction effect was primarily driven by the right amygdala. Analysis of amygdala subregions revealed significant interactions in the basolateral and central amygdala, with decreases in activity after the nature walk. No significant environment-by-time interactions were found in the ACC or dlPFC. While behavioral measures didn't show significant changes, perceived restorativeness was significantly higher after the nature walk, and participants reported greater enjoyment of the nature walk compared to the urban walk.

The findings provide strong evidence for the salutogenic effects of nature on stress-related brain regions. The decrease in amygdala activity after the nature walk suggests that nature exposure can actively mitigate stress responses at a neural level, rather than simply being the absence of urban stressors. The consistency of the effect across both the fearful faces and social stress tasks suggests a generalizable effect of nature on amygdala responsiveness to various stressors. The results are consistent with the Stress Recovery Theory (SRT), which emphasizes nature's stress-reducing properties, rather than Attention Restoration Theory (ART), which focuses on cognitive restoration. The study also points to the potential of nature exposure as a preventative measure against mental illness, particularly in the context of increasing urbanization. The findings may help explain previously observed differences in amygdala activity and integrity between urban and rural dwellers, suggesting that repeated exposure to nature could have long-term positive effects on brain structure and function.

This study demonstrates a causal link between acute nature exposure and decreased amygdala activity, supporting the salutogenic effects of nature on stress-related brain regions. A one-hour walk in nature may buffer the negative impacts of urban environments and serve as a preventative measure against mental health issues. Future research should investigate specific features of natural environments driving these effects, consider different types of natural environments, and explore cultural variations in responses to nature.

The study's limitations include the lack of a perceptual control task to confirm that masked stimuli in the FFT were not consciously perceived. It remains unclear which specific aspects of the natural environment drive the amygdala response. The potential influence of encountering other people during the walks is not fully accounted for. Further research is needed to explore the effects of different types of natural environments and to examine potential cultural differences in responses to nature exposure.