Hyperventilation Effects on Bodily Self-Consciousness

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.