The results of the present PRISMA scoping review are presented in Table 1 (Description of the studies) and Table 2 (Study goal, psychological and physiological measures, and main results).
Table 1
Description of the studies.
First author (year) | Sample size | Gender | Age | Region | Setting |
Blanke (2022) | 26 | 7 Females, 19 Males | Mdn = 55, IQR = 18 (total sample) | Switzerland | Inpatients |
Blum (2019) | 60 | 31 Females, 29 Males | M = 33.5, SD = 9.4 (total sample) | Germany | Outpatients |
Blum (2020) | 72 | 56 Females, 16 Males | M = 21.6, SD = 4.3 (total sample) | Germany | Outpatients |
de Zambotti (2022) | 52 | 32 Females, 20 M Males | M = 18.4, SD = 0.7 (total sample) | United States | Outpatients |
Hu (2021) | 40 | 21 Females, 19 Males | M = 28, SD = 4 (total sample) | United States | Outpatients |
Kniffin (2014) | 63 | 63 Females, 0 Males | M = 18.97, SD = 1.20 (total sample) | United States | Outpatients |
Lan (2021) | 20 | 10 Females, 10 Males | M = 20.6, SD = 2.3 (control group) M = 22.0, SD = 1.2 (experimental group) | Taiwan | Outpatients |
Russel (2014) | 60 | 48 Females, 12 Males | M = 18.8, SD = n.s. (control group) M = 19, SD = n.s. (experimental group) | United States | Outpatients |
Shiban (2017) | 29 | 24 Females, 5 Males | M = 43, SD = 9.96 (control group) M = 34.2, SD = 9.81 (experimental group) | Germany | Outpatients |
Soyka (2016) | 21 | 10 Females, 11 Males | M = 29, SD = n.s. AR = 20–45 | Germany | Outpatients |
Stromberg (2015) | 43 | 35 Females, 8 Males | AR = 18–27 | United States | Outpatients |
Tinga (2019) | 60 | 37 Females, 23 Males | M = 22.07, SD = 3.03 (total sample) | The Netherlands | Outpatients |
Weerdmeester (2021) | 112 | 11 Males, 101 Females | M = 20.84, SD = 2.42 (control group) M = 20.58 SD = 2.51(experimental group) | The Netherlands | Outpatients |
Weerdmeester (2022) | 67 | 48 Females, 16 Males, 3 n.s. | M = 21.27, SD = 2.84 (total sample) | The Netherlands | Outpatients |
M = mean |
SD = standard deviation |
Mdn = median |
IQR = interquartile range |
AR = age range |
n.s. = not specified |
Table 2
Study goal, psychological and physiological measures, and main results.
First author (year) | Study goal | Study design | Psychological measurements | Physiological measurements | Results |
Blanke (2022) | The study aimed to evaluate the efficacy and feasibility of a VR intervention based on breathing synchronization via gender-matched virtual bodies to alleviate dyspnea in patients recovering from COVID-19 pneumonia | RCT (cross-over) Participants were randomly assigned either to the “asynchronous - synchronous” sequence or the “synchronous - asynchronous” sequence. | • Breathing comfort and discomfort (ad-hoc items) • Breathing awareness and agency (ad-hoc items) | • RR • RRV | The study demonstrated that VR intervention improved patients' breathing comfort and satisfaction, indicating its potential as a safe respiratory rehabilitation tool. |
Blum (2019) | The study aimed to determine if incorporating VR in biofeedback therapy using heart rate variability (HRV) improves relaxation and attentional resources. | RCT (between subjects) Participants were divided into two groups, with one receiving treatment through a standard computer screen and the other through a VR headset in a virtual beach setting. | • Relaxation (State Trait Anxiety Inventory) • Relaxation self-efficacy (ad-hoc VAS scale) • Mind wandering (Cognitive Interference Questionnaire) • Focus on the present moment (State Mindfulness Scale) • Attentional resources (modified Stroop task) | • HRV • RMSSD • Cardiac coherence | The results showed that both implementations effectively increased relaxation, but there was no significant difference between them. However, the VR-based implementation led to greater improvement in relaxation self-efficacy, focus on the present moment, and reduced mind wandering compared to the standard implementation. Participants in the VR-based implementation also performed better in a Stroop task, indicating improved attention. There was no significant difference between the two implementations in terms of cardiac coherence and vagal tone. |
Blum (2020) | The study aimed to investigate the efficacy of a VR-based diaphragmatic breathing biofeedback algorithm. | RCT (between subjects) Participants were assigned to a focused breathing exercise with and without respiratory biofeedback in VR. | • Focus on the breath (ad-hoc VAS scale) | • HRV • RMSSD • LF and HF • Abdominal movement | Participants in the feedback group reported greater success in maintaining their attention on the breath compared to the control group. Objective measurements showed a higher relative share of abdominal movements in the feedback group, indicating increased utilization of the diaphragm. The feedback group also exhibited a slower breathing rate and increased heart rate variability, suggesting a more calm and regular breath with enhanced parasympathetic activity. |
de Zambotti (2022) | To investigate the effects of a VR-based mind-body approach on reducing bedtime arousal in adolescents with insomnia | Counterbalanced Participants completed two counterbalanced sessions: an intervention session involving nature-based VR-guided meditation and paced breathing, and a control session without any intervention (watching tv or reading a book). | • Cognitive arousal (Pre-Sleep Arousal Scale) • Alertness, positive and negative mood and sleepiness (Daytime Insomnia Symptom Scale) | • EEG • HR • HRV • LF and HF • Salivary cortisol • SBP and DBP | The study found that the VR-guided meditation and paced breathing intervention led to increases in HRV, indicating vagal activation, as well as changes in EEG activity, including suppression of alpha power and increases in high-frequency synchronization (beta and gamma activity). These changes reflected attentional processes and information processing during meditation. Pre-to-post intervention analyses showed reductions in heart rate and cortisol levels, suggesting relaxation and downregulation of the autonomic nervous system. The intervention did not significantly affect cognitive arousal or EEG measures related to cortical hyperarousal. |
Hu (2021) | The aim of the study was to compare the effectiveness of the two methods in modulating the patients’ pain thresholds in the same study design and clinical environment | RCT (between-subjects) The study involved healthy subjects, with one group engaging in mindfulness-based (TMB) and another in VR-based (VRB) through an in-house developed 3D VR lung that synchronized with breathing cycles in real-time, | • Pain (McGill Pain Questionnaire) • Serenity and fatigue (Positive and Negative Affect Schedule) | • fNIRS • HRV • Pain threshold | Both interventions effectively increased pain thresholds in the participants, although they had different effects on serenity scores. The TMB group showed increased serenity accompanied by increased fatigue, while the VRB group did not show a significant change in serenity scores. The brain activation patterns during pain stimulation suggested that TMB modulates attention and contextual evaluation of internal sensory events, inhibiting sensory-discriminative pain processing. VRB induces immersive exteroception, diminishing functional connections with the S1 and weakening its pain processing function. |
