Abstract:

The body ownership illusion (BOI) is associated with multisensory integration, including visuo-tactile, visuo- proprioceptive, tactile-proprioceptive, visual-interoceptive integration and so forth. A series of studies have demonstrated that BOI may be able to reduce acute and chronic pain. There are several key factors that influence the induction of subjective feeling of ownership over the fake body parts, such as synchronicity of multisensory integration, space between the real and fake body parts, the physical appearance of the fake body parts, a first-person perspective. These factors have also been proved to be critical in the effect of analgesia of BOI. In addition, BOI can be modulated by the perceived body/limb size, transparency of the body/limb and so on. The influence of these factors on pain is complex, which differs between healthy subjects and patients with chronic pain, various chronic pain states and the relevant body perception disturbances. Altered body representation represents one of the mechanisms associated with the analgesic impact of BOI. The neural mechanism of BOI on pain, however, still remains unknown, we propose that it is associated with cross-modal mirror therapy and visual analgesia. Thus based on the neural mechanisms of these two phenomenon, we suggest that the “body matrix” and the “pain matrix” might be involved. The concept of “body matrix” was proposed to interpret the induction of the sense of ownership and the complex relationship between representation of the body in the brain and the integrity of the body itself, which includes the insula and posterior parietal cortex. It processes and further integrates the sensory stimuli like visual, tactile, proprioceptive, interoceptive signals from the environment. To be specific, the induction of the sense of ownership could first activate the body matrix and then suppress the pain matrix in acute pain. In chronic pain, the analgesic effect may be related with the reversal of the cortical representation and functional disturbances. Moreover, the posterior parietal cortex may play a key role in these integrations, which needs to be further validated in fine-designed protocols. In summary, most of the researches support a link between embodied illusion and pain disorders, yet several studies didn’t find the analgesic effect of BOI. It might result from the experimental procedure that the artificial/fake hand was covered to avoid visual feedback, or may due to differences in the experimental settings of the control group. Future research topics may include the following aspects. Firstly, future research might want to consider more about the role of interoception in multisensory integration paradigms as exteroceptive signals may not suitable in patients with allodynia who are extremely sensitive to tactile stimuli. The usage of interoception enables the induction of sense of ownership without any tactile stimuli. Thus taking into account of interoception allows a better understanding of the contribution of interoception in bodily illusion and in pain modulation. Secondly, future studies need to identify the cognitive mechanism and neural substrates underlying the effect of BOI on pain. For instance, whether BOI analgesic effect has shared neural mechanisms with mirror therapy and visual analgesia needs further interrogation. Thirdly, how BOI affects pain varies differently in acute pain and chronic pain, and in different chronic pain conditions. Future research needs to shed further light on the identification of this complex phenomenon. Finally, robust and new paradigms of body ownership illusion are needed to further explore the full potential of BOI to modulate pain and to be applied in pain management in clinical populations.

Key words: body ownership, rubber hand illusion, pain, multisensory integration, visual analgesia