Modulation of laser-evoked potentials by experimental cutaneous tonic pain

The present study aimed to investigate whether tonic cutaneous pain exerts any effect on the cortical processing of nociceptive input and if this effect may involve only body parts in pain. Tonic cutaneous pain was obtained in nine healthy human subjects by infusion of a hypertonic saline (5%) in the s.c. tissue over the hypothenar muscles (10 ml/h for 20 min). Nociceptive cutaneous CO2 laser-evoked potentials were recorded after stimulation of the right hand dorsum, which was adjacent to the painful area, and the right perioral region, corresponding to the adjacent cortical sensory area. Laser-evoked potentials were obtained before saline injection, at the peak pain and 20 min after pain disappeared. During saline infusion, the laser-evoked pain to right hand stimulation was reduced and the vertex laser-evoked potentials (N2a-P2, mean latency 181 ms and 319 ms for the N2a and the P2 potentials, respectively), which are generated in the anterior cingulate cortex, were significantly decreased in amplitude compared with the baseline. Moreover, the topography of these potentials was modified by cutaneous pain, shifting from the central toward the parietal region. Dipolar modeling showed that the dipolar source in the anterior cingulate cortex moved backward during saline infusion. This result suggests that cutaneous pain may modify the relative activities of the anterior and posterior anterior cingulate cortex parts, which are thought to be devoted to encode different aspects of pain sensation. No laser-evoked potential change was observed after stimulation of the right perioral region, suggesting that functional changes in the nociceptive system are selective for the painful regions and not for areas with cortical proximity.