Some chronic pain states are associated with a distortion of the perceived size or shape of the painful area, and multisensory illusions that disrupt these dimensions can modulate pain in healthy controls and people with painful disorders. Illusory hand resizing has recently been found to relieve pain in hand OA, raising the possibility that the illusion corrects some underlying perceptual disturbance. We evaluated this possibility by measuring perceived hand size in healthy controls and those with painful hand OA before and after illusory hand resizing. The aim was to investigate whether people with painful hand OA have distorted representations of hand size and whether these representations are malleable. We hypothesized that hand OA is associated with a distorted mental representation of the painful hand and that perceived hand size can be modulated via multisensory illusion.
Twelve volunteers with painful hand OA and 12 healthy age-matched controls performed three tasks (hand stretch, hand shrink and no illusion) in a randomized order then estimated the size of their hand using an adjustable photographic image.
Our hypotheses were supported: under normal conditions, perceived hand size was smaller for the OA group than for healthy controls, consistent with a distorted mental representation of the painful hand. Furthermore, illusory stretching increased perceived hand size in both groups, while illusory shrinking decreased perceived hand size in healthy controls but not in the OA group.
These results suggest that hand OA is associated with a distorted mental representation of the painful hand and are consistent with the idea that the pain relief offered by multisensory illusions may work via normalization of this distortion.