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A Review of the Use of Virtual Reality in the Treatment of Phantom Limb Pain
sensors were attached to either the residual arm or the leg of the user so that movements of the stump were translated into movements of the virtual limb. Prior to beginning a goal-directed activity, participants performed a series of physical actions with their stump so that the gesture-based system could be calibrated. Afterward, movements of the stump were interpreted as physical expressions of a modeled gesture and determined probabilistically. The first environment interpreted motion for a missing arm. Patients were required to grasp an apple resting upon a table. The achievement of this goal was comprised of a number of actions – namely to reach, grasp, retrieve and replace the apple. In the second environment, for participants with a lower limb amputation, the user saw a bass drum as they might view it while sitting on a chair. Here, participants were required to complete four goal-related actions – raising the leg, performing a forward, pressing action of the foot on the pedal, releasing the pedal, and returning to a rest position. The system developed to interpret the movements of physical performance was dynamically recalibrated so that it was responsive to changes in physical performance. Participants, recruited through consultants in pain and prosthetics, were told that the project was experimental and if there was any effect on their pain it was likely to be short-term. They were also told that although a reduction in phantom limb pain was hoped for, the reverse could occur. The group with lowerlimb amputations was comprised of participants between the ages of 27-72 years old, with a mean of 49 years old. The upper-limb group was made up of participants between the ages of 36-82 years old, with a mean of 56 years old. They were taking, or had taken, a variety of analgesics. Some had also tried acupuncture, hypnosis, and Cognitive Behavioral Therapy (CBT) pain management. Of the seven participants with lower-limb amputations, Cole et al. reported that five gained a sense of agency for the virtual arm and did so usually within half an hour. Along with this sense of agency, participants described distinct perceptions. One man, with severe PLP in some of his fingers and elbow, reported a “buzzing” feeling in his first two fingers as he controlled the virtual arm to make a grasp movement. Another participant felt touch sensation when picking up the apple, so that he experienced sensations not just of movement but of exteroceptive touch also. With the merger of an experienced sense of virtual agency and sensation, pain was reduced. One participant remarked, “Now, when I move the fingers there is still pressure but there is no pain. They are not being ripped off or squashed.” Another stated, “When I move and feel the arm, it does not tingle. Pain disappears into the background and merges into the movement sensation.” A third participant developed such agency following the trial that her experience of her fingers being held in a painful
clawed position changed to one in which they began to open and the associated pain reduced. Moreover, this pain reduction was of a larger magnitude than she had previously experienced with use of a mirror-box. Of the seven participants with lower-limb amputations, four experienced significant reductions in pain. These experiences ranged from gaining control over the virtual leg to stronger phenomenological changes in the physical location, orientation of and touch by the phantom limb. For example, one participant commented, “I can feel the movement in the missing leg and maybe feel touch too. Once I am on the pedal I relax and feel my foot coming off it. It is second nature as though moving my full leg. The prosthesis is always a prosthesis, this is different. Here I am moving the foot. And at the moment the toes have sensation and though there is slight cramping in the toes, there is no pain” (Cole et al. 2009). Participants related how they “forgot” about or did not realize that their pain had ebbed away during the task. One said, “Until you mentioned it, I had not realized it was gone. One minute it was there and then, concentrating on the task, I did not realize it was gone.” While another said that being in the virtual environment “lightens the pain” and went on to state about the virtual limb, “I know it is not my leg and yet it feels as though it is.” Once he stopped moving the pain returned “within a second or two, but equally when I move and feel it is me, the pain reduces.” Another participant felt the touch of the drum pedal on his phantom foot. aUgmEntEd VirtUal rEality: thE dUBlin PsychoProsthEtics groUP The Dublin Psychoprosthetics Group point out that there are methodological constraints inherent in the use of conventional mirrors, including the task symmetry in bimanual movement of anatomical and reflected limbs, the dependant nature of visual feedback on the movement of an intact limb and the lack of phenomenological correspondence between the intact anatomical limb and the often idiosyncratic topography of phantom limbs, namely, “irregularly shaped” phantoms (O’Neill et al., 2003). Therefore, they sought to develop a system that would enable the control of a virtual phantom by a remaining corresponding anatomical limb. Potentially, this system could be adapted so that it produced a virtual representation tailored to the phenomenological experience of a phantom limb by the person using the system. The solution they arrived at was an Augmented Reality system for unilateral upper-limb amputees (Desmond et al., 2006; O’Neill et al., 2003). This consisted of a three-dimensional graphic representation of an arm controlled by a wireless data
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