This study examines how voluntary fast, deep breathing (hyperventilation) affects the way healthy adults perceive their own body. Participants wear a virtual-reality (VR) headset and see a virtual body; a glowing halo around it either pulses exactly in time with their own breathing or is time-shifted. During the study, breathing, heart activity, blood-oxygen level, the carbon-dioxide content of exhaled air, and brain activity (EEG, measured non-invasively from the scalp) are recorded continuously, and participants complete questionnaires about their experience. The aim is to understand whether and how altered breathing changes the interplay between what people see and what they feel inside their body, and thereby the sense of being located within one's own body. The study uses a 2×2 within-subjects design crossing Breathing (normal vs. hyperventilation) with Visual synchrony (synchronous vs. asynchronous), with the asynchronous and normal-breathing conditions serving as within-subject controls. No medication is administered and no medical device is being tested; the instruments used serve only for monitoring and measurement. Participation involves a screening visit (including a short, \~5-minute guided-hyperventilation tolerance test) and an experimental visit.
Background and rationale. Bodily self-consciousness (BSC) - the sense of owning a body and being located within it - depends on integrating exteroceptive (e.g., visual) and interoceptive (e.g., respiratory, cardiac) signals. Voluntary hyperventilation induces marked interoceptive and physiological changes and can alter states of consciousness. This study investigates whether hyperventilation modulates the visually driven manipulation of BSC, using a respiratory-visual full-body illusion. Design. Single-centre experimental study with a 2x2 within-subjects design crossing Breathing (normal vs. voluntary paced hyperventilation) and Visual synchrony (a virtual body whose surrounding luminous halo is modulated synchronously vs. asynchronously with the participant's real-time respiration, recorded via a respiratory belt). The block order is fixed, with the normal-breathing block always preceding the hyperventilation block for safety and physiological reasons; within each block, the order of the synchronous and asynchronous VR exposures is randomised across participants from a computer-generated permuted-block list prepared before enrolment. The asynchronous and normal-breathing conditions serve as within-subject controls. Intervention. Voluntary, guided paced hyperventilation is delivered as four cycles of approximately 10 minutes of deepened breathing under audio guidance (including a sustained-hyperventilation window for EEG), with continuous physiological monitoring: end-tidal CO2 (capnography), pulse oximetry (SpO2 and heart rate), ECG, and direct observation by trained study staff. After each breathing cycle, participants view for about 2 minutes a virtual body (avatar) positioned about 2 m ahead and seen from behind, in a first-person perspective, with the halo modulated synchronously or asynchronously with breathing. A normal, quiet-breathing condition in the same VR environment serves as the comparison. Procedures. The study comprises two visits: (1) a screening visit with informed consent, health-history review, baseline physiology, and a short (\~5-minute) VR-guided hyperventilation familiarisation/tolerance test; and (2) an experimental visit (\~188 minutes) with the full protocol. A urine pregnancy test is performed at screening for participants of childbearing potential. Safety is governed by predefined tiered stop criteria and a recovery/discharge checklist, with an on-site emergency physician available for escalation. No medicinal product is administered and no medical device is under investigation; all instruments are used within their intended purpose for monitoring and measurement. The study is an "other clinical trial" under Swiss law (Chapter 4 ClinO), risk category B, reviewed by the Cantonal Ethics Committee Bern (BASEC 2026-01385).
Study Type
INTERVENTIONAL
Allocation
RANDOMIZED
Purpose
BASIC_SCIENCE
Masking
NONE
Enrollment
35
Participants breathe deeper and faster than normal, maintaining continuous cyclic breathing without pauses between inhalation and exhalation, paced by a pre-recorded standardised voice guide in German or English with background music. The hyperventilation block comprises four cycles, each with a 10-minute guided paced-hyperventilation induction, a 2-minute VR exposure and a 3-minute post-exposure stack (BSC-Q, Mental Imagery Task, distress NRS); cycle 1 additionally includes a 2-minute hyperventilation-state Mental Imagery Task pre-test between the induction and the VR exposure. A 5-minute sustained-hyperventilation resting-state EEG window is collected between cycles 2 and 3, with the voice guide replaced by a soft drum pacing tone. Cumulative hyperventilation is approximately 45 minutes, followed by a single supervised recovery. EtCO2, SpO2, heart rate and heart rate variability (ECG) are monitored continuously under the protocol's tiered stop criteria.