Kniffin (2014) | This article discussed the use of virtual reality (VR) to explore how self-regulatory skills, such as diaphragmatic breathing, can influence female participants' reactions to a high-risk encounter with an aggressive male | RCT (between-subjects) Female undergraduate students were randomly assigned to either diaphragmatic breathing training or attention control. | • Negative affect (negative affect items from the Positive and Negative Affect Schedule) • Level of confidence of resisting and avoiding sexually risky situations (Sexual self-efficacy rating) | • HR • HRV • RR | The study found that training in diaphragmatic breathing led to slower respiration rates and higher HRV compared to the control group. Participants in the breathing group had stable self-regulation capacity, while the control group showed gradual improvement over time. During the stressful VR role-play, both groups reported increased negative affect, but contrary to expectations, the control group reported higher self-efficacy for managing the confrontation. Following the VR role-play, the breathing group exhibited higher HRV and slower respiration rates. |
Lan (2021) | The aim of the study was to evaluate the effectiveness of a VR multimodal system, providing users with tactile feedback based on real-time sensor data from pressure sensors, in improving the user’s breathing control, sleepiness and changing EEG | Between-subjects Participants were assigned to the feedback group or the control group with no feedback | - | • RR • EEG • Eye-blinking frequency | Participants in both the control and feedback groups improved their breathing rate. The feedback group showed a significant decrease in breathing rate compared to the control group. The standard deviation of breathing rates also decreased over time for both groups. However, no statistical difference was observed in the theta-to-alpha power ratio, indicating that the system did not significantly affect the meditation aspect. Eye-blinking frequency was used as an indicator of sleepiness, and the feedback group had a significantly lower frequency, suggesting that the system helped reduce sleepiness during training. |
Russel (2014) | The study aimed to apply diaphragmatic breathing as a self-regulatory strategy and measure its effects on physiological, cognitive, and behavioral responses in a motion sickness-inducing environment | RCT (between subjects) Participants were randomly divided into the diaphragmatic breathing condition, received instructions on diaphragmatic breathing techniques, and the control condition, receiving instructions on being aware of their surroundings as an attention control manipulation. | • Motion sickness management self-efficacy (ad hoc items) • Motion sickness (Motion Sickness Assessment Questionnaire) • Motion sickness during VR (self-rating scale) | • HR • HRV • RR | The diaphragmatic breathing protocol increased parasympathetic nervous system tone, slowed respiration rate, and decreased development of motion sickness symptoms as compared to the control condition. In the diaphragmatic breathing condition, a significant positive correlation indicates that an increased respiratory rate was associated with more motion sickness symptoms. |
Shiban (2017) | The goal of the study was to investigate the role of a relaxation technique, diaphragmatic breathing (DB), as a coping mechanism during VR exposure therapy (VRET) of aviophobia | RCT (between-subjects) Participants were randomly assigned to one of the two treatment groups: VRET (informed about the importance of exposure without distraction) or VRET + DB (receiving information about DB as a coping strategy during exposure). | • Fear of flying (Fear of Flying Scale) • Self-efficacy (ad-hoc items) • Fear during the VR flights (ad-hoc VAS scale) | • HR • SCL • RR | Both the VRET group and the VRET + DB group showed a significant reduction in fear of flying, with a slightly greater reduction observed in the VRET + DB group. Patients in both groups experienced a reduction in subjective fear ratings and heart rate during exposure therapy sessions, indicating the effectiveness of VRET. The study also found that heart rate decreased during exposure trials, while skin conductance level remained relatively constant, reacting only at the onset of turbulence. Self-efficacy did not differ significantly between the groups, indicating that other cognitive and physiological factors may contribute to the enhanced treatment effects observed in the VRET + DB group. |
Soyka (2016) | This research aimed to evaluate the efficacy of a VR application, specifically focusing on an underwater virtual world for paced breathing techniques, compared to standard breathing techniques, for stress management | Between-subjects Participants were divided into the Under Water Environment (UWE) condition and the control group (JELLY condition) | • Stress level before and after going into virtual reality (ad hoc item) • Perceived relaxation (ad hoc item) | • HR • HRV | No differences were found in relaxation, |
Stromberg (2015) | This study aimed to investigate the effectiveness of a visually and audibly mediated slow diaphragmatic breathing (DB) intervention in a motion sickness inducing environment | RCT (between subjects) Participants were randomly assigned to either the experimental group (trained in slow DB) or the control group (briefed on attention control manipulation). | • Motion sickness (Motion Sickness Assessment Questionnaire) • Motion sickness during VR (ad-hoc items) • Self-efficacy (ad-hoc items) | • HR • HRV • RR | The results showed that subjects trained in slow DB exhibited lower respiration rates and higher respiratory sinus arrhythmia (RSA) values compared to the control group. They also reported fewer motion sickness symptoms. The slow DB intervention was effective in reducing motion sickness severity. |
Tinga (2019) | The aim of this study was to assess the effectiveness of respiratory biofeedback during VR meditation in reducing subjective tension and objective arousal after stress. | RCT (between subjects) Participants were randomly assigned to a respiratory biofeedback condition (visual feedback paired to breathing), a control feedback placebo condition (visual feedback not paired to breathing) or a control no-feedback condition (no visual feedback) | • Subjective arousal (ad-hoc VAS scale) | • HR • HRV • RMSSD • EEG | The results showed that subjective and objective arousal decreased during meditation in all conditions, indicating the overall effectiveness of VR in relaxation exercises. However, respiratory biofeedback was not the most effective in reducing arousal. The reduction in arousal (on all outcome measures combined and HR specifically) was largest in the control feedback placebo condition, suggesting that respiratory biofeedback had no additional value and was even less effective than control feedback placebo. |
