Phantom limb pain (PLP) is a type of chronic pain that follows limb amputation, brachial plexus avulsion injury, or spinal cord injury. Treating PLP is a well-known challenge. Currently, virtual reality (VR) interventions are attracting increasing attention because they show promising analgesic effects. However, most previous studies of VR interventions were conducted with a limited number of patients in a single trial. Few studies explored questions such as how multiple VR sessions might affect pain over time, or if a patient's ability to move their phantom limb may affect their PLP. Here we recruited five PLP patients to practice two motor tasks for multiple VR sessions over 6 weeks. In VR, patients "inhabit" a virtual body or avatar, and the movements of their intact limbs are mirrored in the avatar, providing them with the illusion that their limbs respond as if they were both intact and functional. We found that repetitive exposure to our VR intervention led to reduced pain and improvements in anxiety, depression, and a sense of embodiment of the virtual body. Importantly, we also found that their ability to move their phantom limbs improved as quantified by shortened motor imagery time with the impaired limb. Although the limited sample size prevents us from performing a correlational analysis, our findings suggest that providing PLP patients with sensorimotor experience for the impaired limb in VR appears to offer long-term benefits for patients and that these benefits may be related to changes in their control of the phantom limbs' movement.
As a part of self-consciousness, sense of agency (SoA) refers to our sensation that we initiate actions that lead to ensuing perceptual consequences. Previous neuroimaging studies manipulated voluntary actions to create differential levels of SoA, and found that the SoA network largely overlaps with the sensorimotor network. It is thus unclear whether effects observed in these regions are attributable to SoA or some specific aspects of action processes (e.g., intentionality of action) related to voluntary action control, or both. To disentangle the effect of action processes, here we utilized virtual reality (VR) technology to induce an illusive SoA in the absence of voluntary action, and examined it by functional magnetic resonance imaging (fMRI). Participants' experience of controlling an avatar hand elicited a persistent increase in temporal binding, a behavioral marker of SoA, when they merely observed the avatar hand's movement in the scanner. fMRI results showed that the SoA with the non-action was associated with a cluster centered in right angular gyrus, extending to right inferior parietal lobule and the right precuneus. These high-level associative areas have been implicated in SoA with voluntary actions for their role in perceptual-motor mismatch detection and action awareness. Other traditional SoA areas, especially those governing action intentionality and motor planning, were not implicated. Hence, controlling a virtual body can generate a salient SoA whose neural correlates are distinct from those supporting action control but partially overlapping with those governing post voluntary-action SoA processes. These findings also highlight the role of angular gyrus in giving rise to the self-consciousness changes associated with virtual body embodiment in VR and metaverse.
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