Pins & Needles: Towards Limb Disownership in Augmented Reality

The seemingly stable construct of our bodily self depends on the continued, successful integration of multisensory feedback about our body, rather than its purely physical composition. Accordingly, pathological disruption of such neural processing is linked to striking alterations of the bodily self, ranging from limb misidentification to disownership, and even the desire to amputate a healthy limb. While previous embodiment research has relied on experimental setups using supernumerary limbs in variants of the Rubber Hand Illusion, we here used Augmented Reality to directly manipulate the feeling of ownership for one's own, biological limb. Using a Head-Mounted Display, participants received visual feedback about their own arm, from an embodied first-person perspective. In a series of three studies, in independent cohorts, we altered embodiment by providing visuotactile feedback that could be synchronous (control condition) or asynchronous (400ms delay, Real Hand Illusion). During the illusion, participants reported a significant decrease in ownership of their own limb, along with a lowered sense of agency. Supporting the right-parietal body network, we found an increased illusion strength for the left upper limb as well as a modulation of the feeling of ownership during anodal transcranial direct current stimulation. Extending previous research, these findings demonstrate that a controlled, visuotactile conflict about one's own limb can be used to directly and systematically modulate ownership - without a proxy. This not only corroborates the malleability of body representation but questions its permanence. These findings warrant further exploration of combined VR and neuromodulation therapies for disorders of the bodily self

The temporal limits of agency for reaching movements in augmented virtuality

The sense of agency (SoA) describes the feeling of being the author and in control of one’s movements. It is closely linked to automated aspects of sensorimotor control and understood to depend on one’s ability to monitor the details of one’s movements. As such SoA has been argued to be a critical component of self-awareness in general and contribute to presence in virtual reality environments in particular. A common approach to investigating SoA is to ask participants to perform goal-directed movements and introducing spatial or temporal visuomotor mismatches in the feedback. Feedback movements are traditionally either switched with someone else’s movements using a 2D video-feed or modified by providing abstracted feedback about one’s actions on a computer screen. The aim of the current study was to quantify conscious monitoring and the SoA for ecologically valid, three dimensional feedback of the participants’ actual limb and movements. This was achieved by displaying an Infra-Red (IR) feed of the participants’ upper limbs in an augmented virtuality environment (AVE) using a head-mounted display (HMD). Movements could be fed back in real-time (46ms system delay) or with an experimental delay of up to 570ms. As hypothesized, participant’s SoA decreased with increasing temporal visuomotor mismatches (p<.001), replicating previous findings and extending them to AVEs. In-line with this literature, we report temporal limits of 222±60ms (50% psychometric threshold) in N=28 participants. Our results demonstrate the validity of the experimental platform by replicating studies in SoA both qualitatively and quantitatively. We discuss our findings in relation to the use of virtual and mixed reality in research and implications for neurorehabilitation therapies

The Real Hand Illusion: Inducing disownership of the biological limb in virtual reality and the involvement of the right parietal cortex

‘What does it mean to own a body?’ is a fundamental question regarding the nature of the self. The bottom-up perceptual and higher order processes of how we distinguish what is our own body from the environment have been of great interest in recent years. The now classical ‘rubber hand illusion’ revealed how embodiment of a like life rubber hand can be achieved through manipulating visuo-tactile information. However, the RHI and subsequent iterations have often failed to replicate the phenomenology of disorders such as Xenomelia and Somatoparaphrenia, where individuals do not report ownership of a limb(s), rather than the misattribution of ownership to an extrabodily object. Therefore, we have developed the ‘Real Hand Illusion’, which endeavours to reduce ownership of one’s own biological limb through a virtual reality illusion. Participants viewed their hand being stroked by a paintbrush in the virtual environment, in the illusory condition, there was a 400ms visual latency leading to a disruption of multisensory integration. Feelings of ownership were reduced in the illusory condition as measured by a self-report questionnaire generated by the researchers. We also sought to investigate whether right parietal regions are involved in the processing of multisensory data, as has been suggested through clinical cases of individuals with body ownership disorders. Electrical stimulation of the P4 region using tDCS successfully modulated feelings of ownership. Cathodal stimulation conditions resulted in significantly higher sensations of ownership than anodal conditions. We have therefore developed a novel virtual paradigm which more closely reflects the phenomenology of disorders such as Xenomelia. We have also provided further evidence for the involvement of right sided parietal regions in processing multisensory information, and subsequently leading to feelings of ownership