Weerdmeester (2021) | The primary aim of this study was to assess the effectiveness of a VR biofeedback video game called DEEP in reducing anxiety symptoms. The study also examined changes in cognitive appraisals such as self-efficacy, locus of control, and threat-challenge appraisals, and their relationship to anxiety regulation. | RCT (between subjects) Participants were randomly assigned to DEEP (experimental condition) or a phone-based guided breathing application (control condition) | • State-trait anxiety (State Trait Anxiety Inventory) • Self-efficacy (competence subscale of the Player Experience of Need Satisfaction) • Locus of control (self-agency subscale of the Appraisal Questionnaire) • Threat challenge (ad-hoc items) | - | The results showed that all participants experienced a decrease in trait anxiety symptoms after using the applications, and this reduction was maintained at a 3-month follow-up. However, DEEP did not outperform the control application, suggesting that the additional elements in DEEP did not provide additional benefits beyond the guided breathing exercise provided by the control application. Changes in self-efficacy, locus of control, and threat-challenge appraisals were related to anxiety regulation. DEEP players showed an increase in self-efficacy over the course of the training, while their locus of control became more external. Participants in both conditions appraised the training as challenging rather than threatening, and these challenge appraisals were related to stronger decreases in anxiety. |
Weerdmeester (2022) | The study aimed to investigate the effectiveness of including visualizations that mirror changes in breathing in a biofeedback video game intervention (DEEP) for anxiety regulation. | RCT (between subjects) Participants were randomly assigned to the experimental (DEEP with breathing mirroring) or control condition (DEEP without breathing mirroring). | • State-trait anxiety (State Trait Anxiety Inventory) • Physiological arousal (competence subscale of the Player Experience of Need Satisfaction) • Locus of control (self-agency subscale from the Appraisal Questionnaire) | - | The presence of these visualizations did not result in stronger decreases in anxiety and arousal compared to participants who did not receive this additional visual feedback. The reinforcement technique tied to players' movement and the focus on deep diaphragmatic breathing may have been sufficient for anxiety regulation. The study also examined the relationship between cognitive appraisals (self-efficacy and locus of control) and anxiety. Participants who experienced higher self-efficacy had lower anxiety levels, and internal locus of control was related to lower post-play anxiety. However, these relationships were less pronounced for self-reported physiological arousal. |
RCT = randomized controlled trial; RR = respiratory rate; RRV = respiratory rate variability; HR = heart rate; HRV = heart rate variability; RMSSD = root mean square of successive differences between normal heartbeats; LF = low-frequency power; HF = high-frequency power; EEG = electroencephalogram; SCL = skin conductance level; SBP = systolic blood pressure; DBP = diastolic blood pressure; fNIRS = functional near-infrared spectroscopy. |
Only 14 studies of virtual reality combined with breathing exercises that included mental health outcomes met the eligibility criteria. Five studies were conducted in the United States (de Zambotti et al., 2022; Hu et al., 2021; Kniffin et al., 2014; Russel et al., 2014), 4 in Germany (Blum et al., 2019, 2020; Shiban et al., 2017; Soyka et al., 2016), 3 in The Netherlands (Tinga et al., 2019; Weerdmeester et al., 2021, 2022), one in Switzerland (Blanke et al., 2022), and one in Taiwan (Lan et al., 2021). Eight studies were published from 2014 to 2020, 3 were published in 2021 and 3 in 2022. Regarding the presentation of the results, we dedicated a paragraph to the study design and the participants involved. In addition, we analyzed the interventions’ length and the measures used. Then we mapped the use of VR, the types of breathing, and the integration of VR and breathing in the interventions. Finally, we analyzed the outcomes considered by the 14 studies.
The results are presented by delineating the study design and participants involved, followed by a description of the intervention length and the measures used for data collection. The usage of VR and breathing in the interventions is then mapped out, highlighting different types of breathing, as well as how specific breathing techniques are integrated with VR. Outcomes are discussed in terms of relaxation, arousal reduction, breathing awareness and focus on breath, and reduction in physical symptoms. Finally, the overall user experience is examined to provide comprehensive insights into the effectiveness and acceptability of the interventions.
Insert Table 1 and Table 2 here
3.1 Study design and participants
Eleven out of the 14 included studies were randomized controlled trials, and 3 were controlled studies. Moreover, all studies had pre- and post-intervention measurements, and a follow-up measurement was conducted only by Weerdmeester et al. (2021) after three months and by Shiban et al. (2017) after one year. Across all 14 studies, a total of 722 participants were studied. Ten studies included young adult participants with a mean age between 18 and 29 years old, while 3 studies included participants with a mean age ranging from 33 to 55 (Blanke et al., 2022; Blum et al., 2019; Shiban et al., 2017). One study did not report the sample’s age (Stromberg et al., 2015).
Ten out of 14 studies had more female participants, while two studies had the same number of male and female participants (Lan et al., 2021; Shiban et al., 2017). Moreover, one study had only female participants (Kniffin et al., 2014), and another had more male participants (Blanke et al., 2022). Eight out of 14 studies involved healthy participants. Thirteen studies were conducted in an outpatient setting. One study involved patients recovering from COVID-19 pneumonia with persistent dyspnea (Blanke et al., 2022). Other research involved participants with insomnia (de Zambotti et al., 2022), motion sickness (Russel et al., 2014; Stromberg et al., 2015), aviophobia (Shiban et al., 2017), and severe anxiety levels (Weerdmeester et al., 2021).
3.2 Interventions’ length
Regarding the length of the interventions, in most studies participants received a single session (Blanke et al., 2022; Blum et al., 2019; 2020; Kniffin et al., 2014; Russel et al., 2014; Soyka et al., 2014; Stromberg et al., 2015; Tinga et al., 2019; Weerdmeester et al., 2022), while in other studies they received two (de Zambotti et al., 2022; Hu et al., 2021); four (Weerdmeester et al., 2021); five (Shiban et al., 2017); or eight sessions (Lan et al., 2021). Furthermore, the VR experience was from 5 to 10 minutes long for the majority of the interventions (Blanke et al., 2022; Tinga et al., 2019; Blum et al., 2019; 2020; Hu et al., 2020; Kniffin et al., 2014; Russel et al., 2014; Soyka et al., 2014; Stromberg et al., 2015; Weerdmeester et al., 2022). In three studies, it was 20 minutes long (de Zambotti et al., 2022; Lan et al., 2021; Shiban et al., 2017). Moreover, only in Weerdmeester et al. (2021), the VR experience lasted from 30 to 60 minutes.
