Dipole source analyses of early median nerve SEP components 1 obtained from subdural grid recordings

30 31 Objective: The median nerve N20 and P22 SEP components constitute the initial 32 response of the primary somatosensory cortex to somatosensory stimulation of the 33 upper extremity. Knowledge of the underlying generators is important both for basic 34 understanding of the initial sequence of cortical activation and to identify landmarks 35 for eloquent areas to spare in resection planning of cortex in epilepsy surgery. We 36 now set out to localize the N20 and P22 using subdural grid recording with special 37 emphasis on the question of the origin of P22: Brodmann area 4 versus area 1. 38 Methods: EEG dipole source analysis of the N20 and P22 responses obtained from 39 subdural grids over the primary somatosensory cortex after median nerve stimulation 40 was performed in four patients undergoing epilepsy surgery. 41 Results: Based on anatomical landmarks, equivalent current dipoles (ECDs) of N20 42 and P22 were localized posterior to (n=2) or on the central sulcus (n=2). In three 43 patients, the P22 dipole was located posterior to the N20 dipole, whereas in one 44 patient, the P22 dipole was located on the same coordinate in anterior-posterior 45 direction. On average, P22 sources were found to be 6.6 mm posterior [and 1 mm 46 more superficial] compared to the N20 sources. 47 Conclusions: These data strongly suggest a postcentral origin of the P22 SEP 48 component in Brodmann area 1 and render a major precentral contribution to the 49 earliest stages of processing from the primary motor cortex less likely. 50