Dipole source analyses of early median nerve SEP components obtained from subdural grid recordings

The median nerve N20 and P22 SEP components constitute the initial response of the primary somatosensory cortex to somatosensory stimulation of the upper extremity. Knowledge of the underlying generators is important both for basic understanding of the initial sequence of cortical activation and to identify landmarks for eloquent areas to spare in resection planning of cortex in epilepsy surgery. We now set out to localize the N20 and P22 using subdural grid recording with special emphasis on the question of the origin of P22: Brodmann area 4 versus area 1. Electroencephalographic dipole source analysis of the N20 and P22 responses obtained from subdural grids over the primary somatosensory cortex after median nerve stimulation was performed in four patients undergoing epilepsy surgery. Based on anatomical landmarks, equivalent current dipoles of N20 and P22 were localized posterior to (n = 2) or on the central sulcus (n = 2). In three patients, the P22 dipole was located posterior to the N20 dipole, whereas in one patient, the P22 dipole was located on the same coordinate in anterior-posterior direction. On average, P22 sources were found to be 6.6 mm posterior [and 1 mm more superficial] compared with the N20 sources. These data strongly suggest a postcentral origin of the P22 SEP component in Brodmann area 1 and render a major precentral contribution to the earliest stages of processing from the primary motor cortex less likely.