Within-subject control condition for the Breathing factor. Participants breathe at their natural spontaneous rate while receiving audio guidance from the same pre-recorded standardised voice guide and background music used in the hyperventilation condition, so that the auditory environment is held constant across conditions and only the breathing instruction differs. Delivered as two 8-minute normal-breathing cycles (3 minutes spontaneous breathing in VR, a 2-minute VR exposure, and a 3-minute post-exposure stack of BSC-Q, Mental Imagery Task and distress NRS), one with synchronous and one with asynchronous respiratory-visual feedback, seated in the experimental chair wearing the head-mounted display.
Respiratory-visual VR paradigm delivering contingent feedback; this is the measurement method through which bodily self-consciousness is assessed. Seated participants wear a head-mounted display showing a first-person view of a three-dimensional virtual body positioned about 2 metres in front of the viewpoint and seen from behind. A respiratory belt at the level of the umbilicus drives, in real time, a luminous halo surrounding the virtual body: the halo becomes increasingly visible during inspiration, reaches maximum luminosity at peak inspiration and fades during expiration, in synchrony with the participant's actual breathing. Each exposure lasts 2 minutes (120 seconds). Each participant receives one synchronous exposure in the normal-breathing block and two in the hyperventilation block.
Respiratory-visual VR paradigm delivering non-contingent feedback; the control level of the Synchrony factor. Setup is identical to the synchronous condition (seated, head-mounted display, virtual body about 2 metres in front seen from behind, respiratory belt at the umbilicus), except that the halo modulation is phase-shifted and frequency-adjusted to 80% or 120% of the measured breathing frequency, so that the visual feedback never corresponds to the participant's respiratory state. Each exposure lasts 2 minutes (120 seconds). Each participant receives one asynchronous exposure in the normal-breathing block and two in the hyperventilation block.
University of Fribourg, Molecular Psychiatry Laboratory, (at Réseau fribourgeois de santé mentale, RFSM/FNPG)
Villars-sur-Glâne, Switzerland
Mean Rating of Perceived Breathing Location in the Virtual Body: Bodily Self-Consciousness Questionnaire Item 1 (BSC-Q1)
Self-reported agreement with the statement "It seemed as if I was feeling my respiration in the virtual body", item 1 of the Bodily Self-Consciousness Questionnaire (BSC-Q). Rated on a 7-point Likert scale from -3 (totally disagree) to +3 (totally agree); minimum -3, maximum +3. Higher scores indicate that the participant's own breathing is felt more strongly to be located in the virtual body (a stronger illusion). This is a mechanistic outcome; neither a higher nor a lower score is a better or worse clinical outcome. Rated after each of the 6 virtual reality (VR) exposures (2 normal-breathing exposures, 4 hyperventilation cycles); the two hyperventilation cycles at each synchrony level are averaged within participant. Reported as the mean rating in each of the 4 conditions (Synchronous/Asynchronous x Normal breathing/Hyperventilation). Primary analysis: the Synchrony x Breathing interaction in a within-subjects linear mixed-effects model.
Time frame: Experimental visit (single visit, approximately 188 minutes); rated after each of the 6 VR exposures
Mean Drift in Perceived Self-Location Toward the Virtual Body, Measured With the Mental Imagery Task (MIT)
Behavioural measure of perceived self-location, reported in centimetres. Participants stand still in virtual reality and watch a ball approach along the anterior-posterior body axis; the ball then disappears and they press a button when they imagine it reaching their own position. The position of the hidden ball along that axis at the moment of the button press is recorded as the estimate of perceived self-location (method of Nakul et al., Sci Rep 2020;10:6802, which records self-location as an anterior-posterior coordinate and reports drifts in centimetres). One value is reported per participant per condition: the drift, calculated as the coordinate recorded after the VR exposure minus the participant's own matched breathing-state pre-test coordinate. Positive values indicate relocation toward the virtual body; larger positive values indicate a stronger illusion. Secondary endpoint, analysed as the Synchrony x Breathing interaction in a mixed-effects model.
