Space-based bias of covert visual attention in Complex Regional Pain Syndrome

Some patients with Complex Regional Pain Syndrome (CRPS) report that movements of the affected limb are slow, more effortful, and lack automaticity. These symptoms have been likened to the syndrome called hemispatial neglect that sometimes follows brain injury, in which patients exhibit attentional impairments and problems with movements affecting the contralesional side of the body and space. Psychophysical testing of CRPS patients has found evidence for spatial biases when judging visual targets distanced at two metres, but not in directions that indicate reduced attention to the affected side. In contrast, when judging visual or tactile stimuli presented on their own body surface, or pictures of hands and feet within arms’ reach, CRPS patients exhibited a bias away from the affected side. What is not yet known is whether CRPS patients only have biased attention for bodily-specific information in the space within arms’ reach, or whether they also show a bias for information that is not associated with the body, suggesting a more generalized attention deficit. Using a temporal order judgement (TOJ) task we found that CRPS patients processed visual stimuli more slowly on the affected side (relative to the unaffected side) when the lights were projected onto a blank surface (i.e., when no bodily information was visible), and when the lights were projected onto the dorsal surfaces of their uncrossed hands. However, with the arms crossed (such that the left and right lights projected onto the right and left hands, respectively), patients’ responses were no different than controls’. These results provide the first demonstration of a generalized attention bias away from the affected side of space in CRPS patients that is not specifically related to bodily information. They also suggest a separate and additional bias of visual attention away from the affected hand. The strength of attention bias was predicted by scores on a self-report measure of body perception distortion; but not by pain intensity, time since diagnosis, or affected body-side (left or right). At an individual level, those patients whose upper limbs were most affected had a higher incidence of inattention than those whose lower limbs were most affected. However, at a group level, affected limb (upper or lower) did not predict bias magnitude; nor did three measures designed to assess possible asymmetries in the distribution of movements across space. In conclusion, it is suggested that inattention in near space in CRPS may arise in parallel with a distorted body perception