Dynamic brain topography of somatosensory evoked potentials and equivalentdipoles in response to graded painful skin and muscle stimulation

The differential effects of painful stimulation of skin vs. muscle on the c erebral electrophysiology have been poorly described. This study examined t he somatosensory evoked potentials (SEPs) and the associated dipole models of non-painful and graded painful electrical stimulation applied to the ski n and muscle in 20 healthy subjects. With the psychophysical stimulus-respo nse functions determined, the skin stimulation showed a steeper slope than muscle stimulation. For both types of stimulation, the SEPs indicated a sim ilar temporo-spatial activation sequence: F4/N90-P4/P95, Fc2/N135, Cz/P250, Cz/P300, and Cz/N460. The SEP amplitudes increased significantly with the stimulus intensities in these components. The peak SEP latencies of skin st imulation were in general shorter than that of muscle stimulation. The SEP amplitudes to skin stimulation were significantly larger than those caused by muscle stimulation at every stimulus intensity level, except the early m id-latency component. In this case, muscle stimulation caused higher amplit udes over the contralateral parietal-frontal sites. For both types of stimu lation, the topographic maps were quite similar. Equivalent dipole modeling revealed identical site parameters (<1.0 cm) between skin and muscle stimu lation. However, the electrical skin stimulation did not correlate with the pain intensity. Pain intensity, in contrast, was uniquely associated with the Cz/P250 amplitudes for the muscle stimulation. It is concluded that non -nociceptive and nociceptive electrical stimuli applied to skin and muscle are processed in the common cerebral areas, but exhibit differential SEP ef fects.