Proprioceptive-augmented virtual reality influences the kinematic properties of movement imitation

BACKGROUND: Virtual Reality (VR) has been proposed as a tool in rehabilitation to create more realistic, interactive, and engaging environments that can enhance the sense of embodiment, and thus possibly the effect of the treatment itself. Previous research has shown that combining movement observation with somatosensory stimulation may further improve training outcomes. The present study aimed to examine whether a protocol integrating the observation of human movements within an VR environment with proprioceptive stimulation, thereby establishing a Proprioceptive-Augmented virtual Reality (PAR) paradigm, could enhance participants' ability to imitate the dynamic properties of movements presented in VR. METHODS: Twenty-three healthy young adults were asked to imitate the features of a flexion-extension forearm movement observed in an immersive VR environment, either while receiving or not receiving 80 Hz muscle tendon vibration that induced an illusory sensation of movement congruent with the observed direction (PAR). The imitation task was performed with the stimulated forearm either after the stimulation had ended (OFFLINE imitation) or concurrently with it (ONLINE imitation). Participants' ability to reproduce the avatar's mean and maximal velocities was assessed (reproduction error). Root means square error (RMSE) between avatar and participants' trajectory was calculated. In addition, a questionnaire evaluating the sense of embodiment was administered after the OFFLINE imitation task under both conditions, as well as the evaluation of the vividness evoked by PAR and VR. RESULTS: In the OFFLINE task the reproduction error was lower in PAR than in VR, while the opposite was obtained in the ONLINE task. In the OFFLINE task no difference emerged between PAR and VR RMSE values, whilst in the ONLINE task PAR RMSE was higher than that in VR. PAR evoked a higher sense of embodiment and higher vividness compared to VR when considering all the domain tested (body ownership, sense of agency, location of the body). CONCLUSIONS: These findings indicate that proprioceptive stimulation in the PAR condition positively affected movement reproduction during the OFFLINE imitation task, likely benefiting from the heightened sense of embodiment and vividness elicited in this condition. This suggests that muscle tendon vibration can enhance the perception of self-motion in VR, enabling richer interaction strategies and offering promising prospects for sensorimotor rehabilitation.