Discussion
The majority of research exploring the impact of nature on ANS mechanisms has included an exercise component which limits the conclusions drawn about the contribution of the “nature” component.
(15, 17, 39) As exercise has such positive effects on health parameters, it is often hard to isolate the effects solely from the nature component. Therefore, this study looks at the nature contribution to ANS function without complicating the effects by combining it with exercise.
The main finding of this study was that HRV as a marker of ANS function increased during stress recovery, if nature scenes were viewed prior to a stressor, compared to built scenes. This is the first study to suggest that simply viewing scenes of nature prior to a stressor enhances recovery of ANS function poststressor.
The interaction of nature and recovery from stress has been studied previously.
(30, 32) However, previous research has assessed the restorative effects of viewing or interacting with nature during the actual recovery period following a stressor.
(29, 30, 32) For example, viewing video footage of nature scenes for 10 min directly after being exposed to a film of stressful images increased heart period (i.e., decreased HR), suggested to be due to enhanced parasympathetic system activity.
(30) Viewing nature through a window during a 5 min rest period following cognitive tasks was also more effective at reducing HR.
(29) Both authors postulate that these observations are a consequence of cognitive recovery or attention restoration occurring while nature scenes are viewed. The results of the present study suggest greater HRV during viewing nature scenes as contributed to by both sympathetic and parasympathetic systems. It is only during the recovery period that parasympathetic activity alone is greater in the nature views condition. Therefore, viewing nature scenes may encourage future healthy stress responses and recovery patterns and could act as a vital tool in preventive health.
The secondary research question relates to the duration of the nature dose. Research shows that a 5 min dose of nature offers the greatest increases in self-esteem and mood.
(31) Physiologically, 5 min of viewing images of nature is known to increase HRV.
(16) However, it is unknown whether an additional 5 min would enhance the initial changes that occur in the first 5 min dose. Within a laboratory setting, it appears that the second 5 min of exposure is less effective in inducing HRV changes. The strength of the current study is the use of 5 min segments for HRV analysis which is recommended in short-term analysis, i.e., less than 24 h.
(22) Furthermore, the division of exposure to nature images into 5 min segments enabled the observation of increased HRV as anticipated in an initial 5 min dose, thus supporting previous work.
(16) Comparisons in the current study suggest an additional 5 min of exposure to nature does not enhance the greater HRV seen during the initial 5 min dose.
In agreement with previous research,
(10, 31) the current study observed improvements in self-esteem with exposure to nature. This finding suggests enhanced self-esteem associated with viewing nature shows robustness against exposure to a mild stress. In the current study, there was no change in mood associated with condition although this has been shown in previous studies.
(10, 31, 39) The changes in mood observed in these studies were measured using the profile of mood states (POMS).
(10, 31, 39) POMS contains 5 subscales associated with negative mood and 1 subscale for positive mood. This restricts POMS to predominantly reflect changes of negative mood rather than positive mood. In the current study, negative affect, measured using PANAS, showed negative mood improved irrespective of condition, therefore not replicating previous observations using POMS. The use of positive affect scores from PANAS was to ascertain if changes in positive mood occur that may not be so clearly identified using POMS. Positive mood did not differ between conditions nor did it change over time. In contrast, a meta-analysis of five studies did observe improvements in positive affect
(8) suggesting the manipulation in the current study was not strong enough to elicit positive changes. The use of PANAS in laboratory research of this nature might not be appropriate.
Previous literature lacks discussion as to the potential mechanisms behind observed changes in physiological function while viewing nature. One potential mechanism to explain alterations in physiological measures could be attributable to the restorative properties of the nature scenes. The concept of attention restoration occurring after exposure to nature has previously been demonstrated by way of improved performance in attention related cognitive tasks.
(20, 32) Viewing scenes of nature for 10 min, following a period of mentally fatiguing tasks, improved performance in a backward digit-span memory task.
(32) In the current study, there were no such observations of altered cognitive ability, i.e., performance in the mental task, with different viewing conditions. This may be attributable to the lack of a mentally fatiguing task prior to the intervention, but the nature scenes acted as an effective buffer to ANS function during recovery.
