Nature experience reduces rumination and subgenual prefrontal cortex activation

I do about 2 hours daily, my rumination is about my social life and where I'm going next, not my disability. But have they tested a 90 minute walk in nature swatting mosquitoes? For us stroke survivors this is a real problem, no ability to swat mosquitoes landing on the good arm. How often is your doctor or therapist getting you out into nature to stop your rumination of the problems you are having from your stroke?

Nature experience reduces rumination and subgenual prefrontal cortex activation

Gregory N. Bratman, J. Paul Hamilton, Kevin S. Hahn, Gretchen C. Daily, and James J. Gross

PNAS July 14, 2015 112 (28) 8567-8572; published ahead of print June 29, 2015 https://doi.org/10.1073/pnas.1510459112

1. Contributed by Gretchen C. Daily, May 28, 2015 (sent for review March 9, 2015; reviewed by Leslie Baxter, Elliot T. Berkman, and Andreas Meyer-Lindenberg)

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Significance

More than 50% of people now live in urban areas. By 2050 this proportion will be 70%. Urbanization is associated with increased levels of mental illness, but it’s not yet clear why. Through a controlled experiment, we investigated whether nature experience would influence rumination (repetitive thought focused on negative aspects of the self), a known risk factor for mental illness. Participants who went on a 90-min walk through a natural environment reported lower levels of rumination and showed reduced neural activity in an area of the brain linked to risk for mental illness compared with those who walked through an urban environment. These results suggest that accessible natural areas may be vital for mental health in our rapidly urbanizing world.

Abstract

Urbanization has many benefits, but it also is associated with increased levels of mental illness, including depression. It has been suggested that decreased nature experience may help to explain the link between urbanization and mental illness. This suggestion is supported by a growing body of correlational and experimental evidence, which raises a further question: what mechanism(s) link decreased nature experience to the development of mental illness? One such mechanism might be the impact of nature exposure on rumination, a maladaptive pattern of self-referential thought that is associated with heightened risk for depression and other mental illnesses. We show in healthy participants that a brief nature experience, a 90-min walk in a natural setting, decreases both self-reported rumination and neural activity in the subgenual prefrontal cortex (sgPFC), whereas a 90-min walk in an urban setting has no such effects on self-reported rumination or neural activity. In other studies, the sgPFC has been associated with a self-focused behavioral withdrawal linked to rumination in both depressed and healthy individuals. This study reveals a pathway by which nature experience may improve mental well-being and suggests that accessible natural areas within urban contexts may be a critical resource for mental health in our rapidly urbanizing world.

• environmental neuroscience
• nature experience
• rumination
• psychological ecosystem services
• emotion regulation

Never before has such a large percentage of humanity been so far removed from nature (1); more than 50% of people now live in urban areas, and by 2050, this proportion will be 70% (2). What are the potential mental health implications of this demographic shift? Although urbanization has many benefits, it is also associated with increased levels of mental illness, including anxiety disorders and depression (3⇓–5). Causal mechanisms for this increased prevalence of mental illness are likely manifold and are not well understood (6, 7).

One aspect of urbanization that has attracted research attention in recent years is a corresponding decrease in nature experience (8, 9). Using a variety of methodologies, researchers have demonstrated affective and cognitive benefits of nature experience, thereby contributing to an evolving understanding of the types of psychological benefits of which humanity may be deprived as urbanization continues. Correlational findings show that growing up in rural vs. urban settings is associated with lesser stress responsivity (3). A recent longitudinal study, tracking the well-being and mental distress of more than 10,000 people over a period of nearly two decades demonstrates a significant positive effect of proximity to greenspace on well-being (9). This effect traces to living location within the same individuals as they moved closer or further from greenspace. Other correlational studies reveal that window views that include natural elements (compared with window views that do not) are associated with superior memory, attention, and impulse inhibition (10), as well as greater feelings of subjective well-being (11). These correlational findings are buttressed by experimental findings showing, for example, that nature experience (usually in urban greenspace) can improve memory and attention (12) and increase positive mood (13). Experimenters also have used psychophysiological methods to characterize the ways in which images and sounds of the natural environment lead to decreased stress and negative emotion after participants have been subjected to stressful stimuli (14, 15). Taken together, these and numerous other studies provide compelling evidence that nature experience may confer real psychological benefits.

Although this body of work is now substantial, there remains a fundamental yet unanswered question: by what mechanism(s) might nature experience buffer against the development of mental illness? One possible mechanism—and our focus here—is a decrease in rumination, a maladaptive pattern of self-referential thought that is associated with heightened risk for depression and other mental illnesses (16⇓–18) and with activity in the subgenual prefrontal cortex (sgPFC) (19). The sgPFC has been shown to display increased activity during sadness (20) and the behavioral withdrawal and negative self-reflective processes tied to rumination in healthy (21) and depressed (22⇓–24) individuals.