3.3 Measures
We have identified several measures used in the 14 studies: validated questionnaires, ad hoc questionnaires, physiological measures, and user experience measures. Seven studies mainly used validated questionnaires to measure pre- and post-intervention psychological outcomes such as anxiety (Blum et al., 2019), depression (Weerdmeester et al., 2021; 2022), negative affect (de Zambotti et al., 2022; Hu et al., 2021; Kniffin et al., 2014), relaxation (Blum et al., 2019), focus on breath (Blum et al., 2020), cognitive arousal (de Zambotti et al., 2022), regulatory skills in high-risk situations (Kniffin et al., 2014), and pain (Hu et al., 2021). On the other hand, seven studies mainly used ad hoc questionnaires to measure stress (Soyka et al., 2016), subjective arousal (Tinga et al., 2019), fear of flying (Shiban et al., 2017), and motion sickness (Russel et al., 2014; Stromberg et al., 2015). Other outcomes such as self-efficacy in managing motion sickness (Russel et al., 2014; Stromberg et al., 2015), breathing awareness (Blanke et al., 2022), and breathing control were examined (Lan et al., 2021).
Several studies also collected pre- and post-intervention physiological measures such as heart rate (HR) (Kniffin et al., 2014; Russel et al., 2014; Soyka et al., 2016; Stromberg et al., 2015; Tinga et al., 2019), heart rate variability (HRV) (Blum et al., 2019; 2020; de Zambotti et al., 2022; Kniffin et al., 2014; Russel et al., 2014; Shiban et al., 2017; Soyka et al., 2016; Tinga et al., 2019), root mean square of successive differences between normal heartbeats (RMSSD) (Blum et al., 2019; 2020; Tinga et al., 2019), respiratory sinus arrhythmia (RSA) (Stromberg et al., 2015), blood pressure (de Zambotti et al., 2022), tidal volume (Blanke et al., 2022), skin conductance level (SCL) (Shiban et al., 2017), and brain activity (de Zambotti et al., 2022; Tinga et al., 2019). Furthermore, 6 out of 14 studies also used respiration rate (RR) as a pre- and post-intervention physiological measure to test the efficacy of the technique itself (Blanke et al., 2022; Blum et al., 2020; Kniffin et al., 2014; Lan et al., 2021; Russel et al., 2014; Stromberg et al., 2015).
A few studies (n = 7) further analyzed the impact of the interventions proposed on users’ experiences and perceptions in relation to the interventions using various types of measures. Most studies used self-reported measures to gauge users’ perceptions of the interventions. Blanke et al. (2022) used an ad hoc questionnaire to evaluate patients' acceptance and feasibility of intervention. Blum et al. (2019) measured participants' liking, effort to comply, and perceived discomfort using Likert scales. De Zambotti et al. (2022) employed a Likert scale to measure participants' discomfort and difficulties. Lan et al. (2021) administered a questionnaire to assess user satisfaction and willingness to continue using the system. Soyka et al. (2016) utilized ad hoc items to measure fun, task difficulty, time perception, and willingness to use the interventions in the future. Blum et al. (2020) were the only ones using a validated questionnaire, the User Experience Questionnaire (UEQ), that measures attractiveness, perspicuity, novelty, and stimulation.
Some studies measured the perceived realism of the environments, including Kniffin et al. (2014), who utilized a 7-point scale to measure the perceived realism of the interaction; Russell et al. (2014), who employed a 7-point Likert scale to rate the realism of a virtual video; and Stromberg et al. (2015), who used a 7-point scale to rate the perceived realism of the environment.
These diverse measurement approaches allowed researchers to capture participants' experiences and perceptions across a range of factors, including acceptance, satisfaction, liking, discomfort, realism, and future usage intentions.
3.4 Mapping the use of virtual reality
VR technology offers a wealth of opportunities for creating immersive environments that can be combined with breathing techniques for various purposes, as will be discussed in the following sections (see Table 3).
Table 3
VR device, VR setting, integration with breathing techniques, and biofeedback.
First author (year) | VR device | Biofeedback | VR experience Description | Type of breathing Intervention |
Blanke (2022) | Smartphone-based VR headset (Zeiss VR ONEPLUS + Samsung Galaxy s8) | no | Virtual body Participants were instructed to look at a virtual body of the same gender lying next to them in a similar position during the virtual environment experience. | Normal breathing Participants received synchronous or asynchronous feedback of their breathing via a gender-matched virtual body. The visuo-respiratory stimulation lasted 5 minutes. In the synchronous condition, the virtual body illuminated with maximal radiance at the end of inspiration and minimal at the end of expiration. In the asynchronous condition, the visual stimulation was randomly generated with a phase-shift and frequency modulation. |
Blum (2019) | Tethered VR headset (Oculus CV1) | yes | Natural environment Participants experienced a virtual beach scenery at sunset, including various elements such as palms, rocks, and a campfire, accompanied by ambient instrumental background music and sounds of the ocean and campfire. | Diaphragmatic breathing Participants performed diaphragmatic breathing at a rate of 6 breaths per minute with auditory guidance while receiving either standard graphical feedback (control group) or VR feedback (experimental group) during a single 10-minute heart rate variability biofeedback session. |
Blum (2020) | Tethered VR headset (Oculus CV1) | yes | Natural environment Both experimental and control groups experienced a virtual environment of a stylized natural landscape with hills, rocks, flowers, and swaying trees. | Diaphragmatic breathing Certain elements, such as flowers, parts of trees, and rocks, were designed to change color either randomly (control group) or according to participant's respiration (experimental group). Participants completed a single session practicing slow diaphragmatic breathing for 7 minutes in VR while either performing breathing exercise or receiving a respiratory biofeedback |
de Zambotti (2022) | Tethered VR headset (Oculus Rift) | yes | Natural environment The study utilized the Nature Treks VR app Version 1.71 to provide a virtual environment of a forest during an orange sunset with birds and crickets. | Diaphragmatic breathing Participants either practiced slow diaphragmatic breathing in a VR-guided meditation or engaged in quiet activities for 20 minutes in two non-consecutive evening sessions before bedtime. |
Hu (2021) | Tethered VR headset (Oculus Rift) | no | Virtual body 3D virtual lungs | Diaphragmatic breathing Participants were randomly assigned to either interoceptive breathing focusing sessions using VR technology (VRB group), watching a VR display of a 3D lungs image that synchronized with their inhaling and exhaling cycles or traditional mindful breathing (TMB group), imagining their breathing inflating ande deflating in an abstract manner. During the experimental week, on the first and seventh days, both groups had a 10-minute breathing practice and 20 trials of the thermal quantitative sensory test (tQST) in-person in the lab. From the second to the sixth day, participants practiced home self-guided exercises. In the VRB group, participants watched a 3D VR display of synchronized inhaling and exhaling cycles for 10 minutes and listened to their breathing sounds using headphones. In the TMB group, participants imagined their breathing inflating and deflating abstractly. Both groups were instructed to perform diaphragmatic breathing. |
