A novel finger illusion reveals reduced weighting of bimanual hand cortical representations in people with complex regional pain syndrome

Complex regional pain syndrome (CRPS) is associated with deficits in sensorimotor control. Herein we have used a novel finger illusion to investigate whether CRPS is associated with reduced weighting of bimanual hand representations. The illusion normally induces a compelling feeling that the hands are close together when in fact they are 12 cm apart. People with CRPS and age, gender, and dominant hand-matched controls tested the illusion in the midline then on either side of the midline. The illusion had 2 variants; the passive pincer-grip position, without contact (no grasp condition) and with contact (grasp condition) of the artificial finger. The primary outcome was the perceived vertical distance between the index fingers. Twenty people with CRPS and 20 controls participated (mean age 44.4 ± 11.7 years). During the no grasp condition, participants with CRPS perceived the vertical distance significantly closer to the actual 12 cm (mean 8.0 cm, 95% confidence interval 6.5-9.5 cm), than controls did (mean 6.4 cm, 95% confidence interval 5.5-7.2 cm]). That is, the illusion was weaker in people with CRPS than in controls during no grasp. There was no such difference during grasp; that is, both groups showed the predicted illusion response. There was no effect of hand placement relative to midline or relative to the opposite hand. We conclude that people with unilateral CRPS have lower weighting of bimanual hand representation than controls have, independent of hand location. However, adding additional cutaneous input returns those with CRPS to the expected performance. We suggest the results have clear clinical and research implications.