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

Dipolar source modeling might help in clarifying whether somatosensory evok ed potentials (SEPs) after electrical stimulation at painful intensity cont ain any information related to the nociceptive processing. SEPs were record ed after left median nerve stimulation at three different intensities: inte nse but nonpainful (intensity 2); slightly painful (pain threshold; intensi ty 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 st imulus intensities, no significant difference was found. In the later epoch (40-200 ms), a posterior parietal dipole and two bilateral sources probabl y located in the second somatosensory (SII) areas increased significantly t heir dipole moments when the stimulus was increased from 2 to 4 and became painful. Since no difference was found when the stimulus intensity was incr eased from 4 to 6, the observed increase of the dipolar strengths is probab ly related to a variation of the stimulus quality (nonpainful vs, painful), rather than of the stimulus intensity per se. Our findings lead us to conc lude that a large convergence of nociceptive and non-nociceptive afferents probably occurs bilaterally in the SII areas, (C) 2000 John Wiley & Sons, I nc.