Fig. 1

Upper panel The outlines of the brain, skull, and scalp are extracted from each magnetic resonance imaging (MRI) slice (slice thickness 2 mm) and saved as x, y, and z coordinates. Three-layered (scalp, skull, and brain) head models are then reconstructed from the x, y, and z coordinates. Conductivities of the electrodes for the brain (0.3300 s/m), skull (0.0041 s/m), and scalp (0.3300 s/m) are then calculated before the dipole is calculated. Somatosensory-evoked potentials (SEPs) elicited by electrical stimulation to the left median nerve are measured in a subject. Dipole analysis is performed from an individual subject’s SEP with his/her own head model, and these dipole locations are superimposed on the Montreal Neurological Institute (MNI) head model. Lower panel Outlines of the brain, skull, and scalp are extracted from slices of the standard MNI template, and a three-layered head model is reconstructed from the coordinates obtained from each slice. Conductivities are also taken into account for the dipole calculations. All SEPs of all subject are averaged, and dipole analysis is performed from this grand averaged SEP with the MNI model. Rt right, Lt left