Virtual Agency, Embodiment and Analgesia in Phantom Limb Pain Jonathan Cole, Greg Austwick, Richard Wynne, Jian Zhang and Chris Dawson. University of Bournemouth

Phantom Limb Sensation and Awareness “the subjective report of the awareness for a non-existing or deafferented body part in a mentally otherwise competent individual.” Weinstein, 1969. Up to 100%, i.e. it is normal. Vivid - try to walk on phantom leg Wide range of sensations, pressure, warmth, tickle Embodied and Owned Phantom Limb Sensation - ‘exteroceptive and or proprioceptive sensations; tingling, pressure, movement etc.’ Phantom Limb Awareness - ‘ a general awareness of the presence of the limb.’ Hunter et al, Brain, 2003

Phantom Limb Pain a majority of patients Not only after amputation - brachial plexus avulsion, nerve block spinal cord injury moderate to severe in round 60% of those affected; in spinal injury pain in 60-65%, severe in 20-25%. In round 15% interferes with social activity, in 20% work and sleep (Kooilman, et al, Pain, 2000, 87, 33-41, Rose et al, Pain, 1988, 34, 101-102). ‘Injury does not stop life but pain can.’

Pathogenesis of Chronic Pain Deafferentation, equivalent to tinnitus, Overlander - studies on shingles Central reorganisation and plasticity, Flor - SEP and functional imaging after amputation Mismatch between motor intention and sensory return, leading to pain associated with internal forward models unrestrained by peripheral feedback, or a breakdown in integrity of agency Ramachandran, Sirigu, Frith

Treatments for Phantom Limb Pain: imagined or observed movement of the phantom. 1. Below Knee Amputee swimming and playing golf. 2. Mirror box:

Virtual re-Embodiment Limb construction in virtual space Motion capture at stump to drive virtual limb leading to… Virtual re-Embodiment and pain relief?

Computer Graphics Computer graphics is now advanced enough, because of computer gaming technology to; allow the believable representation of human limbs in real-time. be affordable, by PC skinning, skeletal modelling and boundaries

Nvidia demo using modern shader technology

A finished Polygon Model, ready for texturing and rigging

A polygon model of a hand that has been exported into Milkshape where ‘bones’ are added

The bones are used to deform the polygon model – the animation is done by manipulating the bones.

Modelling of hand with bounding spheres for collision detection

Motion Capture. Ascension Bird series; magnetic motion capture PCI Bird lab Nest of Birds in field up to 4 sensors, one source, sensitivity +/- 1mm in 1.8m radius each sensor provides position and orientation information in 3d, sampled at 103Hz, lag <10msec Runs off a Windows PC using MS Direct 3d for rendering (interface for graphics card, NVidia Quadro FX series. (Open GL supported in future.) NVidia stereoscopic extension to run a pair of I-glasses (VR specs – IO Systems, USA.)

The environment

Transformations between gesture and avatar. Motion of the gesture is used proportionally to alter the rotations of the major joints of the avatar arm to perform the required simulated gesture, aiming for a natural appearance and hence for embodiment. The grasp pre-set at a set distance between apple and hand which opens and shuts, using collision detection. Temporal lag between motion and avatar ~ 16 ms

Gesture complexity. Set avatar motion and grasp, and a single vector of motion capture, total motion ~ 40 mm, resting distance error +/- 2mms. A single subject motion produces a single action. Error from temporal and positional relation between the extent of shoulder movement and the extent of avatar movement. Is an action goal directed or towards the avatar?

