Study finds “green” images have restorative benefits, adding to growing evidence base for the hypothesis that exposure to nature, independent of how it is encountered, can be beneficial for human health and well-being.
van den Berg MM, Maas J, Muller R, et al. Autonomic nervous system responses to viewing green and built settings: differentiating between sympathetic and parasympathetic activity. Int J Environ Res Public Health. 2015;12(12):15860–15874.
This randomized crossover trial had participants view sets of images of nature-filled “green” and contemporary “built” urban environments both before and after experiencing a social-cognitive stressor task. Sets were shown for 5 minutes (8 seconds per image) on a computer screen in front of the participant in an otherwise empty room. None of the images in either set displayed people, animals, or signs of environmental neglect. Psychophysiological stress was produced via the Montreal Imaging Stress Test (MIST), a validated, screen-based arithmetic stress-induction task with false feedback indicating below-average performance.
All participants (N=46; 25 female; mean age=21 years) were Dutch university students who self-reported as healthy with no significant history of chronic disease, heavy medication use, or use of tobacco.
Upon arrival, participants provided information regarding potential covariates, such as current subjective mood, stress level, and previous 24-hour physical activity.
Participants were connected to both electrocardiogram (ECG) and impedance cardiogram (ICG) continuously throughout the experiment to allow measurement of autonomic nervous system (ANS) function as follows:
- Parasympathetic nervous system (PNS) activity was assessed by respiratory sinus arrhythmia (RSA), a common physiological indicator of vagal tone.1
- Sympathetic nervous system (SNS) activity was assessed by cardiac pre-ejection period (PEP), the time between left ventricle depolarization and aortic opening. PEP decreases with increasing SNS activity.2
Subjective interpretation of the two setting types was determined by post-exposure rating using the validated Restoration Outcome Scale (ROS).
When examining order effect, it was found that participants who viewed the built scenes first had muted responses to green scenes in the second viewing set. To account for this, only data from the first viewing sets were compared (ie, crossover data was not used).
After adjusting for covariates including baseline differences in RSA and PEP, the following key results were found:
- RSA: When viewed after the MIST stressor, green scenes increased RSA (+31.82ms ± 7.02ms) compared to viewing built scenes (+11.39ms ± 6.41ms) in a manner that was statistically significant [F(1,41)=4.45, P=0.04, ηp2=0.1]. However, neither scene type had any significant effect on stress outcome when viewed before the stressor [F(1,43)= 0.61, P>0.44, ηp2=0.01].
- PEP: Scene type did not have any significant effect on PEP when viewed either before [F(1,43)=0.02, P=0.9, ηp2=0.0] or after [F(1,43)=0.55, P=0.46, ηp2=0.01] the MIST stressor.
Analysis of subjective ROS revealed that green setting was considered significantly more restorative than urban setting [F(1,45) = 153.61, P<0.001, ηp2=0.77].
This experiment was designed to address 3 issues:
- Whether viewing green vs built scenes can have a buffering effect on ANS function (ie, a protective ability to lessen stress response when viewed before a stressor)
- Whether viewing green vs built scenes can have a restorative effect on ANS function (ie, a therapeutic ability to lessen stress response when viewed after a stressor)
- Whether viewing green vs built scenes can have an effect on PNS vs SNS function
The results suggest that viewing scenes of green space may be most effective for addressing stress after a stressful event has occurred, as shown by increased RSA as an indicator of parasympathetic activity. This fits with findings from many other studies demonstrating the restorative ANS effect of contact with nature compared to modern urban settings3-4 particularly in the Shinrin-yoku or “forest-air bathing” literature.5-6 Increases in PNS activity after exposure to stressors have substantial health impacts via modulation of allostatic load and significantly influence processes such as cardiovascular and digestive function, blood sugar regulation, immune status, and mental health.7-8
Regarding the buffering effect of nature on stress, the results of this study do not support the idea that viewing natural images can reduce the physiological impact of post-viewed stressors. This may not be surprising, as physiological activation of SNS and reduction in PNS activity in response to stressors is a survival requirement via the fight-or-flight response. Muting of this response through any means, especially viewing the ubiquitous natural scenes that were our ancestors’ constant surroundings, would be an evolutionary liability to survival. For now, while spending time in nature may be beneficial for restorative reasons, the experience does not appear to “immunize” someone against future stresses. It may be that longer-term nature exposures produce structural and functional nervous system alterations, similar to the neuroplastic changes that result from regular meditation practice,9 but at this point that idea is purely speculative and extends beyond the scope of the current study.
This study is encouraging because it suggests natural imagery alone is sufficient to achieve physiological benefit.
The data did not support the hypothesis that viewing natural scenes increases PEP, indicating no reduction in sympathetic activity. This is at odds with other experimental studies in this research area6,10 and is most likely a result of low statistical power due to the just-mentioned crossover design flaw. It has been stated that this growing field of research significantly lacks a meta-analysis to determine true efficacy and clinical impact, though the widely varying study designs, interventions, and biomarkers used in these studies currently make such an effort impossible.11
In addition to the design issues mentioned above, this study has a few other limitations. First, order effect data removes the value of having a crossover trial and limits the statistical power of the findings. Second, this study uses computer screen images of green and built settings, rather than actual outdoor exposures as occur in the real world. This limits the study’s applicability for supporting true natural places as experiences for fostering health and well-being. However, much of the foundational research in this area has been conducted using photo or video exposures.4,12-13 The information from this study is encouraging because it suggests natural imagery alone is sufficient to achieve physiological benefit. This is important for many groups, including office workers, inner-city residents, and homebound individuals with mobility issues for whom access to outdoor green spaces may be limited.
The artificial nature of this study does not diminish the decades of research and thousands of years of human experience demonstrating the restorative effect of actual outdoor natural spaces.14 This study adds to the evidence base for the biophilia hypothesis: exposure to nature, independent of how it is encountered, can be beneficial for human health and well-being.15