A threat to a virtual hand elicits motor cortex activation

Abstract

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.

Avatar Embodiment. Towards a Standardized Questionnaire

“…Despite being the least prevalent category on the reviewed experiments, behavioral responses are often measured in addition to using questionnaires. More importantly several experiments have shown correlations between questionnaires and physiological and behavioral responses (Maselli and Slater, 2013;González-Franco et al, 2014) thus supporting the relevance of questionnaires. Note that presenting response questions after each condition in within subject studies might reveal the aim of the experiment and influence the participant response to the next condition.…”

“…It is important to emphasize how radically different the effects of the experience can be depending on the degrees of embodiment that participants experience. Aspects such as participant's behavior or their physiology are heavily influenced by the embodiment score (González-Franco et al, 2014;Padrao et al, 2016;Slater and Sanchez-Vives, 2016). The embodiment itself can be modulated by the type, race and look of the avatar participants embody (Hershfield et al, 2011;Kilteni et al, 2013;Peck et al, 2013).…”

“…The physiometric measures support embodiment when people respond to actions performed on the virtual body in the same way as if the action was performed on the own body. For example, using EEG the same region in the brain that responds to threat was activated when an embodied virtual hand was stabbed with a virtual knife (González-Franco et al, 2014). Even though the evaluation of a perceptual illusion via a subjective questionnaire that is delivered after the fact may not render the best results (Slater, 2004), questionnaires are still the most prevalent metric, due to versatility and ease of use.…”

“…Even though the evaluation of a perceptual illusion via a subjective questionnaire that is delivered after the fact may not render the best results (Slater, 2004), questionnaires are still the most prevalent metric, due to versatility and ease of use. Additionally, several embodiment experiments that use questionnaires as well as quantitative measures have shown correlations between the objective effects of the experiment and the subjective embodiment levels of the participants as extracted from the questionnaires (González-Franco et al, 2014;Padrao et al, 2016). Therefore, comprehensive questionnaires may render a reasonable embodiment measure.…”

“…For example, some adaptations account for a whole-body substitution in which the body is collocated and in front of a virtual mirror (Slater et al, 2010b;Borland et al, 2013), compared to a dislocated body part shown in experiments with the RHI (IJsselsteijn et al, 2006;González-Franco et al, 2014). Yet, the many variations of the questionnaire in the literature make it impossible to compare results across experiments.…”