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Natural Medicine Journal

July 3, 2024

Green Scenes and Mental Health

A comparison of natural vs urban images

Kurt Beil

ND, LAc, MPH
Nature images improve brain blood flow and subjective experience in depressed and anxious patients.

Reference

Mizumoto T, Ikei H, Hagiwara K, et al. Mood and physiological effects of visual stimulation with images of the natural environment in individuals with depressive and anxiety disorders. J Affect Disord. 2024;356:257-266.

Study Objective

“To elucidate the effects and physiological changes of visual stimuli from natural environments…on patients with depression and anxiety disorders” 

Key Takeaway

Exposure to nature imagery can improve subjective and objective measures of depression and anxiety.

Design

Randomized crossover trial. Though true blinding is not possible for interventions of this kind, participants were unaware of the purpose of this study, and investigators were unaware of the experimental group during data analysis. 

Participants

Individuals with pre-existing diagnoses of various severities of depression (n=30, severity determined by the Beck Depression Inventory, BDI) or anxiety (n=30, severity determined by the State-Trait Anxiety Inventory, STAI). All participants were undergoing outpatient treatment in Ube, Yamaguchi Prefecture, Japan. No patients had any visual impairments except possible use of corrective lenses, and none had suicidal ideation. Age ranged from 20 to 64 years (mean 45.4 years, standard deviation [SD] 10.4 years). Female n=39, male n=21.

Intervention

Exposure to 2 sets of high-quality photos, 1 of natural-vegetation images and 1 of urban buildings. Each set consisted of 12 images shown for 15 seconds each (3 minutes total). Crossover design provided each participant with experience with both photo sets in randomized order.

Study Parameters Assessed

Investigators collected subjective data using the Chen-Hagiwara Mood Test (CHAMT), a novel instrument of 3 visual analog scales measuring emotional valence and arousal regarding “Comfortable,” “Relaxed,” and “Vigorous” states.

They collected objective data using 2 biomarkers:

  1. Near-infrared spectroscopy (NIRS) of oxygenated hemoglobin (Oxy-Hb) concentration measured relative to blood flow in the left and right prefrontal cortex (PFC)
  2. Fingertip accelerated plethysmography measured heart rate variability (HRV) for both high frequency (ie, parasympathetic) and the ratio of low-to-high frequency (LF/HF) (ie, sympathetic) autonomic nervous system activity.

Primary Outcome

Within-subject changes in subjective (CHAMT) and objective (Oxy-Hb and HRV) measurements

Key Findings

Subjective mood (CHAMT) was significantly improved for all participants after viewing the nature vs urban pictures. 

Participants with depression reported greater scores for the “Comfortable” (t=4.659, P<0.001, d=0.851), “Relaxed” (U=20.000, z=−4.272, P<0.001, d=0.929), and “Vigorous” (U=82.000, z=−2.757, P=0.006, d=0.478) scales. 

Participants with anxiety reported greater scores for “Comfortable” (U=44.000, z=−3.485, P< 0.001, d=0.825), “Relaxed” (U=35.000, z=−3.701, P<0.001, d=0.885), and “Vigorous” (t=5.047, P<0.001, d=0.921) scales. 

After participants viewed the nature pictures, there was a significant negative correlation between severity of anxiety and all 3 mood items (Comfortable r=–0.438, P<0.001; Relaxed r=–0.313, P=0.015; Vigorous r=–0.275, P=0.033), suggesting participants with greater anxiety received less subjective benefit (though still more significant than urban viewing) . 

Objective measures detected significant increases to Oxy-Hb concentration in both the left orbital PFC (t=2.355, P=0.025, d=0.430) and the right orbital PFC (U=98.000, z=−2.766, P=0.006, d=0.545) from viewing nature vs urban pictures in the depression group only. No significant Oxy-Hb concentration changes were detected with the anxiety group overall. However, a significant negative correlation (rho=−0.437, P=0.016) was detected between left PFC Oxy-Hb and changes in “comfortable” mood.

No significant between-group differences were detected in either HF (parasympathetic) or LF/HF (sympathetic) HRV measures for either the depression or anxiety participants.

Participants with comorbid depression and anxiety (n=10) reported significant improvements in the Relaxed mood scale but no other outcome measures. (Data not reported.)

Transparency

Authors stated there were no financial conflicts to disclose.