Rumination is a prolonged and often maladaptive attentional focus on the causes and consequences of emotions—most often, negative, self-relational emotions (25). This pattern of thought has been shown to predict the onset of depressive episodes (17), as well as other mental disorders (26). Positive or neutral distraction (vs. maladaptive distractions such as binge drinking of alcohol) has been shown to decrease rumination (27). To be effective in decreasing rumination, these positive or neutral distractions must be engrossing, to maintain the shift of attention onto the distracting stimuli (27). From this perspective, we aimed to observe whether a 90-min nature experience has the potential to decrease rumination. In addition to gathering self-report measures, we examined brain activity in the sgPFC, an area that has been shown to be particularly active during the type of maladaptive, self-reflective thought and behavioral withdrawal that occurs during rumination (19). This behavioral and neural evidence—when taken together—would provide convincing evidence for a change in rumination resultant from nature experience.

We quantified the impacts of a brief nature experience on rumination and neural activity in the sgPFC through a controlled experiment, comparing changes that occur in a 90-min nature walk to those in a 90-min urban walk. We hypothesized that we would observe decreased rumination and decreased neural activity in the sgPFC for urban residents who experienced a nature walk, whereas we would not observe such a decrease in those who experienced an urban walk. We obtained measures of individuals’ self-reported levels of rumination using the rumination portion of the Reflection Rumination Questionnaire (RRQ) (28). We documented activity in the sgPFC by using a neuroimaging method called arterial spin labeling (ASL, presented more fully in Methods), through which we measured regional cerebral blood flow (rCBF): the volume of cerebral blood passing through the region of interest. This technique can detect effects associated with longer-lasting psychological phenomena such as rumination, in contrast to momentary, reactive emotional responses such as a startle response (29).

Thirty-eight healthy participants took part in the study. Although rumination is often studied in the context of clinically depressed individuals, we studied participants with no history of mental disorder to broaden the applicability of our findings. Our sample comprised individuals residing in urban environments. We posited that these individuals, although currently healthy, would enter the study with a somewhat elevated level of rumination resulting from the ongoing and chronic stressors associated with urban experience, and their corresponding deprivation of regular contact with nature. We therefore hypothesized that a nature experience would reduce the baseline rumination levels of such participants, compared with those who had an urban experience.

On arrival at our laboratory, each participant completed a self-report measure of rumination (RRQ) and underwent our scanning procedure. We then randomly assigned each participant to a 90-min walk in either a natural environment (19 participants) or urban environment (19 participants). The nature walk took place near Stanford University, in a greenspace comprising grassland with scattered oak trees and shrubs. The urban walk took place on the busiest thoroughfare in nearby Palo Alto (El Camino Real), a street with three to four lanes in each direction and a steady stream of traffic (Fig. S1). After the walk, each participant returned to the laboratory and provided a second, follow-up self-report of levels of rumination (RRQ) before undergoing a second resting-state ASL scan. Transportation to and from the walk was via a car ride of 15-min duration (for both walks). Participants were given a smartphone and told to take 10 photographs during their walk (Fig. S2). These photographs were used to verify that participants went on the walk. We also tracked the phone itself during the walk, as further verification that the correct route was taken by each participant.

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Fig. S1.

Satellite maps of (A) the nature walk and (B) the urban walk. Both walks were a total of 5.3 km.

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Fig. S2.

Sample photographs taken by participants during (A and B) the nature walk and (C and D) the urban walk.

Results

To analyze the impact of nature experience on self-reported rumination, we conducted a two-way ANOVA, with time as a within-subjects factor (before vs. after the walk) and environment as a between-subjects factor (nature walk vs. urban walk). This analysis revealed an interaction between time and environment [F(1,35) = 3.51, P = 0.07, η2p

= 0.09]. Consistent with our hypothesis, follow-up t tests indicated that our results were driven by a decrease in self-reported rumination for the nature group but not for the urban group (Fig. 1A). There was a simple effect of time for the nature group [t(17) = −2.69, P < 0.05, d = 0.34; mean change pre- to postwalk = −2.33, SE = 0.55; mean score prewalk = 35.39, SE = 1.60; mean score postwalk = 33.06, SE = 1.61], with decreases from pre- to postwalk. There was no such effect for the urban group (mean score prewalk = 30.11, SE = 2.61; mean score postwalk = 30.16, SE = 2.50).