Kniffin (2014) | Tethered VR headset (eMagin 2800 3D visor) | no | Real-world setting The study involved participants having an interaction with a male avatar controlled by a male experimenter in an adjacent room within a virtual reality setting. The virtual scene featured an automobile interior with bucket seats and an exterior view simulating a rainy evening with the automobile parked near a lake. The interaction began with small talk, and the actor made sexual advances, which escalated into more aggressive behavior and anger towards the participant. | Diaphragmatic breathing Participants were randomly assigned to either a breathing or attention control condition and listened to instructions before an 8–12-minute VR interaction with a male avatar. Participants in the breathing condition received audio instructions on diaphragmatic breathing and were instructed to use the technique throughout the experiment at a pace of 3–7 breaths per minute. |
Lan (2021) | Tethered VR headset (HTC Vive) | yes | Virtual body A 3D human body model and a realistic facial model based on a fontal face photo supplied by the participant. | Abdominal breathing During the virtual reality experiment, participants were asked to use abdominal breathing and focus on slowing down their breathing rate. The experimental group was given a 3D human body model that simulated the changing of body temperature with a virtual line that changed thickness and color based on their breathing rate, while the control group received no feedback. They had 8 training sessions, 4 times a week for 2 weeks, each lasting 20 minutes. In each training session, they were encouraged to slow down their breathing as much as possible. |
Russel (2014) | 3D head mounted goggle set with over the ear headphones | no | Real-world setting The virtual experience consisted of a 10-min color video of ocean swelled in a stormy sea. The participants experienced a fluctuating view of a stormy dark blue horizon and that then followed the wave downwards until only water was visible | Diaphragmatic breathing Depending on their assigned condition, participants were either briefed on diaphragmatic breathing techniques or received an attention control manipulation involving instructions on being aware of their surroundings. Both groups then experienced a 10-minute VR simulation of an ocean swelled in a stormy sea and were asked to use the given instructions |
Shiban (2017) | VR headset (HMD V6) | no | Real-world scenario The VR environment simulated a passenger compartment of a Boeing 73. | Diaphragmatic breathing Participants received either virtual reality exposure therapy or virtual reality exposure therapy with diaphragmatic breathing. The latter group received breathing instructions during the exposure session and were instructed to inhale through the nose for 4 seconds, exhale through the mouth for 6seconds, and maintain a respiration frequency of 6 cycles per minute. |
Soyka (2016) | Not specified | no | Symbolic scenario Jellyfish moving in an underwater environment | Normal breathing Participants were randomly assigned to either an Under Water Environment (UWE) condition or a control group (JELLY condition) and received a single 10-minute virtual reality session. In the virtual underwater environment, participants saw a jellyfish moving up and down in the 6 seconds paced breathing rhythm. |
Stromberg (2015) | Not specified | no | Real-world scenario The VR experience showed a boat in rough seas | Diaphragmatic breathing Participants were assigned to an experimental or control group and received a single 50-minute session including instructions, a breathing practice session, and a VR experience of a boat in rough seas. The experimental group received training in slow diaphragmatic breathing (DB) techniques, while the control group received attention control manipulation. |
Tinga (2019) | Tethered VR headset (Oculus Rift DK2) | yes | Symbolic scenario White cloud moving towards and away from participants | Mindful breathing Participants were randomly assigned to a respiratory biofeedback condition, a control feedback placebo condition, or a control no-feedback condition. All groups received a single session including arousal induction and a 6-minute VR audio guided meditation focused on mindful breathing. The respiratory biofeedback group controlled a cloud in the virtual environment with their respiration, while the control feedback placebo group saw the cloud moving automatically, and the control no-feedback group only saw a blue background. |
Weerdmeester (2021) | Thetered VR headset (HTC Vive) | yes | Gamified task (DEEP) Virtual underwater world | Diaphragmatic breathing: Participants with high scores on anxiety, stress, or depression were randomly assigned to the DEEP or phone-based guided breathing conditions. Both conditions received multiple sessions, including a 1-hour session and a 30-minute session in a relaxing environment (training phase 1), followed by a 30-minute session and a 1-hour session in an exposure environment (training phase 2). In the DEEP condition, participants experienced a virtual underwater world with biofeedback during diaphragmatic breathing. In the control condition, participants were asked to breathe through their diaphragm and match their breathing to instructions. |
Weerdmeester (2022) | Tethered VR headset (HTC Vive) | yes | Gamified task (DEEP) Virtual underwater world | Diaphragmatic breathing Participants played a 10-minute single session of DEEP, a virtual biofeedback video game. They were instructed to use deep and calm diaphragmatic breathing during the game, allowing them to move through the virtual underwater world. In the experimental condition, participants were shown visualizations that mirrored their breathing. |
For instance, VR has been used to simulate real-world scenarios to train people on managing their breathing in different settings (n = 4). By practicing breathing exercises in a safe and controlled virtual environment that replicates uncomfortable conditions they may encounter in real life, individuals can improve their skills and confidence. Shiban et al. (2017) used VR to help people with aviophobia by simulating the experience of flying. Kniffin et al. (2014) used VR to teach female participants how to manage respiration and internal state in a situation where a male avatar was making aggressive advances. Similarly, Russel et al. (2014) and Stromberg et al. (2015) used VR to reproduce real-world scenarios, such as a stormy ocean and a boat in rough seas, respectively, to help people cope with motion sickness through diaphragmatic breathing.
Another advantage of using VR is the ability to create gamified tasks that make practicing breathing techniques fun and enjoyable (n = 2). Weerdmeester et al. (2021, 2022) incorporated game-like elements to make participants practice breathing techniques in response to changes in the virtual environment. These elements could include interactive elements (e.g., expanding and contracting circles) within the virtual environment that prompt participants to regulate their breathing in specific ways.
VR further allows users to escape from their real-life stressors and immerse themselves in controlled, calming environments (n = 3). This typically involves being in a calming natural environment, such as a forest, beach, or mountain setting, where visual and auditory cues can help to promote a sense of calm and reduce distractions from the outside world, such as a virtual forest environment with birds and crickets (de Zambotti et al., 2022), a virtual beach at sunset (Blum et al., 2019), or a stylized natural landscape with rocks and flowers, displaying hills and swaying trees in the background (Blum et al., 2020). In these environments, participants can practice slow diaphragmatic breathing through either a guided meditation (de Zambotti et al., 2022) or by controlling environmental elements’ changes with their breath (Blum et al., 2019; Blum et al., 2020).
