GENERATOR MECHANISM OF PAIN-RELATED EVOKED-POTENTIALS FOLLOWING CO2-LASER STIMULATION OF THE HAND - SCALP TOPOGRAPHY AND EFFECT OF PREDICTIVE WARNING SIGNAL

In order to clarify the generator mechanism of pain-related evoked pot entials (pain EPs), we studied the scalp topography of the pain EPs fo llowing CO2 laser stimulation of hand dorsum by using balanced stemove rtebral electrodes as the noncephalic reference in 11 normal volunteer s. We also examined the effects of predictive warning signal (light-em itting diode) on the pain EPs. In both the warned and unwarned conditi ons, all of the 22 hand stimulations showed a large negative component (N2) at the peak latency of about 213 ms followed by a large positive component (P2) at the peak latency of about 329 ms. A preceding small negative component (N1) at the peak latency of about 148 ms was detec ted in 12 of the 22 hand stimulations in the warned condition and in 1 3 of the 22 hand stimulations in the unwarned condition. P2 was signif icantly larger and occurred earlier in the warned condition than in th e unwarned condition, whereas other components did not differ between the two conditions, suggesting that an increased attention of the subj ect to the stimulus influenced the generator mechanism of the P2 compo nent. With regards to the scalp topography, N2 was maximal at Cz and w idespread transversely to both sides, whereas P2 was maximal at Cz or Pz and spread more posteriorly than N2. These findings suggest that P2 is generated by a different mechanism from N2 and is most likely asso ciated with pain-related cognitive function. N1 was localized to the c ontralateral central and midtemporal areas, confirming that the nocice ptive inputs are perceived in the sensory cortex in humans. The questi on as to whether the N1 component is generated in the hand area of the primary somatosensory cortex or in the secondary somatosensory cortex , or in both areas, remains to be solved.