Time frame: Experimental visit (single visit, approximately 188 minutes); measured after each of the 6 VR exposures, each baseline-corrected against a matched breathing-state pre-test
Mean Rating of Perceived Breathing Synchrony With the Virtual Body: Bodily Self-Consciousness Questionnaire Item 2 (BSC-Q2)
Self-reported agreement with the statement "I felt as if the virtual body was breathing with me", item 2 of the Bodily Self-Consciousness Questionnaire (BSC-Q). Rated on a 7-point Likert scale from -3 (totally disagree) to +3 (totally agree); minimum -3, maximum +3. Higher scores indicate a stronger sense that the virtual body was breathing together with the participant. This is a mechanistic outcome; neither a higher nor a lower score is a better or worse clinical outcome. Rated after each of the 6 virtual reality (VR) exposures (2 normal-breathing exposures, 4 hyperventilation cycles); the two hyperventilation cycles at each synchrony level are averaged within participant. Reported as the mean rating in each of the 4 conditions (Synchronous/Asynchronous x Normal breathing/Hyperventilation). Secondary endpoint, analysed with a mixed-effects model analogous to the primary endpoint and reported with uncorrected p-values.
Time frame: Experimental visit (single visit, approximately 188 minutes); rated after each of the 6 VR exposures
Mean Rating of Self-Identification With the Virtual Body: Bodily Self-Consciousness Questionnaire Item 3 (BSC-Q3)
Self-reported agreement with the statement "I felt as if the virtual body was my body", item 3 of the Bodily Self-Consciousness Questionnaire (BSC-Q). Rated on a 7-point Likert scale from -3 (totally disagree) to +3 (totally agree); minimum -3, maximum +3. Higher scores indicate stronger self-identification with the virtual body. This is a mechanistic outcome; neither a higher nor a lower score is a better or worse clinical outcome. Rated after each of the 6 virtual reality (VR) exposures (2 normal-breathing exposures, 4 hyperventilation cycles); the two hyperventilation cycles at each synchrony level are averaged within participant. Reported as the mean rating in each of the 4 conditions (Synchronous/Asynchronous x Normal breathing/Hyperventilation). Secondary endpoint, analysed with a mixed-effects model analogous to the primary endpoint and reported with uncorrected p-values.
Time frame: Experimental visit (single visit, approximately 188 minutes); rated after each of the 6 VR exposures
Mean Rating of Being Located Outside the Physical Body: Bodily Self-Consciousness Questionnaire Item 4 (BSC-Q4)
Self-reported agreement with the statement "I felt as if I was outside my physical body", item 4 of the Bodily Self-Consciousness Questionnaire (BSC-Q). Rated on a 7-point Likert scale from -3 (totally disagree) to +3 (totally agree); minimum -3, maximum +3. Higher scores indicate a stronger sensation of being located outside the physical body. This is a mechanistic outcome; neither a higher nor a lower score is a better or worse clinical outcome. Rated after each of the 6 virtual reality (VR) exposures (2 normal-breathing exposures, 4 hyperventilation cycles); the two hyperventilation cycles at each synchrony level are averaged within participant. Reported as the mean rating in each of the 4 conditions (Synchronous/Asynchronous x Normal breathing/Hyperventilation). Secondary endpoint, analysed with a mixed-effects model analogous to the primary endpoint and reported with uncorrected p-values.
Time frame: Experimental visit (single visit, approximately 188 minutes); rated after each of the 6 VR exposures
Mean Rating of Separation From the Physical Body: Bodily Self-Consciousness Questionnaire Item 5 (BSC-Q5)
Self-reported agreement with the statement "I felt as if I was separated from my physical body", item 5 of the Bodily Self-Consciousness Questionnaire (BSC-Q). Rated on a 7-point Likert scale from -3 (totally disagree) to +3 (totally agree); minimum -3, maximum +3. Higher scores indicate a stronger sensation of being separated from the physical body. This is a mechanistic outcome; neither a higher nor a lower score is a better or worse clinical outcome. Rated after each of the 6 virtual reality (VR) exposures (2 normal-breathing exposures, 4 hyperventilation cycles); the two hyperventilation cycles at each synchrony level are averaged within participant. Reported as the mean rating in each of the 4 conditions (Synchronous/Asynchronous x Normal breathing/Hyperventilation). Secondary endpoint, analysed with a mixed-effects model analogous to the primary endpoint and reported with uncorrected p-values.
Time frame: Experimental visit (single visit, approximately 188 minutes); rated after each of the 6 VR exposures
Mean Rating on the Control Item: Bodily Self-Consciousness Questionnaire Item 6 (BSC-Q6)
Self-reported agreement with the statement "It seemed as if I had more than two bodies", item 6 of the Bodily Self-Consciousness Questionnaire (BSC-Q). Rated on a 7-point Likert scale from -3 (totally disagree) to +3 (totally agree); minimum -3, maximum +3. BSC-Q6 is a pre-specified control item on which no synchrony effect is expected; it detects response bias and does not index the illusion. Higher scores indicate stronger endorsement of an experience not expected to occur. This is a mechanistic outcome; neither a higher nor a lower score is a better or worse clinical outcome. Rated after each of the 6 virtual reality (VR) exposures (2 normal-breathing exposures, 4 hyperventilation cycles); the two hyperventilation cycles at each synchrony level are averaged within participant. Reported as the mean rating in each of the 4 conditions (Synchronous/Asynchronous x Normal breathing/Hyperventilation). Secondary endpoint, reported with uncorrected p-values.
