Laser-evoked potentials after painful hand and foot stimulation in humans: evidence for generation of the middle-latency component in the secondary somatosensory cortex.

The vertex potential (N2, P2) of the laser-evoked potential (LEP) is preceded by a small negativity (N1). The role of the secondary somatosensory cortex (SII) in generation of the N1 is established for the upper but not for the lower limb. We therefore investigated the N1 after painful radiant heat stimulation of hand and foot dorsum in 22 subjects. LEPs were recorded from the scalp with midline and temporal electrodes. After hand stimulation N1 was maximal in the contralateral temporal lead (mean peak latency 156 +/- 23 ms). After foot stimulation N1 was maximal in the same lead (200 +/- 22 ms). In the ipsilateral temporal lead, N1 appeared significantly smaller and later. N2 and P2 were maximal in midline electrodes for both stimulus sites. The latency shift between hand and foot stimulation was identical for all three components. These results suggest a contribution of temporo-parietal cortex (e.g. SII) to the N1 generation for stimulation of upper and lower limb.