The systematic review will follow the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement (21, 22). Moreover, the protocol has been already registered on PROSPERO, with the registration number CRD42020213197.
Eligibility Criteria
The eligibility criteria followed the PICOS checklist (Population, Intervention, Comparison, Outcome, Study Design) described by PRISMA (21) (Table 1). Concerning the population, the included studies will involve the non-clinical population. Therefore, all the studies dealing with a population affected by physiological (e.g., Alzheimer, cancer, HIV) or psychological (PTSD, anxiety, stress-related disorders) will be excluded.
The interventions considered eligible will relate the exposure to virtual nature through 360° images, biophilic elements, VR pre-recorded videos, or immersive environment, excluding the augmented reality. No restrictions will be considered about the possibility of inducing the stress. Moreover, this systematic review does not frame the intervention into a psychological perspective, thus cognitive behavioural techniques (CBT) and mindfulness program are excluded.
Regarding the comparison, the studies considered eligible will compare the use of virtual nature with non-virtual landscapes, urban landscapes, real nature landscape conditions, or other kind of stimuli (e.g., sound-related stimuli). The studies that compare also physical exercise or movement will be excluded.
The measured outcome will be the reduction of stress, the restoration and the relaxation effects, measured objectively (through physical measures) or subjectively (e.g., self-report questionnaires on stress levels). Physical measures will include the heart-rate variability (HRV), systolic and diastolic blood pressure variation, salivary alpha amylase activity, salivary cortisol, heart rate, galvanic skin responses. No exclusion criteria are forecasted. For the psychological responses, the preferred criteria will be the use of standardized questionnaire about stress.
The studies considered eligible are all the intervention studies with quantitative outcomes, from scientific journals and indexed conference papers. Reviews, meta-analyses, abstracts, book reviews, and editorials will be excluded.
Table 1
Eligibility Criteria according to Population, Intervention, Comparison, Outcome, and Study Design (PICOS) criteria;
PICOS
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Inclusion Criteria
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Exclusion Criteria
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Population
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Non-clinical population
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Population affected by physiological (e.g., Alzheimer, cancer, HIV) or psychological (PTSD, anxiety, stress-related disorders) diseases.
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Intervention
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Virtual nature through 360° images, biophilic elements, VR pre-recorded videos, or immersive environment
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Augmented reality, Cognitive Behavioural Theory, Mindfulness program
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Comparison
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Comparison between the use of virtual nature with non-virtual landscapes, urban landscapes, real nature landscape conditions, or other kind of stimuli (e.g., sound-related stimuli)
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VR combined with physical exercise or movements
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Outcome
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Physiological stress reduction
Psychological stress reduction (standardised questionnaires preferred)
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No restrictions
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Study design
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Quantitative studies
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Reviews, meta-analyses, abstracts, book reviews, and editorials will be excluded.
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Information Source
The information sources will include Web of Science, PubMed, and Scopus. Data screening will concern the selection of manuscript based on the title and topic of natural virtual reality and stress reduction.
Search Strategy
The following groups of keywords will be searched in combination with the Boolean indicator AND:
Keywords1: “virtual reality”, “virtual nature”, “virtual landscape”, “immersive environment”;
Keywords2: “natural environment”, “biophilic environment”, “forest”, “nature”;
Keywords3: “stress reduction”, “stress relief”, “restoration”, “restorative”, “relaxation”;
Each search keyword will be used in combination with another keyword term of the other groups on PubMed, Web of Science, or Scopus, as showed in the following string example: ((“virtual reality” OR “virtual nature” OR “virtual landscape” OR “immersive environment”) AND (“natural environment” OR “biophilic environment” OR “forest” OR “nature”) AND (“stress reduction” OR “stress relief” OR “restoration” OR “restorative” OR “relaxation”).
Study record
Data management
The manuscripts will be collected by two independent Authors (AG and AN), that will perform all the steps to ensure that the manuscript selection will be not affected by the researcher characteristics. First, the selection will be performed through EndNote (EndNote version X7.5; Thompson Reuters, New York, USA) and the function “find duplicated” will be selected. Afterwards, a manual selection of the eligible manuscripts will be performed by the two investigators, considering the main aim of the systematic review, following three steps: the title screening, the abstract screening, and the full-text screening. The discrepancies between the two investigators will be solved by the involvement of the other investigators (AB and PN). The investigators will be aware of all the information contained within the manuscripts, such as study title, authors, and affiliations. A PRISMA flow diagram will be used to represent the search strategy and the resulting outcomes.
Selection process
The studies will be considered if they are written in English, independently from the country of origin of the study. The selection process will concern the acquisition of each study and then its examination according to the inclusion and exclusion criteria.
Data collection process
Data collection will be performed according to the following procedure. Information about the lead author, year of publication, type of study, study design (between/within participants) sample size, conditions forecasted by the study, stress induction procedure, objective measures of stress, subjective measures of stress will be extracted and included into a Microsoft Excel (Microsoft Corp, Redmond, Washington) spreadsheet.
Risk of Bias
The risk of bias will be evaluated through the Cochrane risk of bias tool (23), which considers six potential sources of bias, namely: selection bias, performance bias, detection bias, attrition bias, reporting bias, and other bias. Each source is identified by one or more items whose assessment is divided in two parts: a free-text judgment of the risk of bias and a judgment on a scale of high, low, or unclear risk of material bias. Two independent Authors (AG and AB) will complete the tool, and a rank correlation will be performed to evaluate the agreement between studies. Relevant discrepancies will be solved through the involvement of the other two Authors (PN and AN).
Data synthesis
All the collected information will be summarized through tables, with the aim of showing the general effect produced by virtual nature on stress reduction. Tables will include information about each study, the principal methodological features, and the main results delivered. The considered outcomes within the current systematic review will concern the physiological and psychological effectiveness of VR nature. A narrative description of the outcomes, both from physiological and psychological measures, will be provided. Moreover, a description of other features significantly affecting the results (such as sound of nature, brightness level of the landscape) will be discussed.