Patient, sex and age   Injury and duration of pain VAS pain, maximum and minimum MPQ rank and (weighted) Electrode placement VA VS Pain relief, VAS 1 F, 83 Rt forequarter amputation for tumour, 1996. Pain since 8 to 2, half day each 46, (47.9) Chest wall Yes 8 to 4.5 2 M, 37 Rt C6-T1 root avulsion 2000. Pain since. 7-8 2 hours per day, 2-3 rest 46, (57.0) mid-humerus stump No 3 M, 69 Lt brachial plexopathy 1997. Pain from 6 months. 8-9 to 3-4 round half time each 44, (49.2) Elbow 7-8 to 1-2 4 M, 36 Rt forequarter amputation, 1996. Pain since. 9 to 4. Severe most of the time. 41, (46.6) 8-9 to 2 5 M, 72 Rt mid-humerus amputation, 2005. Pain/Discomfort since 4 to 1-2 (less than half the time) 16, (17) 4 to 0 6 F 61 Lt forequarter Amputation, 2005. pain developing. 5 but sporadic only 11, (11.2)   VAS - visual analogue scale. MPQ – McGill Pain Questionnaire score. VA and VS – virtual agency and virtual sensation.

Phenomenology of virtual agency. A difference between ‘just’ seeing the avatar move and intending its movement, in terms of both the mental 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.’ This often did not involve the whole arm at once. ‘When trying to move the hand the fingers are stiff and seem to resist movement.’

Patient 4 - agency and analgesia. Pain Time, Phenomenology VAS mins 4 most of time before 4 10 ‘novel feeling of moving the arm, hand still clenched’ 0 20 ‘can feel and move phantom arm and pain disappears into background’ 0 15 ‘can feel and move arm, can feel hand, joints and knuckles and skin in hand when grasping, elbow less. It moves quickly when I do’ 0 30 ‘when trying to move the fingers are stiff... I can feel the elbow coming from the stump and the hand on the end of it, but the two do not match up’ 0 50 ‘the arm is now a gentle presence’

Patient 5 - agency and analgesia; Day 1. Pain Time, Phenomenology VAS mins Pain 8 most of time, up to 10 at times and for a third of the time 4. 8 0 10 ‘can move the fingers a bit but not the rest’ 20 ‘the arm moves well with the fingers, it is still painful but is taking the pressure off’ 6 30 ‘the wrist is beginning to move and I can feel a pressure in the arm trying to move’ 2 15 ‘when I move the fingers and hand there is pressure but no pain, they are not being ripped off or squashed’

Patient 5 - agency and analgesia; Day 2. Pain VAS Time, mins Phenomenology 9 0 2-3 20 can move hand and fingers and feel them them 2-3 40 became aware of some elbow pain once hand pain reduced 8-9 0 2-3 10 focussing on elbow once hand controlled 20 elbow straightened 2 40 can move fingers and hand with no pain there pain now in elbow 8-9 c60 balance between mental concentration on agency

Perception and Intention Planner Motor command Movement Proprioceptive re-afference Visual re-afference efference Sensory bias Forward model comparator Comparator Movement independent sensory input ? Modified from Tsakiris and Haggard 2005

Perception and Intention Planner Motor command Movement comparator efference Forward model Comparator Proprioceptive re-afference Sensory bias Visual re-afference Movement independent sensory input ? PAIN Modified from Tsakiris and Haggard 2005

Perception and Intention Planner Motor command Movement Pain Perception reduced comparator efference Forward model Comparator Proprioceptive re-afference Sensory bias Visual re-afference ‘VIRTUAL’ Movement independent sensory input Movement dependent sensory input Modified from Tsakiris and Haggard 2005

Two failures of virtual agency and analgesia. The recovery of intention and action needed considerable mental effort, at least during these trials over, mostly, two days. As patients’ concentration tired then so did virtual agency and embodiment, and their pain returned. Typically during a day’s trial they would develop agency within 30 minutes or so and then have several hours with pain reduction. Two with no virtual agency: they had had paralysis of the arm for 5 and 18 years before amputation. (Ramachandran’s ‘learned paralysis?’) Interestingly one of these patients felt her phantom arm move in a mirror-box, and being touched by her other one in the mirror. Passive movement and sensation may be different to agency, active movement.

Acknowledgement. This work was supported by a Showcase Award from The Wellcome Trust.