VR also provides the possibility of creating symbolic scenarios (n = 2). Tinga et al. (2019), for example, used a white cloud to symbolically represent their breathing, in particular by approaching it when they were inhaling and distancing itself when exhaling. Similarly, Soyka et al. (2016) used an animation of a jellyfish so that participants could follow its rhythmic movements to control their breath.
Finally, VR can provide synchronous or asynchronous bodily visual feedback (n = 3), using either full virtual bodies (Blanke et al., 2022; Lan et al., 2021) or specific body parts, such as 3D lungs (Hu et al., 2021). The use of bodily visual feedback in VR can help users to better understand and control their own movements and bodily sensations, which can be useful for practicing breathing techniques.
These environments are integrated with various breathing techniques to achieve different goals, such as controlling internal status and reducing clinical symptoms.
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3.5 Mapping the use of breathing
3.5.1 Types of breathing
Regarding breathing, several studies used diaphragmatic breathing at a frequency of 6 breaths per minute (Blum et al., 2019; de Zambotti et al., 2022; Shiban et al., 2017; Stromberg et al., 2015; Weerdmeester et al., 2021, 2022) or at a frequency of 3–7 breaths per minute (Kniffin et al., 2014). Tinga et al. (2019) employed mindful breathing, whereas Blanke et al. (2022) asked participants to breathe normally while receiving synchronous (intervention) and asynchronous (control) feedback of their breathing. Only one study has not specified the type of breathing used (Soyka et al., 2016).
3.5.2 Breathing techniques and their integration with VR
Two methods can be distinguished for helping participants breathe properly. The first method consists of giving breathing instructions before the VR experience, and the second one consists of giving breathing instructions during the VR experience. The latter method, in turn, includes using pacers that participants listen to or watch to breathe properly during the VR experience or using biofeedback during the VR experience, through which participants can interact with the virtual environment and change it according to their breath.
For example, regarding the first method, participants were asked to listen to a short set of audio instructions training in diaphragmatic breathing before the VR exposure. Then, they were instructed to use these skills throughout their remaining time in the experiment (Kniffin et al., 2014; Russel et al., 2014; Stromberg et al., 2015; Tinga et al., 2019). Interestingly, in one study, participants were asked to practice breathing at home for four days following the given instructions (Hu et al., 2021).
Concerning the second method, pacers were introduced to guide participants’ breathing during the VR experience. For example, de Zambotti et al. (2022) provided pre-recorded breathing audio files for inhaling (person breathing in through the nose) and exhaling (person breathing out through the mouth). In other studies, participants were asked to follow the rhythmic motion of a jellyfish with their breathing (Soyka et al., 2016) or an oval that expanded (inhale) and retracted (exhale) during the VR experience (Stromberg et al., 2015). Furthermore, audible cues to breathe appropriately during the virtual scenario were introduced by Stromberg et al. (2015) and Shiban et al. (2017). In Hu et al. (2021), researchers gave orientation on the breathing pattern beforehand, and the participants could visualize the 3D lungs reproducing their breathing cycles.
Biofeedback was also employed to help participants breathe during the VR experience. For instance, in Blum et al. (2019), the cloud coverage in the sky was associated with breathing and heart rate variability: the better the latter parameter, the clearer the sky became. In another study, the cloud moved towards the participant while breathing in and away from the participant while breathing out (Tinga et al., 2019). In Blum et al. (2020), elements of the VR environment, such as flowers, parts of trees, and rocks, were able to change their color whenever the participant was exhaling. The color change lasted as long as the respective exhalation. In one study, to simulate the changing of body temperature, a virtual line in the center of the virtual body was created. The thickness and colors of this line changed according to the breathing rates. When the participant breathed slower, this central line became thicker, and its color changed from yellow to red (Lan et al., 2021). In two studies using an underwater virtual environment, up- and forward-forces were applied with each inhale, and a speed boost was applied with each exhale. Moreover, the plants changed in size and illumination according to inhalation or exhalation. During deep and calm exhalations, participants could also use a flashlight to illuminate a dark cave. Another visual feedback was provided by an expanding and contracting circle (i.e., getting bigger with the inhale and smaller with the exhale) in the players’ field of vision (Weerdmeester et al., 2021; Weerdmeester et al., 2022).
3.6 Outcomes
Breathing has been used to promote different goals. For example, it was employed to promote relaxation, decrease arousal levels, improve breathing control and focus on breath, and decrease clinical and physical symptoms.
3.6.1 Relaxation
In Blum et al. (2019), breathing, in combination with heart-rate variability biofeedback, has been used to promote relaxation, relaxation self-efficacy, focus on the present and attentional resources, and reduce mind wandering after a cognitive stressor task. Specifically, the healthy participants were randomized into two groups. The VR-based heart rate variability biofeedback group (VR-BF) experienced a 10-minute virtual beach scenery with accompanying sounds and music while performing slow diaphragmatic breathing. The standard heart rate variability biofeedback group (Standard-BF) received abstract feedback in the form of graphical geometrical indicators. Physiological measures such as HRV, RMSSD, and heart coherence were measured using a Polar H7 chest strap, while relaxation self-efficacy, mind wandering, focus on the present, and attentional resources were assessed using questionnaires and computerized tasks. Results showed that the VR-BF intervention increased self-efficacy, relaxation and focus on the present over time and promoted a reduction in reaction times and in mind-wondering compared to the Standard-BF intervention. In other research, breathing has been used to enhance relaxation and reduce stress after a stressful task in healthy participants. For example, Soyka et al. (2016) randomly assigned healthy participants to either the Under Water Environment (UWE) condition or the control group (JELLY condition). Both groups received a single 10-minute session of VR, but only the UWE group was immersed in an underwater environment and instructed to follow the rhythmic motion of a jellyfish with their breathing. The control group only saw the jellyfish without any surrounding environment. Both groups heard the same underwater sounds, and a piezo-electric crystal respiration effort sensor was placed around participants' chests to detect breathing. HR and HRV were also measured with a blood pulse sensor on the left index finger (Brain Products). Participants were asked to rate their level of relaxation and enjoyment after the VR session. Results showed no significant differences in relaxation between the two groups, but the UWE condition was rated as more fun and more likely to be used at home than a traditional breathing technique.