Time frame: Experimental visit (single visit, approximately 188 minutes); rated after each of the 6 VR exposures
Mean Score on the Oceanic Boundlessness Dimension of the 5-Dimensional Altered States of Consciousness Rating Scale (5D-ASC)
Dimension score from the 5-Dimensional Altered States of Consciousness Rating Scale (5D-ASC), a 94-item retrospective self-report instrument completed once at the end of the experimental session, within 30 minutes of the end of the hyperventilation block, and referring to the hyperventilation block as a whole. Every item is rated on a 0-100 visual analogue scale and each dimension score is expressed on that same scale; minimum 0, maximum 100. This outcome measure reports the Oceanic Boundlessness dimension only; the other four 5D-ASC dimensions are reported as separate outcome measures. Higher scores indicate a more pronounced altered state on this dimension. The measure is descriptive and phenomenological; neither a higher nor a lower score is a better or worse clinical outcome. Secondary endpoint, reported descriptively and exploratorily with uncorrected p-values.
Time frame: Experimental visit (single visit, approximately 188 minutes); completed once at the end of the session, within 30 minutes of the hyperventilation block
Mean Score on the Anxious Ego Dissolution Dimension of the 5-Dimensional Altered States of Consciousness Rating Scale (5D-ASC)
Dimension score from the 5-Dimensional Altered States of Consciousness Rating Scale (5D-ASC), a 94-item retrospective self-report instrument completed once at the end of the experimental session, within 30 minutes of the end of the hyperventilation block, and referring to the hyperventilation block as a whole. Every item is rated on a 0-100 visual analogue scale and each dimension score is expressed on that same scale; minimum 0, maximum 100. This outcome measure reports the Anxious Ego Dissolution dimension only; the other four 5D-ASC dimensions are reported as separate outcome measures. Higher scores indicate a more pronounced altered state on this dimension. The measure is descriptive and phenomenological; neither a higher nor a lower score is a better or worse clinical outcome. Secondary endpoint, reported descriptively and exploratorily with uncorrected p-values.
Time frame: Experimental visit (single visit, approximately 188 minutes); completed once at the end of the session, within 30 minutes of the hyperventilation block
Mean Score on the Visionary Restructuralization Dimension of the 5-Dimensional Altered States of Consciousness Rating Scale (5D-ASC)
Dimension score from the 5-Dimensional Altered States of Consciousness Rating Scale (5D-ASC), a 94-item retrospective self-report instrument completed once at the end of the experimental session, within 30 minutes of the end of the hyperventilation block, and referring to the hyperventilation block as a whole. Every item is rated on a 0-100 visual analogue scale and each dimension score is expressed on that same scale; minimum 0, maximum 100. This outcome measure reports the Visionary Restructuralization dimension only; the other four 5D-ASC dimensions are reported as separate outcome measures. Higher scores indicate a more pronounced altered state on this dimension. The measure is descriptive and phenomenological; neither a higher nor a lower score is a better or worse clinical outcome. Secondary endpoint, reported descriptively and exploratorily with uncorrected p-values.
Time frame: Experimental visit (single visit, approximately 188 minutes); completed once at the end of the session, within 30 minutes of the hyperventilation block
Mean Score on the Auditory Alterations Dimension of the 5-Dimensional Altered States of Consciousness Rating Scale (5D-ASC)
Dimension score from the 5-Dimensional Altered States of Consciousness Rating Scale (5D-ASC), a 94-item retrospective self-report instrument completed once at the end of the experimental session, within 30 minutes of the end of the hyperventilation block, and referring to the hyperventilation block as a whole. Every item is rated on a 0-100 visual analogue scale and each dimension score is expressed on that same scale; minimum 0, maximum 100. This outcome measure reports the Auditory Alterations dimension only; the other four 5D-ASC dimensions are reported as separate outcome measures. Higher scores indicate a more pronounced altered state on this dimension. The measure is descriptive and phenomenological; neither a higher nor a lower score is a better or worse clinical outcome. Secondary endpoint, reported descriptively and exploratorily with uncorrected p-values.
