Inside virtual reality, users can embody avatars that are collocated from a first-person perspective. When doing so, participants have the feeling that the own body has been substituted by the self-avatar, and that the new body is the source of the sensations. Embodiment is complex as it includes not only body ownership over the avatar, but also agency, co-location, and external appearance. Despite the multiple variables that influence it, the illusion is quite robust, and it can be produced even if the self-avatar is of a different age, size, gender, or race from the participant's own body. Embodiment illusions are therefore the basis for many social VR experiences and a current active research area among the community. Researchers are interested both in the body manipulations that can be accepted, as well as studying how different self-avatars produce different attitudinal, social, perceptual, and behavioral effects. However, findings suggest that despite embodiment being strongly associated with the performance and reactions inside virtual reality, the extent to which the illusion is experienced varies between participants. In this paper, we review the questionnaires used in past experiments and propose a standardized embodiment questionnaire based on 25 questions that are prevalent in the literature. We encourage future virtual reality experiments that include first-person virtual avatars to administer this questionnaire in order to evaluate the degree of embodiment.

Advances in computer graphics algorithms and virtual reality (VR) systems, together with the reduction in cost of associated equipment, have led scientists to consider VR as a useful tool for conducting experimental studies in fields such as neuroscience and experimental psychology. In particular virtual body ownership, where the feeling of ownership over a virtual body is elicited in the participant, has become a useful tool in the study of body representation in cognitive neuroscience and psychology, concerning how the brain represents the body. Although VR has been shown to be a useful tool for exploring body ownership illusions, integrating the various technologies necessary for such a system can be daunting. In this paper, we discuss the technical infrastructure necessary to achieve virtual embodiment. We describe a basic VR system and how it may be used for this purpose, and then extend this system with the introduction of real-time motion capture, a simple haptics system and the integration of physiological and brain electrical activity recordings.

This paper reports an experiment that investigated people's body ownership of an avatar that was observed in a virtual mirror. Twenty subjects were recruited in a within-groups study where 10 first experienced a virtual character that synchronously reflected their upper-body movements as seen in a virtual mirror, and then an asynchronous condition where the mirror avatar displayed prerecorded actions, unrelated to those of the participant. The other 10 subjects experienced the conditions in the opposite order. In both conditions the participant could carry out actions that led to elevation above ground level, as seen from their first person perspective and correspondingly in the mirror. A rotating virtual fan eventually descended to 2m above the ground. The hypothesis was that synchronous mirror reflection would result in higher subjective sense of ownership. A questionnaire analysis showed that the body ownership illusion was significantly greater for the synchronous than asynchronous condition. Additionally participants in the synchronous condition avoided collision with the descending fan significantly more often than those in the asynchronous condition. The results of this experiment are put into context within similar experiments on multisensory correlation and body ownership within cognitive neuroscience.

We report an ex periment where participants observed an attack on their virtual body as ex perienced in an immersive virtual reality (IVR) system. Participants sat by a table with their right hand resting upon it. In IVR they saw a virtual table that was registered with the real one, and they had a virtual body that substituted their real body seen from a first person perspective. The virtual right hand was collocated with their real right hand. Event-related brain potentials (ERPs) were recorded in two conditions, one where the participant's virtual hand was attacked with a knife and a control condition where the knife only struck the virtual table. Significantly greater P450 potentials were obtained in the attack condition confirming our ex pectations that participants had a strong illusion of the virtual hand being their own, which was also strongly supported by questionnaire responses. Higher levels of subjective virtual hand ownership correlate with larger P450 amplitudes. Mu-rhythm Event Related Desynchronization (ERD) in the motor cortex , and Readiness Potential (C3-C4) negativity were clearly observed when the virtual hand was threatened -as would be ex pected if the real hand was threatened and the participant tried to avoid harm. Our results support the idea that eventrelated potentials (ERPs) may provide a promising non-subjective measure of virtual embodiment. They also support previous ex periments on pain observation and are placed into contex t of similar ex periments and studies of body-perception and body-ownership within cognitive neuroscience.

How do we recognize ourselves as the agents of our actions? Do we use the same error detection mechanisms to monitor self-generated vs. externally imposed actions? Using event-related brain potentials (ERPs), we identified two different error-monitoring loops involved in providing a coherent sense of the agency of our actions. In the first ERP experiment, the participants were embodied in a virtual body (avatar) while performing an error-prone fast reaction time task. Crucially, in certain trials, participants were deceived regarding their own actions, i.e., the avatar movement did not match the participant's movement. Self-generated real errors and false (avatar) errors showed very different ERP signatures and with different processing latencies: while real errors showed a classical frontal-central error-related negativity (Ne/ERN), peaking 100ms after error commission, false errors elicited a larger and delayed parietal negative component (at about 350-400ms). The violation of the sense of agency elicited by false avatar errors showed a strong similarity to ERP signatures related to semantic or conceptual violations (N400 component). In a follow-up ERP control experiment, a subset of the same participants merely acted as observers of the avatar correct and error movements. This experimental situation did not elicit the N400 component associated with agency violation. Thus, the results show a clear neural dissociation between internal and external error-monitoring loops responsible for distinguishing our self-generated errors from those imposed externally, opening new avenues for the study of the mental processes underlying the integration of internal and sensory feedback information while being actors of our own actions.

The aim of this paper is to further the understanding of embodiment by 1) analytically determining the components defining embodiment, 2) increasing comparability and standardization of the measurement of embodiment across experiments by providing a universal embodiment questionnaire that is validated and reliable, and 3) motivating researchers to use a standardized questionnaire. In this paper we validate numerically and refine our previously proposed Embodiment Questionnaire. We collected data from nine experiments, with over 400 questionnaires, that used all or part of the original embodiment 25-item questionnaire. Analysis was performed to eliminate non-universal questions, redundant questions, and questions that were not strongly correlated with other questions. We further numerically categorized and weighted sub-scales and determined that embodiment is comprised of interrelated categories of Appearance, Response, Ownership, and Multi-Sensory. The final questionnaire consists of 16 questions and four interrelated sub-scales with high reliability within each sub-scale, Chronbach’s α ranged from 0.72 to 0.82. Results of the original and refined questionnaire are compared over all nine experiments and in detail for three of the experiments. The updated questionnaire produced a wider range of embodiment scores compared to the original questionnaire, was able to detect the presence of a self-avatar, and was able to discern that participants over 30 years of age have significantly lower embodiment scores compared to participants under 30 years of age. Removed questions and further research of interest to the community are discussed.

In Virtual Reality (VR) it is possible to induce illusions in which users report and behave as if they have entered into altered situations and identities. The effect can be robust enough for participants to respond “realistically,” meaning behaviors are altered as if subjects had been exposed to the scenarios in reality. The circumstances in which such VR illusions take place were first introduced in the 80's. Since then, rigorous empirical evidence has explored a wide set of illusory experiences in VR. Here, we compile this research and propose a neuroscientific model explaining the underlying perceptual and cognitive mechanisms that enable illusions in VR. Furthermore, we describe the minimum instrumentation requirements to support illusory experiences in VR, and discuss the importance and shortcomings of the generic model.

No abstract