The magnetic and electric fields agree with intracranial localizations of somatosensory cortex

We measured the magnetoencephalogram (MEG), electroencephalogram (EEG), and electrocorticogram (ECoG) after stimulation of contralateral median nerve in four patients with partial epilepsy evaluated for surgery. Quantitative localization estimates from equivalent source modeling were compared with locations of central fissure in hand sensorimotor area determined by cortical stimulations, intraoperative photographs, and examination after excision in frontal lobe. We also measured MEG and EEG in nine control subjects. MEG and EEG localizations were within 2.5 cm of the estimated location of central fissure in all 13 subjects. In the three patients who had complete mapping of all three fields, the average distance of localizations from central fissure was approximately 4 mm in both MEG and EEG, 3 mm in ECoG, and 3 mm in combined MEG and EEG. MEG was simpler than EEG, which was simpler than ECoG. MEG resolved ambiguities in both EEG and ECoG. The combination of the three fields added information about the spatiotemporal activity of somatosensory cortex. Localization of central fissure was essential to surgical treatment.