To date, there are only a handful of studies that have measured or inferred changes in the ANS
(14, 16-18) associated with nature. We suggest that the different components of the environmental stimulus, e.g., visual, cognitive, emotional, and restorative properties, induce changes in the regulation of different areas throughout the brain thus altering ANS function. The findings of this study, alongside previous studies,
(14, 16-18) suggest a top-down mechanism originating in higher centers of the brain. Evidence for this was in part obtained from a study which utilized fMRI while viewing urban scenes.
(40) The urban scenes caused increased activity in the amygdala compared to viewing nature scenes.
(40) This action is likely to cause alterations in ANS control such as those seen in the current study,
(41) through changes in parasympathetic and sympathetic outputs. Inhibition of the parasympathetic nervous system arises from the frontal cortex, and the pathways pass through the amygdala and then to the
nucleus tractus solitarii and
nucleus ambiguus.
(41) The prefrontal cortex is prominent in threat-avoidance situations, causing inhibition on HR via the vagus nerve.
(42) Therefore, during periods of threat, parasympathetic activity is decreased (increasing HR). The results of the current study suggest the absence of threat during nature viewing, without decreases in parasympathetic activity, while during built views overall variability decreased in the first 5 min. This interaction may be primarily due to alterations in both the frontal cortex and the amygdala.
Visual properties of an image may also play a role, as the composition of a picture can alter activity in the visual cortex. Images of nature are less aversive and uncomfortable when examining their spectral properties compared to built images.
(43) Indeed, recent research suggests that the primitive characteristic of color, in particular the “greenness”, of a nature image is associated with improved mood.
(39) Through color perception and reduced impact on the visual system, images of nature may evoke lower activity in the amygdala and visual cortex
(40) culminating in increased parasympathetic activity as seen in the current study.
It is unknown how long the physiological changes that nature evokes are maintained, but it will be vital to explore this further, especially if nature is to be considered as a therapy. In the current study, unlike previous studies, nature exposure was experienced 10 min before the stressor, not during the stressor or immediately following the stressor. The evidence from this study suggests that there does indeed appear to be a buffering effect of nature. A stronger stimulus, i.e., within the environment itself, may prolong the buffering effect and also induce greater changes in cardiovascular measures. This may also be the case when nature is combined with exercise (“green exercise”). Exercising while viewing nature reduces BP in the 5 min following the exercise period in comparison to viewing built images.
(15) These effects again may be even greater following “real” exposure to nature. Indeed, recent research suggests that adrenaline, noradrenaline, and BP still remain reduced in the evening following a daytime walk in a forest field.
(18) Interestingly, forest walking increases natural killer cell activity for a period of 30 days in males and 7 days in females.
(44)
This study has some limitations that should be addressed in future research. The impact of individual beliefs on the regulation of emotions when viewing the different environments, and how this effects physiological modulations, is unknown and could pose a mediating factor to the effectiveness of viewing nature on improving stress recovery. In order to quantify individual relationships with nature, the nature relatedness scale
(46) could be used to indicate experience, beliefs, and contact with nature. Complementary information could be gained by noting home postcode, and thus, the surrounding area could be assessed in terms of land usage to explore the potential impact it may have on participants’ perception of nature. The present study collected postcode data but does not have a sufficient population size to draw conclusions about all potential subsets. The majority of studies to date, including the present study, use extreme examples to depict natural and urban environments in order to examine the influence of nature. Investigating a greater variety of environments (including more urban green spaces) would add population level validity to the results and account for individual landscape preferences. The restorative properties of nature (images or within the location itself) may evoke different psychological, cognitive, and physiological responses. Further studies would benefit from the inclusion of a questionnaire to assess how restorative the scenes or places are perceived to be, as suggested by Hartig, Mang, and Evans,
(47) in combination with physiological and psychological responses to these different images.
The current study suggests that nature itself may evoke physiological responses, which may be in part driven by psychological reactions and restorative properties of nature. Furthermore, the increase in parasympathetic activity in the recovery from a stressor may help to counteract a buildup of psychological stress and thus reduce the impact of stress on physical and mental health. This would likely occur by nature images encouraging a healthier stress recovery pattern.
(45) If nature increases autonomic recovery to stress and thus is an effective coping mechanism, this provides an argument for the need for more nearby nature. A green view through a workplace window, small pockets of greenspace in the home and workplace, and accessible local parks could be effective tools in altering ANS control of the heart. The buffering effect of nature could have particular relevance for the workplace where it may be beneficial to utilize nature during the lunch break, prior to a stressful afternoon, to help enhance recovery of autonomic function.
