Inducing physiological stress recovery with sounds of nature in a virtual reality forest — Results from a pilot study

Highlights

• Stress reactions were induced with virtual reality TSST.

• Virtual reality nature facilitated the recovery from stress.

• Nature sounds combined with virtual nature activated the parasympathetic system.

• Experimental studies on human-nature interactions may use virtual techniques.

Abstract

Experimental research on stress recovery in natural environments is limited, as is study of the effect of sounds of nature. After inducing stress by means of a virtual stress test, we explored physiological recovery in two different virtual natural environments (with and without exposure to sounds of nature) and in one control condition. Cardiovascular data and saliva cortisol were collected. Repeated ANOVA measurements indicated parasympathetic activation in the group subjected to sounds of nature in a virtual natural environment, suggesting enhanced stress recovery may occur in such surroundings. The group that recovered in virtual nature without sound and the control group displayed no particular autonomic activation or deactivation. The results demonstrate a potential mechanistic link between nature, the sounds of nature, and stress recovery, and suggest the potential importance of virtual reality as a tool in this research field.

Introduction

Estimates and predictions of current and forthcoming global burden of disease strongly stress the epidemics of non-communicable diseases, such as cardiovascular and mental disorders [1]. Many of these conditions are related to today's urbanised life-styles, where chronic stress has emerged as a critical risk factor [2]. This underlines the need for research concerned with stress and opportunities for stress recovery, in order to improve public health [3].

Nature and green environments have in several studies been related to stress relief [4], [5] and recent research has indicated increased neurophysiological vulnerability to social stress in an urban compared to a rural population [6].

Different theories have driven research into the correlation of nature and health [7], [8], [9], [10]. Many of these theories are rooted in the tradition of the natural environment's psychological values, but they are also often linked to theories of stress, mental fatigue, and restoration. Recovery in green environments has been proposed as particularly effective due to certain inherent qualities of nature, such as noise reduction and spontaneous induction of positive emotions [11], [12], [13]. Existing theories and studies concerned with health and the natural world reflect a connection between the physiology of stress and the potential health benefits to be derived from nature, although those pathways are not fully revealed.

Natural environments are dynamic settings that may be inconvenient locales for using sophisticated research equipment. By simulating a natural environment in a setting where complex research methods could function under controlled conditions, we might be able to better understand what components of nature are conducive to stress recovery. At the same time, we could also study the physiological mechanisms that operate when humans interact with nature.

However, the question arises whether a simulated natural environment would produce the same effects as a genuine one, and if so what sensory input is necessary to provide this sense of realism? In the past, the greatest emphasis has been placed on static modes of simulation, such as photographs, sketches, or slides [14], [15]. Several studies support the suggestion that descriptive and evaluative responses, as well as preferences, are comparable between simulations and authentic presentations [16], [17]. However, evidence concerning physiological and behavioural responses to the environment is less clear. Compared to static simulations, virtual environments (VEs) provide a more dynamic alternative with greater ecological validity, that is, approximating the real-life situation [18]. The experience of actually being in the place depicted by the medium is also considered to be higher in VEs. The latter phenomenon is referred to as presence in the virtual reality (VR) research community [19], and is something that has an influence on behavioural and physiological response [20]. Presence is believed to be correlated to immersion [21], [22], i.e., the extent to which computer displays are capable of delivering an inclusive, extensive, all-embracing vivid illusion to the human senses. The higher the immersion of a VR system, the better the restorative potential that can be expected from the mediated natural environment [23].

Very little research has been done on whether the quality of a virtual simulation influences its restorative effect [20]. An earlier investigation supported the idea that an increased level of realism can be achieved by adding other modalities (e.g., auditory) than just visual [24].

Soundscape is a complex concept, relating to varied auditory input, such as noise, music, and sounds of nature. Several studies have proven the detrimental health effects of environmental noise, but some research has also considered positive aspects of sound where natural sounds are consistently perceived as pleasant and technological noise as mostly unpleasant. Especially birdsong and sound of water seem to induce positive reactions [25], [26]. Such natural sounds have been used in stressful situations like surgical procedures, and have demonstrated stress-relieving effect via the autonomic nervous system. Several other examples of sounds of nature being used as stress-reducing components exist [27], [28], [29].

A recent functional magnetic resonance imagining (fMRI) study found that a visual context can modulate connectivity of the auditory cortex with other regions of the brain that are implicated in the generation of subjective states, especially tranquility [30]. This suggests a relationship between objective multimodal sensory input and individual mental states.

A variety of tests have been developed to provoke stress reactions for research purposes. The Trier Social Stress Test (TSST) is a highly standardised, validated, and widely used protocol for inducing social stress in laboratory settings [31]. It has consistently been proven to activate the hypothalamus–pituitary–adrenal (HPA) axis and the sympatho–adrenal–medullary (SAM) system [32], [33], [34], along with the corresponding endocrine and cardiovascular responses. It requires the test participant to hold a speech and do an arithmetic problem in front of an audience. The audience consists of three actors who show no emotional response whatsoever to the test participant, making the situation very stressful.

In this study we used a recently developed virtual form of TSST to induce acute stress [35], [36], and explored autonomic and endocrine stress and recovery responses together with subjective ratings of stress Recovery was studied in three different conditions: a virtual forest including congruent sounds; the same virtual forest with no sounds; and a control condition with no virtual forest or sounds.

Autonomic and endocrine stress reactivity was assessed by heart rate, T-wave amplitude, heart rate variability parameters, and saliva cortisol, together with subjective ratings of stress [37], [38], [39], [40].

We hypothesised that stress recovery after a virtual stress provocation could also be facilitated in a virtual green environment, and that stress recovery would be further facilitated by adding sounds of nature to the virtual green environment. We supposed this would be partly due to the effect of such sounds themselves [27] and partly to the resulting increased sense of reality in the virtual environment. We decided to use sounds of birdsong and water, since this had previously been related to feelings of relaxation and those sounds were also connected to the virtual green environment we used — a forest-like setting with a water stream.