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 spaceMotion capture at stump to drive virtual limb leading to…Virtual re-Embodiment and pain relief?

Computer GraphicsComputer 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)

Electrodeplacement

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 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 No 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 Yes Yes 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) Chest wall Yes Yes 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) mid-humerus stump

Yes Yes 4 to 0

6 F 61

 Lt forequarterAmputation, 2005. pain developing.

5 but sporadic only

11, (11.2) Chest wall No Yes No

 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, PhenomenologyVAS mins

4 most of time before 0

4 10 ‘novel feeling of moving the arm, hand still clenched’0 20 ‘can feel and move phantom arm and pain disappears into

background’

00 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’

0 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

Proprioceptive re-afference

Visual re-afference

efference

Sensory bias

Forward model

comparator

Comparator

Movement independent sensory input ? PAIN

Modified from Tsakiris and Haggard 2005

Perception and Intention

Planner Motor command Movement

Proprioceptive re-afference

Visual re-afference ‘VIRTUAL’

efference

Sensory bias

Forward model

comparator

Comparator

Movement dependent sensory input

Modified from Tsakiris and Haggard 2005

Movement independent sensory input

PainPerceptionreduced

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.