Description

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 \[Haggard, 2017; Scandola et al., 2019\]: 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 \[Estevers et al., 2025\].

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) \[Hägni et al, 2008; Giangualano et al, 2025\], cardiac activity for heart rate variability (HRV) \[Ronca et al, 2024; Marìn-Morales, 2021; Hassard 2022\] , and skin temperature \[Salomon et al, 2013; Crivelli et al, 2021\], 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) \[Roth \& Latoschik, 2020)\].

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).