Practice Implications & Limitations

Mental-health issues present significant public health concerns in the modern world. Prevalence of depression and anxiety spiked during the Covid-19 pandemic and remains unacceptably high, especially for those under 40 years of age.1 Mood disorders also cost hundreds of billions of dollars in direct and indirect healthcare costs in the USA.2 Up to 30% of patients with mental-health conditions experience “treatment-resistant” forms,3 increasing the need for “alternative” (ie, complementary and/or integrative medicine) approaches to address these epidemics of mental ill-health.

Nature therapy (NT, aka, “nature-based therapy,” “eco-therapy,” “biophilic medicine”) is a broad category of methods exposing individuals to therapeutic and mentally “restorative” natural environments and stimuli.4,5 There are many varieties of NT application. Some, like the current study, use nature-based visual imagery (eg, pictures, video, virtual reality) or natural sounds and smells to elicit positive health responses.6,7 Other NT methods are holistic, real-world immersions in natural settings such as forest-bathing therapy,8 horticulture therapy,9 and wilderness/adventure therapy.10 All have demonstrated the capacity to significantly benefit mental (and physical) health status. 

The 2 most commonly researched explanatory mechanisms for the mental-health benefits of NT are described by the allostatic load-modulating Stress Reduction Theory11 and the neurocognitive Attention Restoration Theory.12 Both have their origins in E.O. Wilson’s psycho-evolutionary “Biophilia Hypothesis,” which supposes humans have an innate affinity for natural environments due to eons of adaptation to their ancestral living habitat.13

Millenia of anecdotal, direct human experience and decades of empirical evidence demonstrate salutogenic effects of nature on both positive mental-health constructs, such as happiness,14 vitality,15 and wellbeing,16 as well as the mental-illness states of depression and anxiety referenced above. There are also psychophysiological effects of NT, with multiple biomarkers reflecting nature’s positive health impact on cortisol,17 autonomic nervous system activity/HRV,18 brain imaging,19 and immune system and pro/anti-inflammatory cytokines,20 all of which influence mental-health status. 

Prevalence of depression and anxiety spiked during the Covid-19 pandemic and remains unacceptably high, especially for those under 40 years of age.

This current study presents interesting findings, being 1 of the first to demonstrate a relationship between subjective (CHAMT) and objective (Oxy-Hb) indicators of mental health in patients with mental-health diagnoses. The significant beneficial changes in these measures after viewing the nature scenes (but not the urban ones) suggest biophilic responses are occurring. 

The association of an improved subjective response in the nature group with an increase in prefrontal Oxy-Hb for depressed patients but a decrease in Oxy-Hb for anxiety patients shows that psychophysiological responses are not globally “good” or “bad” but must be understood in the context of what is being measured, which is consistent with other findings using this measure.21 Lack of significant differences in HRV results were surprising given the supportive data in most22 but not all23 studies on NT. This and other research24 suggest that Oxy-Hb and other biomarkers (eg, skin conductance respone [SCR], electroencephalogram [EEG]) may be better than HRV as measures of psychophysiological response to environmental stimuli. 

The results of this study are limited by the brief single exposure (15 seconds each, 3 minutes total) to nature and urban photographs. This is not a practical exposure time in real-world conditions. A study examining permanent placement of high-quality nature photographs in frequently viewed spaces (eg, office desk/cubicle, living-room mantle) would provide greater generalizability for real-world application to address chronic mental-health conditions. Conversely, repetitive exposure to similar images (eg, on a computer screensaver) would also provide information about potential long-term/longitudinal mental-health benefits. 

This study adds to the body of evidence demonstrating the therapeutic effect that viewing natural scenery has for individuals wanting to improve their mental health. Practitioners and patients, particularly those with otherwise treatment-resistant depression and/or anxiety, may want to incorporate this and other types of nature therapy into the therapeutic regimen. 

Conflict of Interest Disclosure

There are no conflicts of interest to report.

Categorized Under

Kurt Beil

ND, LAc, MPH

Kurt Beil, ND, LAc, MPH, is a naturopathic physician and licensed acupuncturist and holds a master’s degree in public health focused on the benefits of green space as a sustainable public-health promotion tool. He did his postdoctoral work at the Helfgott Research Institute on biomarker and psychometric assessment of the restorative and therapeutic effects of natural vs built urban environments. Dr Beil has been an advisor to the Children & Nature Network’s “Nature Research Database” and was the founding cochair of the Nature & Health subcommittee of the Intertwine Alliance in Portland. He also moderates a Facebook group NDs for Nature for the naturopathic medicine community on the clinical health benefits of contact with nature. Dr Beil maintains a clinical naturopathic and Chinese medicine practice in Happy Valley, Oregon, focusing on chronic disease. He can be reached via email or www.drkurtbeil.com.

References

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