By Palanisamy Vijayanand
Dismantled in rage by Shiva and reassembled in haste by Vishnu, as popular mythology goes, Ganesha’s fancy looking prosthetic cranium is perhaps the finest example of perfect sensorimotor integration and motor control. Outwitting his brother in the sibling rivalry stakes – to gain the fruit of wisdom from his amused parents – is evidence enough for the superior cognitive disposition of his transplanted head, in particular abstract thinking. Seemingly, the mythological pain physicians were in luck. Like a non-lovely couple in facebook, complaints about phantom sensations, or even pain, by Ganesha didn’t exist. Whether a pachyderm brain would have painfully missed its 3 tonne mass and the 150 kg diet, or whether the human-like peripheral inputs would have changed the tusker’s neuromatrix – one would never know. He certainly was never in need of a mirror-box. No such luck for mere mortals, for they might need it, but all the same unsure about the mechanisms (learned paralysis vs. spatial coupling) and effectiveness of such artificial visual feedback. What if the feedback is multisensory? Or even, what if the experience is provided by an immersive virtual environment? Digital solution for pain and rehabilitation is a fascinating area of research. While it is known that seeing one’s own body could have analgesic effects, it is not known whether this effect is transferable to newly embodied body parts via an alternative reality provided by fully immersive and interactive virtual reality.
Whether and how the illusion of owning a new body part affects human perception, and in particular pain detection, is inadequately known. Martini and colleagues, in a recent study, explored heat pain threshold under different experimental conditions involving ownership of a virtual arm. To create the illusion of ownership of the virtual body, they used multisensory correlations and first-person perspective with respect to a virtual body. A stereoscopic head-mounted display (HMD) allowed participants to experience an immersive virtual environment around them and to see a virtual body collocated with their own from a first-person perspective. When they looked down towards their own body, they could see the virtual body in place of their real own body. An experimenter constantly moved the participant’s right index finger in a flexion-extension fashion. This passive movement was meant to provide the proprioceptive feedback without calling into play the role of agency induced by active movement. Finger tracking was done with sensors and their coordinates in space were computed. Hence, when the participant’s finger was moved, the avatar’s finger could move accordingly, mimicking exactly the same movements at the same time. The pain threshold was tested by thermal stimulation under four conditions: (1) synchronous movements of the real and virtual fingers; (2) asynchronous movements; (3) seeing a virtual object instead of an arm; and (4) not seeing any limb in real world.
After each condition, the HMD was removed and the participants were given a questionnaire, which included different questions to evaluate anxiety, attention, presence in the virtual environment and body ownership for all given conditions. The only condition reporting significant difference with the others was the synchronous condition, suggesting that only when participants had the illusion to own the virtual body (ownership) their pain threshold was effectively higher, while just the vision of an avatar’s body or of an object replacing the body or just being immersed in the virtual environment (presence) did not yield any statistically relevant difference. Importantly, the synchronous movement of the finger not only led to the ownership of the hand but it extended to the entire arm. Virtual reality could decrease pain sensation through a decreased attention to the own body. However, no significant differences in self-reported levels of attention paid to the painful stimulus were found among conditions, suggesting that the difference in pain threshold observed in the synchronous group was probably not due to mere attentional processes. Characteristic of the synchronous condition was that correspondence between the visual, motor and the proprioceptive inputs provided the experience of ‘embodying’ the avatar’s limb, a phenomenon already documented as effective for inducing the virtual hand illusion
Virtual reality technology, by creating sensory environments that can be replicated identically across experiments and that are under the full control of the experimenter, is a versatile tool for pain research. For instance, it has been shown that the manipulation of one’s hand or limb size affects pain, such that seeing the hand smaller decreases pain, while seeing the hand larger increases it. Furthermore, it has already been reported to be effective as an adjunctive analgesic in burn injuries, phantom limb pain and complex regional pain syndromes. The Greeks believed that the brain existed to produce mucous, giving a whole new meaning to blowing one’s brains out. This study is also evidence that the Cartesian dichotomy between body and mind is getting narrower and narrower. Not sure whether Descartes with his fetish for women with crossed eyes (courtesy: Encyclopedia Britannica), would have loved this narrowness as much as that of the pupils of his lady love. In Sanskrit, the word buddhi, variously translated as intelligence, wisdom or intellect, is a feminine noun – all associated with Ganesha. And, it looks like for long he will remain the thinking woman’s crumpet, or modhak.