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

2. 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

4. 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, ).
‘Injury does not stop life but pain can.’

5. 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

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

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

10. 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

11. Nvidia demo using modern shader technology

12. A finished Polygon Model, ready for texturing and rigging

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

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

15. Modelling of hand with bounding spheres for collision detection

16. 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.)

17. The environment

18. 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

19. 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?

22. 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, 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 Pain since.
7-8 2 hours per day, 2-3 rest
46, (57.0)
mid-humerus stump No
3 M, 69
Lt brachial plexopathy 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, Pain since.
9 to 4. Severe most of the time.
41, (46.6)
8-9 to 2
5 M, 72
Rt mid-humerus amputation, Pain/Discomfort since
4 to 1-2 (less than half the time)
16, (17)
4 to 0
6 F 61
Lt forequarter
Amputation, 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.

23. 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.’

24. 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’
‘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’
‘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’
‘the arm is now a gentle presence’

25. 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.
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’
‘the wrist is beginning to move and I can feel a pressure in the arm trying to move’
‘when I move the fingers and hand there is pressure but no pain, they are not being ripped off or squashed’

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

27. 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

28. 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

29. 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

30. 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.

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