Patients with complex regional pain syndrome overestimate applied force in observed hand actions.
BACKGROUND
Movement accuracy is ensured by interaction between motor, somatosensory, and visual systems. In complex regional pain syndrome (CRPS), this interaction is disturbed. To explore CRPS patients' visual perception of actions, we investigated how these patients evaluate the applied force in observed hand actions of another person.


METHODS
Nineteen patients suffering from unilateral upper-limb CRPS and 19 healthy control subjects viewed six different videos of left- and right-hand actions. They were asked to evaluate the applied force in each hand action, as well as their subjective sensations of unpleasantness and pain during the observation.


RESULTS
The patients overestimated the force applied in the videos: the ratings were two times as large as in the control subjects for actions performed with the hand corresponding to the patients' affected hand, and 1.5 times as large for actions corresponding to their healthy hand. The control subjects considered the stimuli neutral and painless, whereas the patients rated them unpleasant. Moreover, the patients felt increased pain during viewing actions performed with the hand corresponding to their affected side. The overestimation of force was related to the elicited unpleasantness and pain, but not to the patients' muscle strength.


CONCLUSIONS
We propose that the overestimation of force is explained both by the pain elicited by the observation and by the abnormal sensorimotor integration that is associated with perception of increased effort. This visually elicited unpleasantness and painfulness may promote avoidance of viewing own actions, further impairing the patients' motor performance.
Department of Neuroscience and Biomedical Engineering, Aalto University, Espoo, Finland.|Clinical Neurosciences, Neurology, University of Helsinki and Helsinki University Hospital, Finland.
J
J
Hotta
Clinical Neurosciences, Neurology, University of Helsinki and Helsinki University Hospital, Finland.|Pain Clinic, Department of Anaesthesiology, Intensive Care and Pain Medicine, University of Helsinki and Helsinki University Hospital, Finland.
H
H
Harno
Department of Neuroscience and Biomedical Engineering, Aalto University, Espoo, Finland.|Turku PET Centre, University of Turku, Finland.
L
L
Nummenmaa
Pain Clinic, Department of Anaesthesiology, Intensive Care and Pain Medicine, University of Helsinki and Helsinki University Hospital, Finland.
E
E
Kalso
Department of Neuroscience and Biomedical Engineering, Aalto University, Espoo, Finland.
R
R
Hari
Department of Neuroscience and Biomedical Engineering, Aalto University, Espoo, Finland.|Clinical Neurosciences, Neurology, University of Helsinki and Helsinki University Hospital, Finland.
N
N
Forss
D000328:Adult; D020918:Complex Regional Pain Syndromes / Q000503:physiopathology*; D005260:Female; D006225:Hand / Q000503:physiopathology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D009043:Motor Activity / Q000502:physiology*; D058748:Pain Perception / Q000502:physiology*; D014796:Visual Perception / Q000502:physiology*; D055815:Young Adult