PAIN-RELATED SOMATOSENSORY-EVOKED POTENTIALS FOLLOWING CO2-LASER STIMULATION OF FOOT IN MAN

Since our previous study of pain somatosensory evoked potentials (SEPs ) following CO2 laser stimulation of the hand dorsum could not clarify whether the early cortical component N1 was generated from the primar y somatosensory cortex (S1) or the secondary somatosensory cortex (SII ) or both, the scalp topography of SEPs following CO2 laser stimulatio n of the foot dorsum was studied in 10 normal subjects and was compare d with that of the hand pain SEPs and the conventional SEPs following electrical stimulation of the posterior tibial nerve recorded in 8 and 6 of the 10 subjects, respectively. Three components (N1, N2 and P2) were recorded for both foot and hand pain SEPs. N1 of the foot pain SE Ps was maximal at the midline electrodes (Ct or CPz) in all data where that potential was recognized, but the potential field distribution w as variable among subjects and even between two sides within the same subject. N1 of the hand pain SEPs was maximal at the contralateral cen tral or midtemporal electrode. The scalp distribution of N2 and P2, ho wever, was not different between the foot and hand pain SEPs. The mean peak latency of N1 following stimulation of foot and hand was found t o be 191 msec and 150 msec, respectively, but there was no significant difference in the interpeak latency of N1-N2 between foot and hand st imulation. It is therefore concluded that N1 of the foot pain SEPs is generated mainly from the foot area of SI. The variable scalp distribu tion of the N1 component of the foot pain SEPs is likely due to an ana tomical variability among subjects and even between sides.