3.6.2 Arousal reduction
Tinga et al. (2019) aimed to examine the effectiveness of respiratory biofeedback during virtual reality meditation in lowering subjective and objective arousal after stress. The healthy participants were randomly assigned to one of the following conditions: (a) a respiratory biofeedback condition in which visual feedback was paired to breathing (i.e., clouds moving towards the participant while breathing in and away from the participant while breathing out); (b) a control feedback placebo condition in which visual feedback was not paired to breathing; (c) control no-feedback condition in which no visual feedback was used. Participants were instructed to follow the audio-guided meditation instructions while keeping their eyes open and then experienced a 6-minute meditation task in VR. Breathing, HR, HRV, and RMSSD were detected using a BioNomadix wireless system, and the researchers found a greater reduction in arousal in the control feedback placebo condition. In other research, breathing has also been employed as a self-regulation strategy during interactions with an aggressive male. For instance, Kniffin et al. (2014) investigated whether diaphragmatic breathing (measured through a strain gauge sensor placed around the participant’s abdomen) could be used to improve healthy women's regulatory skills in high-risk situations. Researchers randomly assigned participants to either a breathing or attention control condition and underwent an 8–12-minute VR interaction with a male avatar in an automobile interior with bucket seats and an exterior view simulating a rainy evening. The breathing group received brief audio instructions training them in diaphragmatic breathing, while the control group listened to a short audio recording and was instructed to pay careful attention. HR, HRV, and RR were measured. The breathing group had a decrease in HRV over the course of the trial and slowed its respiration rate following the VR experience. These results suggest that training in diaphragmatic breathing may improve physiological outcomes after this type of VR exposure.
3.6.3 Breathing awareness and focus on breath
Breathing has also been used to promote breathing awareness. For instance, Lan et al. (2021) aimed to improve participants’ breathing control and sleepiness. They assigned healthy participants to either the feedback group or the control group with no feedback. During the eight VR experiences, they were asked to use abdominal breathing, and a pressure sensor attached to the abdomen was used to detect it. For each participant in the feedback group, a realistic 3D human body model was created with a central virtual line that changed its thickness and color according to the participant’s breathing. A heating pad was sewn into the clothes of the participants to simulate the rising body temperature experienced by well-trained mediators when they enter a deep state of meditation. The study involved 8 training sessions over 2 weeks, with 4 sessions per week, each lasting 20 minutes. Results showed that the feedback group was able to control breathing in a more uniform way during the training and reported fewer eye-blinking events compared to the control group. Another example can be the study of Blum et al. (2020) in which they investigated the efficacy of a developed biofeedback algorithm on fostering focus on breath. Participants were randomized to a control or feedback group. They received a single session of VR breathing exercises in a virtual environment depicting a stylized natural landscape and were instructed to practice slow diaphragmatic breathing. For the feedback group, the simulation was further integrated with a respiratory biofeedback system in VR that made elements change color in response to the participant's exhalation, serving as respiratory feedback. On the other hand, the control group did not receive feedback. Physiological parameters such as HRV, RMSSD, LF, and HF were measured using a Polar H10 chest strap, and the VR-controller on the abdomen was used to detect inhalations and exhalations. Following this intervention, the feedback group increased the use and duration of diaphragmatic movements, reported an easier focus on the breath, a lower respiration rate, and higher RMSSD and LF compared to the control group. In addition, Blanke et al. (2022) focused on alleviating dyspnea in patients recovering from COVID-19 pneumonia and improving their breathing comfort, awareness, and agency. They detected breathing with the Go Direct® Respiration belt and used VR to embody participants in a gender-matched virtual body and to then manipulate the breathing feedback (synchronous or asynchronous with respect to the patient’s chest movements). Participants received both synchronous (intervention) and asynchronous (control) feedback of their breathing. While no significant differences were found between the intervention and control groups, breathing comfort improved after the synchronous feedback intervention compared to baseline, and 91.2% of patients were satisfied with the intervention, with 66.7% perceiving it as beneficial for their breathing.
3.6.4 Clinical symptoms reduction
Breathing has been used to reduce clinical symptoms. Specifically, Weerdmeester et al. (2021, 2022) aimed to reduce psychophysiological symptoms such as anxiety. They investigated the effects of a VR intervention called DEEP, which consisted of a virtual underwater world where clinical participants used slow-paced breathing techniques to influence their movement. A controller belt measured the expansion of participants’ diaphragms. In Weerdmeester et al. (2021), participants trained in a relaxing environment during the first phase (1 hour and 30 minutes) and in an exposure environment (a dark cave with only a dim flashlight that illuminates during deep and calm exhalations) during the second phase (1 hour and 30 minutes) a significant reduction in trait anxiety symptoms from pre- to post-test was found both in the DEEP condition and in the control condition that used a phone-based guided breathing application. Furthermore, DEEP players reported lower self-efficacy and perceived resources in session one, felt less internal control in the third session, and had lower threat-challenge ratios compared to the control group. However, DEEP players showed an increase in self-efficacy over the course of the training, and they were more engaged in the first session compared to those in the control group. Moreover, Weerdmeester et al. (2022) aimed to investigate the extent to which the mirroring visualizations in DEEP increase healthy players' ability to regulate their anxiety and whether the presence of these visualizations would result in higher levels of internal locus of control and self-efficacy. In this study, participants were randomly assigned to either an experimental condition where they played DEEP with visualizations that mirrored their breathing, or a control condition without the mirroring visualizations. After a brief instruction and preparation phase of the Trier Social Stress Test (TSST), participants in both conditions played DEEP for 10 minutes. The results showed that participants in the experimental condition did not show greater reductions in anxiety and arousal and increases in self-efficacy and locus of control compared to the control group. However, the more self-efficacy participants experienced while playing DEEP, the lower their anxiety was immediately after playing, and the stronger their pre- to post-play decrease in anxiety was. Shiban et al. (2017) focused instead on fear of flying reduction. They created a safe and controlled environment to simulate the experience of flying and used this stimulation to examine the effects of diaphragmatic breathing as a coping strategy for aviophobia. The respiration rate was measured with a strap placed around the waist in the three VR flights, each lasting 22.5 minutes. HR and SCL were also measured. The researchers found a greater tendency to overcome the fear of flying in the diaphragmatic breathing group compared to the non-diaphragmatic breathing group. On the other hand, de Zambotti et al. (2022) investigated the effects of a VR-based mind-body approach on reducing bedtime arousal in adolescents with insomnia. The Nature Treks VR app was used to provide a virtual forest environment with birds and crickets, and thoracic and abdominal piezoelectric breathing bands were employed to detect breathing movements. Other physiological parameters such as ECG, HRV, LF, HF, EEG, and systolic and diastolic blood pressure were measured using various sensors. Participants in the experimental group engaged in VR-guided meditation for two non-consecutive evening sessions, with each session lasting 20 minutes. Within the environments, they practiced slow diaphragmatic breathing following a prerecorded breathing audio file for inhaling and exhaling while interacting with the environment by grabbing spheres with symbols to activate a rain sound and a dark mode to further rest their breathing. On the other hand, participants in the control group were asked to engage in a quiet activity. Results showed lower physiological arousal in the experimental condition compared to the control group. The results showed that the intervention resulted in acute autonomic and cortical modulation, leading to lower physiological arousal, heart rate, and cortisol compared to the control condition. Resting-state HR was also significantly lower after the VR-guided meditation and paced breathing intervention session compared to pre-intervention levels and compared to levels preceding and following quiet activities.
