Topography of middle-latency somatosensory evoked potentials following painful laser stimuli and non-painful electrical stimuli.

The aim of this study was to compare cerebral evoked potentials following selective activation of A beta and A delta fibers. In 15 healthy subjects, A beta fibers were activated by electrical stimulation of the left radial nerve at the wrist. A delta fibers were activated by short painful radiant heat pulses, applied to the dorsum of the left hand by a CO2 laser. Evoked potentials were recorded with 15-27 scalp electrodes, evenly distributed over both hemispheres (bandpass 0.5-200 Hz). The laser-evoked potentials exhibited a component with a mean peak latency of 176 msec (N170). Its scalp topography showed a parieto-temporal maximum contralateral to the stimulus side. In contrast, the subsequent vertex negativity (N240), which appeared about 60 msec later, had a symmetrical scalp distribution. Electrically evoked potentials showed a component at 110 msec (N110), that had a topography similar to the laser-evoked N170. The topographies of the N170 and N110 suggest that they may both be generated in the secondary somatosensory cortex. There was no component in the electrically evoked potential that had a comparable interpeak latency to the following vertex potential: for N60 it was longer, for N110 it was shorter. On the other hand, in the laser-evoked potentials no component could be identified the topography of which corresponded to the primary cortical component N20 following electrical stimulation.