Dipolar source modeling of somatosensory evoked potentials to painful and nonpainful median nerve stimulation

Dipolar source modeling might help in clarifying whether somatosensory evoked potentials (SEPs) after electrical stimulation at painful intensity contain any information related to the nociceptive processing. SEPs were recorded after left median nerve stimulation at three different intensities: intense but nonpainful (intensity 2); slightly painful (pain threshold; intensity 4); and moderately painful (intensity 6). Scalp SEPs at intensities 2, 4, and 6 were fitted by a five-dipole model. When the strength modifications of the source activities up to 40 ms were examined across the different stimulus intensities, no significant difference was found. In the later epoch (40-200 ms), a posterior parietal dipole and two bilateral sources probably located in the second somatosensory (SII) areas increased significantly their dipole moments when the stimulus was increased from 2 to 4 and became painful. Since no difference was found when the stimulus intensity was increased from 4 to 6, the observed increase of the dipolar strengths is probably related to a variation of the stimulus quality (nonpainful vs. painful), rather than of the stimulus intensity per se. Our findings lead us to conclude that a large convergence of nociceptive and non-nociceptive afferents probably occurs bilaterally in the SII areas.