Proceedings of The Physiological Society

University College London December 2005 (2006) Proc Physiol Soc 1, C20

Oral Communications

Virtual agency, embodiment and analgesia in phantom limb pain

Cole, Jonathan; Austwick, Greg; Chris, Dawson; Zhang, Jian; Wynne, Richard;

1. Design, Engineering and Computing, University of Bournemouth, Poole, United Kingdom.

Phantom limb pain (PLP) in man may result from loss of sensory input and central plasticity (see Flor et al. 2001), though Ramachandran, following use of a mirror box, suggested that it may result from motor intention unrestrained by and mismatched with sensory return (Ramachandran & Rogers-Ramachandran, 1996). We have developed a virtual arm seen on a screen, or in VR-spectacles, which moves in real-time relation to movement of the amputees stump recorded using a magnetic motion sensor. The arm moves to a table and grasps an apple as the subject guides his stump or shoulder forward and medially. Six patients with severe PLP (aged 32 - 87 and with forequarter to mid-humerus amputations 2 months to 15 years previously) have tried this for several hours over 1-2 days. Four learnt to move the virtual arm and felt their phantom arms move and grasp. With virtual agency and re-embodiment their pain reduced, in three from 79 to 2-4 on a visual analog scale and in one from 4 to 0; (t=3.88, n=4, significant at the 2.5% level, compared with a 30% fall as considered due to distraction, Flor, 2002). Patients commented on the difference between just seeing the avatar move and intending its movement, in terms of both the perceived effort involved and the subsequent perception. It is much heavier and needs more effort to move the virtual arm than just to move the avatar from the shoulder alone. Pain relief required active movement and mental concentration. No effect was seen in two; one had poor motor control of the stump following root avulsion, whilst both had had paralysis of their phantoms (and arms) for years before the trial. This absence of an effect suggests decay in the mechanisms of intention with time, described by Ramachandran (1994) as learned paralysis. These results show that the sense of virtual agency can develop quite rapidly, within 30 min, and be associated with perception in the phantom of both predicted movement and touch. The mechanisms of this effect are unclear but suggest a close relation between agency, sensation and integrity of the body image (see Tsakiris & Haggard, 2005); pain may occur when these break down and be reduced during their temporary, virtual restoration.

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