Short-Term Exposure – Environmental Effects in the Range of Weeks to Hours

The Day2Day Environment Study (Ongoing): How Day-to-Day Variations in Environmental Exposure are Linked to the Human Brain
Investigating Hippocampal Brain Plasticity After a One-Hour Walk in a Forest
Studying Objective and Subjective Stress Indicators in Response to Walks in Mother–Infant Dyads
Inaudible Infrasound Exposure and its Impact on Resting State Functional Connectivity
Conclusion

Most of the existing work in the field of environmental neuroscience is observational/correlational and targeted at associations between long-term environmental factors—usually exposures derived from the home address of participants—and mental as well as brain health. However, recent findings indicate that short-term, everyday variations, such as time spent outdoors on a given day, can be used to assess environmental influence on mental and brain health, even to parameters usually assumed to be relatively stable over time, such as brain structure. The focus on this shorter time scale offers an important window for intervention and micro-longitudinal studies that will help to unravel the causal pathway involved in how potential environmental "active ingredients" affect humans. Ideally these studies home in on environmental variables that have been implicated by observational/correlational evidence in long-term environmental effects in order to limit the number of possible "active ingredients" as evidence for the importance of particular environmental variables crystalizes across the funnel.

The Day2Day Environment Study: How Day-to-Day Variations in Environmental Exposure are Linked to the Human Brain

CEN has designed a multimodal, micro-longitudinal study to examine how day-to-day variations in environmental exposure shape brain structure, function, and well-being (Figure 1). Across up to 27 testing sessions per participant (N = 32, resulting in 781 sessions in total), each consisting of a 24-hour pre-scan period followed by a structural and functional MRI scan, researchers combined neuroimaging with continuous tracking of environmental, physiological, and behavioral parameters. During the pre-scan period, wearable sensors and Geographic Ecological Momentary Assessments (GEMA) with continuous GPS-based geospatial tracking were used to capture air quality, noise, light exposure, land cover, built-environmental characteristics, physical activity, stress, cognition, and affect (Falkenstein et al., 2026).

To uncover relationships between multidimensional exposures and neural and mental outcomes, the group will employ multivariate feature selection algorithms, linear mixed-effects models, and dynamic structural equation models to identify key environmental predictors and to explore interactions among them. At the same time, the data will be used to further investigate the stability of MRI-derived measures to provide insights into the reliability of neuroimaging metrics.

Key Reference

Investigating Hippocampal Brain Plasticity After a One-Hour Walk in a Forest

Visits to natural environments have been shown to benefit mental health and brain function, but whether short-term exposures to nature influence brain structure has not been investigated before. To explore the causal relationship between environmental exposure and hippocampal formation—a brain region known to be highly plastic in animals and humans—, Sudimac and Kühn (2024) conducted an interventional neuroimaging experiment in which 60 participants took a one-hour walk either in a natural environment (a forest) or an urban environment (a busy street) in Berlin (Figure 2). High-resolution hippocampal imaging was acquired immediately before and after each walk. 

They found that participants who walked in the forest setting showed an increase in subiculum volume, a hippocampal subfield previously related to the inhibition of stress-responses, while no structural change was observed following the urban walk. In addition, increases in subiculum volume following the forest walk were linked to lower levels of self-reported rumination. It is important to note that this effect did not withstand conservative Bonferroni correction for multiple testing. However, these findings provisionally suggest that even brief visits to nature may result in measurable alterations in brain structure, with potential benefits for mental health, though further research is needed to confirm this. Currently CEN is following up on this study with a walk study including various types of natural environments.

Key Reference

Studying Objective and Subjective Stress Indicators in Response to Walks in Mother–Infant Dyads

Although there is evidence that nature can improve mood and reduce stress in adults, far less is known about how such benefits might extend across the lifespan, in particular to infancy. Existing studies have shown cross-sectional observational/correlational associations between more greenspaces in residential surroundings and better mental health in childhood, yet these designs cannot establish causality, and the direct impact on infants has not yet been explored. To address this, CEN carried out a field-based intervention study with 72 mother–infant dyads to examine how a walk in a natural versus urban environment in Berlin affected stress metrics. The study was conducted in participants' everyday environment to maximize ecological validity. Mother–infant dyads completed two 45-minute walks, one in a natural setting and one in an urban area (order counterbalanced across dyads). Both subjective and physiological stress markers were assessed as outcome variables, including maternal ratings of stress, salivary cortisol in mothers and infants, and cortisol levels in mothers' breast milk.

The results show that across both settings, mothers and infants showed reductions in salivary cortisol after the walks (Sudimac et al., 2026). Mothers also reported feeling less stressed and in a more positive and pleasant mood afterwards. The type of environment did not differentially impact subjective reports or physiological stress measures; while spending time outdoors helped to reduce stress in both mothers and infants, the specific type of environment did not have an effect in this particular study.

Key Reference

Inaudible Infrasound Exposure and its Impact on Resting State Functional Connectivity

There is a growing interest in the effects of sound, in particular infrasound, on humans due to the increasing number of windfarms (and concomitant complaints by residents). In daily life, humans are regularly exposed to infrasound, whether from natural sources—wind, ocean waves, thunderstorms—or human-made ones—wind farms, heavy machinery, ventilation systems. The effects of prolonged infrasound exposure on brain function and behavior are largely unknown but highly relevant given political efforts to increase renewable energy production amid growing complaints by residents. To address this research gap, Forlim et al. (2024) designed a long-term randomized-controlled trial in which 38 participants were exposed to inaudible airborne infrasound (6 Hz, 80–90 dB) or sham devices during the night (8 h) for four weeks. The study showed that long-term infrasound exposure led to functional connectivity changes within the default mode, sensorimotor, and executive control networks. Notably, in this study the auditory network was unaffected by long-term infrasound exposure. Interestingly, increases in functional connectivity in the right inferior parietal lobe during verum exposure compared to sham were negatively correlated with self-reported annoyance from environmental low-frequency noise, implying that higher annoyance levels may attenuate the response to experimental infrasound exposure. The study is the first to investigate prolonged infrasound exposure effects on brain function.

Key Reference

Conclusion

CEN's short‑term studies are beginning to test causality for prioritized "active ingredients." The "Day2Day" design links 24‑hour exposure composites (GPS, GEMA, sensors) to MRI, enabling multivariate selection of the most predictive short‑term environmental signatures. Controlled "walk out" trials suggest that even one hour in nature can modulate brain structure in hippocampal subfields and self‑reported rumination, while other field interventions (e.g., mother–infant walks) have demonstrated general outdoor stress reduction without clear setting differences. Multi‑week sensory exposures (e.g., inaudible infrasound) have revealed network‑level brain changes, highlighting individual sensitivity as an important moderator. In the funnel, the short‑term level channels evidence from observational/correlational to interventional/causal by isolating which environmental ingredients may matter most. The short-term studies conducted so far do in fact suggest a causal role of greenspace exposure (namely a walk in a forest) on hippocampal brain structure and of noise (infrasound) exposure on brain function.