3.6.5 Physical symptoms reduction
In reviewing the literature, it has emerged that several studies have not only emphasized the psychological outcomes but have also investigated the impact of combining VR with breathing techniques on different physical symptoms.
For instance, some research employed breathing as a physical symptoms reduction strategy such as pain. Hu et al. (2021) aimed to compare the effectiveness of two methods in modulating healthy participants’ pain thresholds. To do this, they randomly assigned participants to one of two conditions: (a) interoceptive breathing focusing sessions using virtual reality 3D lungs that synchronized with the participants’ breathing cycles in real time (VRB) or (b) traditional mindful breathing (TMB). A Braebon plethysmography belt was used to synchronize the 3D lungs with participants’ inhalation and exhalation cycles during their diaphragmatic breathing. Results showed that, after a week of practice, both breathing interventions resulted in a significant increase in pain thresholds, with no significant between-group differences. TMB modulated the prefrontal cortex to facilitate sensory-interoceptive processing of breathing but inhibit sensory-discriminative pain processing. VRB, on the other hand, augmented cortical visual-auditory activations to override pain processing and raise the pain threshold. On the other hand, Russell et al. (2014) and Stromberg et al. (2015) aimed to examine the effect of diaphragmatic breathing strategies on motion sickness. In both studies, the experimental group was debriefed on diaphragmatic breathing and then invited to use it during the 10-minute VR experience (a stormy ocean and a boat in rough sea, respectively). On the other hand, the control group received only attention control manipulation before the VR exposure. A respiration sensor was placed around the abdomen, and a respiratory effort transducer was used to detect breathing. HR and HRV were also measured with three electrodes connected to a BioPac ECG100C electrocardiogram amplifier module. In Russel et al. (2014), results showed that the diaphragmatic breathing protocol increased parasympathetic nervous system tone, slowed RR, and decreased the development of motion sickness symptoms as compared to the control condition. The study also found a significant positive correlation between an increased RR and more motion sickness symptoms in the diaphragmatic breathing condition. Furthermore, in Stromberg et al. (2015), the diaphragmatic breathing group reported lower breathing rates, a higher HRV, and reduced motion sickness severity as compared to the control group.
As we can see, breathing techniques can be used to manage relaxation, arousal, and breathing awareness and reduce both psychological and physical symptoms.
3.7 User experience
Blanke et al. (2022) assessed patients' acceptance and feasibility of VR intervention through an ad hoc questionnaire. The results showed that a majority of the patients (91.2%) were satisfied with the intervention, and 66.7% reported respiratory benefits. Additionally, a significant proportion expressed a desire to continue using the device during recovery and at home. Some patients also indicated a preference for earlier usage during their hospital stay. Overall, the findings suggest positive acceptance and the feasibility of VR intervention.
Blum et al. (2019) assessed participants' overall experience of the biofeedback treatment by measuring liking, effort to comply, or perceived discomfort. The results revealed no significant differences between the groups in terms of effort to comply and perceived discomfort. However, the VR-BF group reported higher liking ratings compared to the Standard-BF group, with a statistically significant difference observed. Blum et al. (2020) assessed users’ experience using the User Experience Questionnaire (UEQ; Laugwitz et al. 2008) with four subscales: attractiveness, perspicuity, novelty, and stimulation. The overall mean values were high in both conditions (breathing exercise and biofeedback), suggesting a satisfying user experience. A separate one-way ANOVA was conducted for each subscale, and there were no significant differences between the two conditions, indicating that the addition of biofeedback did not significantly alter the user experience.
De Zambotti et al. (2022) assessed the potential discomfort and difficulties experienced by participants during a bedtime intervention session involving VR immersion and slow breathing. Participants were asked to rate discomfort and difficulties the following morning on a Likert scale. Most participants perceived low discomfort associated with VR immersion, while slowing down breathing was challenging for 30–40% of the sample.
After the role-play, Kniffin et al. (2014) asked participants to rate the perceived realism of the interaction on a 7-point scale. The results showed that participants in the VR condition rated the interaction as more real than those in the non-VR condition. Additionally, participants in the VR condition reported higher levels of negative affect both during and after the interaction compared to those in the non-VR condition. These findings suggest that VR increases the realism of such role-plays.
In the study conducted by Lan et al. (2021), a questionnaire was administered to gauge user satisfaction and willingness to continue using the system for long-term meditation practice. The results revealed that subjects in the feedback group exhibited stronger agreement across most of the questions (e.g., “I find this breathing control experiment enjoyable”). Notably, they found the multimodal feedback system for meditation training enjoyable, as indicated by their responses to the first question.
Russell et al. (2014), after the experiment, asked participants to rate the realism of a virtual video on a seven-point Likert scale, including how real it looked and how real the boat motion felt. The study found no significant difference in perceived realism between the diaphragmatic breathing condition and the control condition.
Soyka et al. (2016) asked their participants to rate the intervention in terms of fun, task difficulty, time perception, and willingness to use it in the future, using ad hoc items. The results indicated that the underwater world VR experience was perceived as significantly more enjoyable and less monotonous compared to other experiences. On average, participants found it relatively easy to synchronize their breathing with the motion of the jellyfish, although there was considerable variability. As regards time perception, it was observed that the distribution of time perception was skewed towards longer durations in the underwater world experience condition, despite the average perceived time being the same for both groups. Finally, in terms of future usage, individuals in the underwater world experience condition expressed willingness to utilize this tool at home for relaxation, while participants in the jellyfish condition displayed reluctance.
Stromberg et al. (2015) asked subjects to rate the perceived realism of an ocean swells video on a 7-point scale. Participants were asked two questions regarding the video's perceived realism: "During the video, how real did it look?" and "During the video, how real did the motion of the boat feel?" The responses were summed to create a perceived realism scale. The scale aimed to measure how real the video looked and felt to the subjects, but results were not reported.