Atypical influence of biomechanical knowledge in Complex Regional Pain Syndrome-Towards a different perspective on body representation

Part of the multifaceted pathophysiology of Complex Regional Pain syndrome (CRPS) has been ascribed to a lateralized maladaptive neuroplasticity in sensorimotor cortices, a finding that has been corroborated by behavioral studies indicating that CRPS patients indeed present difficulties in mentally representing their painful limb. Hand laterality judgment tasks (HLT) are widely used to measure such difficulties, with the laterality of hand stimuli corresponding to the affected hand judged more slowly than the one of hand stimuli corresponding to the unaffected hand. Importantly, the HLT is also regularly used in the rehabilitation of CRPS and other chronic pain disorders, with the aim to activate motor imagery and, consequently, restoring the cortical representation of the limb. The potential of these tasks to elicit motor imagery is thus critical to their use in therapy. Yet, the influence of the biomechanical constraints (BMC) on HLT reaction time, supposed to reflect the activation of motor imagery, is rarely verified. In the present study we investigated the influence of the BMC on the perception of hand postures and movements. The results of a first experiment, in which a HLT was used, showed that CRPS patients were significantly slower than controls in judging hand stimuli, whether or not the depicted hand corresponded to their affected hand, but that their performance did not differ from controls when they judged non-body stimuli. Results regarding reaction time patterns reflecting the BMC were inconclusive in CRPS and controls, questioning the validity of the task in activating motor imagery processes. In a second experiment we therefore directly investigated the influence of implicit knowledge of upper-limb BMC on perceptual judgments of hand movements with the apparent body movement perception task. Participants judge the perceived path of movement between two depicted hand positions, with only one of the two proposed paths that is biomechanically plausible. While the controls chose the biomechanically plausible path most of the time, CRPS patients did not, indicating that the perception and/or use of the BMC seems to be disturbed in CRPS. These findings show a non-lateralized body representation impairment in CRPS, which might be related to difficulties in using correct knowledge of the bodys biomechanics. Most importantly however, our results, in agreement with previous studies, indicate that it seems highly challenging to measure motor imagery and the indexes of BMC with the classical HLT task, which has important implications for the rehabilitation of chronic pain with these tasks.