Investigating Techniques to Enhance the Sense of Embodiment in Immersive Virtual Reality

This is a prospective, single-center, non-profit interventional study conducted at IRCCS Eugenio Medea (Bosisio Parini, Italy) in collaboration with CNR-STIIMA. The aim is to investigate factors that influence embodiment during immersive virtual reality (VR), in healthy volunteers and in individuals with acquired neurological injuries (spinal cord injury or stroke). The study evaluates the effects of: (1) visuo-tactile stimulation, (2) VR biofeedback synchronized with the participant's heart rate, (3) the degree of active control over the avatar and virtual environment, and (4) familiarity with the performed action.

Each participant completes three experimental days. On Day 1, participants experience multiple short VR conditions, including a baseline condition (no additional stimulation), visuo-tactile stimulation, heart-rate-synchronized biofeedback, and active hand control of the avatar using motion tracking and data gloves. After each condition, participants perform a brief reaction-time task used in the literature as an objective, performance-based measure related to embodiment: brief light stimuli are presented either on the avatar's body or in the surrounding virtual environment, and participants respond as quickly as possible when a light appears. Reaction times to body versus environment stimuli are compared, and the task is repeated after each condition to examine whether performance changes depending on the preceding VR experience.

On Day 2, participants complete immersive VR cycling tasks designed to probe sense of agency and body ownership by manipulating the match between visual feedback and the participant's actual movements (matched vs mismatched feedback) and by comparing active versus passive movement conditions. During Day 2, electroencephalography (EEG) is recorded to investigate neural correlates associated with embodiment and agency under the different VR manipulations.

On Day 3, participants complete first-person VR mobility scenarios (walking or wheelchair) to evaluate embodiment in relation to task familiarity, comparing passive versus active exploration, with and without heart-rate-synchronized biofeedback.

Across all study days, participants wear wearable sensors to record electrodermal activity (EDA), cardiac activity for heart rate variability (HRV), and skin temperature, providing objective physiological measures that may vary with embodiment-related experiences. After each condition on each study day, participants also complete the Virtual Embodiment Questionnaire (VEQ).

Planned analyses will compare experimental conditions and participant groups (within each study day) using both subjective outcomes (VEQ scores) and objective measures (reaction-time task performance, physiological signals, and EEG metrics).

Participants complete multiple immersive virtual reality conditions across three study days. Conditions manipulate factors expected to influence embodiment. The order of conditions is randomized within each study day. Subjective embodiment is assessed after each condition using questionnaire, together with objective physiological measures.

Behavioral: Immersive Virtual Reality Embodiment Manipulations

Participants undergo multiple immersive virtual reality conditions designed to modulate embodiment. Conditions include synchronized visuo-tactile stimulation, heart-rate-synchronized visual biofeedback, varying degrees of active control of the avatar, and tasks with different levels of familiarity. Conditions are presented as short VR scenarios across three study days, with randomized order within each day.

Time frame: Immediately after each VR condition on Day 1, Day 2, and Day 3.

Embodiment-related self-report assessed with the Virtual Embodiment Questionnaire (VEQ). Three subscale scores are computed: Ownership, Agency, and Perceived Change in Body Representation. Each subscale score is calculated as the mean of 4 items rated on a 7-point Likert scale (1 = strongly disagree to 7 = strongly agree). Higher scores indicate stronger perceived ownership of the virtual body (Ownership), greater sense of control over the virtual body/actions (Agency), and stronger perceived change in body representation during VR (Perceived Change in Body Representation).

Time frame: Immediately after each VR condition on Day 1.

Participants perform a VR reaction-time task in which brief light stimuli are presented at predefined locations: on the avatar's right upper limb, left upper limb, right lower limb, left lower limb, or in the virtual environment outside the avatar. Stimuli occur at randomized intervals (1-5 seconds). Participants respond as quickly as possible when a light appears. The outcome is the mean reaction time (milliseconds) computed separately for each stimulus location category. Location-specific averages are used to capture potential asymmetries relevant to the clinical groups (e.g., right/left differences in stroke due to hemiparesis, and upper vs lower limb differences in spinal cord injury due to predominant lower-limb impairment).

Time frame: During each VR condition (60 seconds on Day 1 and Day 3; 120 seconds on Day 2)

Electrodermal activity (EDA), a measure of sympathetic arousal, includes a tonic component (skin conductance level, SCL) and a phasic component (skin conductance responses, SCR). EDA is recorded during the VR sessions. For each VR condition, EDA metrics are computed relative to the baseline. The tonic component is summarized as the mean SCL during the condition. Phasic activity is summarized by SCR measures, including the number of SCR peaks and the mean SCR peak amplitude during the condition. Baseline-referenced values are computed as change from baseline for each metric.

Time frame: During each VR condition (60 seconds on Day 1 and Day 3; 120 seconds on Day 2)

Chest skin temperature will be analyzed as a slow physiological indicator that may vary with embodiment-related experiences in VR; based on prior findings, decreases in peripheral skin temperature have been reported with stronger embodiment. For each VR condition, skin temperature will be summarized as the mean value and condition values will be expressed relative to the baseline period (change from baseline).

Time frame: During each VR condition (60 seconds on Day 1 and Day 3; 120 seconds on Day 2)

Electrocardiogram (ECG) is recorded continuously during each study session; on Day 1 and Day 3, the ECG-derived heart rate is also used to provide real-time heart-rate visual biofeedback in VR. R-peaks are detected to derive the RR-interval time series, with artifact beat correction prior to analysis. Heart rate variability (HRV) is quantified for each VR condition and compared with baseline. Planned HRV metrics include standard time-domain indices (e.g., mean RR, SDNN, RMSSD) and frequency-domain indices (e.g., LF and HF power and LF/HF ratio). In the literature, HRV frequency components have been reported as sensitive to embodiment-related manipulations, with higher LF activity associated with lower embodiment and higher HF activity associated with moderate/high embodiment.

Time frame: During each VR condition on Day 2 (120 seconds)

Electroencephalography (EEG) is recorded during the second study day using the eego mylab system with a 32-channel cap. EEG data will be pre-processed and analyzed in MATLAB using EEGLAB and custom scripts to remove artifacts and restrict the frequency content to the bands of interest. Power spectral density is computed and summarized within standard EEG bands (Delta, Theta, Alpha, and Beta). From band-power features, indices reflecting cognitive state are derived. For each VR condition, EEG-derived indices are compared with the preceding resting baseline segment to quantify condition-related deviations. In prior VR research, spectral features-particularly in the alpha and beta bands, including spatio-spectral patterns-have been investigated as potential correlates of embodiment-related processes.

Studio Dell'Embodiment In Ambienti Di Realtà Virtuale Immersiva