Time frame: Experimental visit (single visit, approximately 188 minutes); completed once at the end of the session, within 30 minutes of the hyperventilation block
Mean Score on the Reduction of Vigilance Dimension of the 5-Dimensional Altered States of Consciousness Rating Scale (5D-ASC)
Dimension score from the 5-Dimensional Altered States of Consciousness Rating Scale (5D-ASC), a 94-item retrospective self-report instrument completed once at the end of the experimental session, within 30 minutes of the end of the hyperventilation block, and referring to the hyperventilation block as a whole. Every item is rated on a 0-100 visual analogue scale and each dimension score is expressed on that same scale; minimum 0, maximum 100. This outcome measure reports the Reduction of Vigilance dimension only; the other four 5D-ASC dimensions are reported as separate outcome measures. Higher scores indicate a more pronounced altered state on this dimension. The measure is descriptive and phenomenological; neither a higher nor a lower score is a better or worse clinical outcome. Secondary endpoint, reported descriptively and exploratorily with uncorrected p-values.
Time frame: Experimental visit (single visit, approximately 188 minutes); completed once at the end of the session, within 30 minutes of the hyperventilation block
Mean Item Score on the Six-Dimensional Extended Bodily Self-Consciousness Scale (6D-xBSC)
Overall score on the 6D-xBSC, an exploratory in-house 41-item scale characterising hyperventilation-induced changes in the bodily self. The questionnaire is completed once, retrospectively, within 30 minutes of the end of the hyperventilation block, and refers to the hyperventilation block as a whole. Every item is rated on a 0-100 visual analogue scale; minimum 0, maximum 100. One value is reported per participant: the mean of the 41 item ratings, expressed on the same 0-100 scale. Higher scores indicate stronger disruption of bodily self-consciousness; the measure is descriptive and phenomenological, so neither a higher nor a lower score is a better or worse clinical outcome. The scale targets six bodily-self dimensions, but its factor structure has not been validated; dimension-level scores are therefore not pre-specified outcome measures and any dimensional scoring will be determined by exploratory analysis after data collection. Secondary endpoint, reported descriptively.
Time frame: Experimental visit (single visit, approximately 188 minutes); completed once at the end of the session, within 30 minutes of the hyperventilation block
Mean Amplitude of the Heartbeat-Evoked Potential (HEP) in Microvolts Over the Frontocentral Electrode Cluster
Amplitude of the heartbeat-evoked potential (HEP): one EEG parameter, reported in microvolts. Scalp EEG (32 channels) and ECG are recorded continuously throughout the experimental visit. EEG epochs time-locked to the ECG R-peak are averaged, and HEP amplitude is the mean voltage of that averaged waveform across one pre-specified post-R-peak time window and one pre-specified frontocentral electrode cluster. No other EEG parameter (for example spectral power) is reported under this outcome measure. One value per participant per condition: epochs are averaged within condition to give one HEP amplitude for each of the four within-subject cells (Synchronous/Normal, Asynchronous/Normal, Synchronous/Hyperventilation, Asynchronous/Hyperventilation), which enter one mixed-effects model with Synchrony and Breathing as within-subject factors. Exploratory secondary endpoint.
Time frame: Experimental visit (single visit, approximately 188 minutes)
Mean Respiratory Rate During the Hyperventilation and Normal-Breathing Blocks
Respiratory rate, reported in breaths per minute, derived from the continuous respiratory-belt and capnography recordings. One value is reported per participant per breathing condition: the mean respiratory rate across the normal-breathing block and across the four guided hyperventilation cycles. Higher values indicate faster breathing; the measure describes the physiological context in which the bodily-self measures were obtained and is not a clinical benefit or harm outcome. Exploratory secondary endpoint, reported descriptively.
Time frame: Experimental visit (single visit, approximately 188 minutes)
Mean Heart Rate Variability (RMSSD) During the Hyperventilation and Normal-Breathing Blocks
Heart rate variability derived from the continuous electrocardiogram, reported as the root mean square of successive RR-interval differences (RMSSD) in milliseconds. One value is reported per participant per breathing condition: the mean RMSSD across the normal-breathing block and across the four guided hyperventilation cycles. Higher values indicate greater beat-to-beat variability; the measure describes the physiological context in which the bodily-self measures were obtained and is not a clinical benefit or harm outcome. Exploratory secondary endpoint, reported descriptively.
Time frame: Experimental visit (single visit, approximately 188 minutes)
This platform is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional.