This study investigates how the sense of embodiment in virtual environments can be enhanced by multisensory feedback related to body movements. In particular, we analyze the effect of combined vestibular and proprioceptive afferent signals on the perceived embodiment within an immersive walking scenario. These feedback signals were applied by means of a motion platform and by tendon vibration of lower limbs, evoking illusory leg movements. Vestibular and proprioceptive feedback were provided congruently with a rich virtual scenario reconstructing a real city, rendered on a head-mounted display (HMD). The sense of embodiment was evaluated through both self-reported questionnaires and physiological measurements in two experimental conditions: with all active sensory feedback (highly embodied condition), and with visual feedback only. Participants' self-reports show that the addition of both vestibular and proprioceptive feedback increases the sense of embodiment and the individual's feeling of presence associated with the walking experience. Furthermore, the embodiment condition significantly increased the measured galvanic skin response and respiration rate. The obtained results suggest that vestibular and proprioceptive feedback can improve the participant's sense of embodiment in the virtual experience.

It is well known by the virtual hand illusion (VHI) that simultaneous and synchronous visuotactile sensory feedback within a virtual environment elicits the feeling of ownership of a virtual hand, by observing for some seconds in a scene a virtual hand being touched while at the same time receiving tactile stimulation on the real hand in the corresponding positions. In this paper, we investigate possible modulations in the feeling of ownership (sensation of owning a virtual hand) and of agency (sensation of owning virtual movements and actions) according to whether or not the participant's own motor acts (1) induce coherent self-activated visuotactile sensory stimulations; and (2) generate plausible consequences in the simulated environment. For this purpose, we elicited the VHI within a group of participants through a cross-modal integration of visuo-tactile sensory stimulations within a dynamic and physically plausible immersive virtual environment, where they were able to perform natural tasks in both passive and active agency conditions. Our results indicate that both feelings of ownership and agency can be achieved in immersive virtual environments, when the subject is realistically interacting and performing natural upper limb movements. We did not observe any significant difference in the VHI in terms of ownership and agency between the active and passive conditions, but we observed that a physically incongruent simulated interaction with the virtual world can lead to a significant disruption of ownership. Moreover, in the passive agency condition, a plausible physical behavior of the virtual hand was sufficient to elicit a partially complete sense of ownership, if measured in terms of proprioceptive drift, even in the presence of an asynchronous visuotactile sensory feedback. All these findings suggest that the multisensory feedback associated with a subject's own actions and the physical plausibility of the environment both act as determinant factors, influencing and modulating the vividness of the VHI.