Evidence for a change in neural processing in phantom limb pain patients

The present study was designed to investigate differences in neural processing of pain-related semantic information in amputees with and without chronic phantom limb pain and healthy controls. One-hundred-and-twenty words (40 neutral, 40 body-related, 40 pain-related) were presented in pseudorandom order to probe group differences in the perception of pain-related information. Visual evoked potentials (VEP) to the words were recorded from 11 scalp locations, EMG was measured at the stump and the contralateral side and bilaterally from the M. corrugator. In addition, heart rate and skin conductance responses were obtained. Both early and late VEP components were altered. Pain-free amputees showed a reduced N100 amplitude compared to phantom limb pain patients and healthy controls whereas the phantom limb pain patients displayed a significantly enhanced late positivity (500-800 msec after word onset). VEP components were unaffected by word type. The peripheral EMG responses were significantly different among the groups: phantom limb pain patients showed larger EMG reactions on the stump than on the contralateral side, whereas pain-free amputees showed the opposite effect. The data suggest an enhanced central and peripheral processing of visual stimuli in phantom limb pain patients.