Figure 2.

The EEG (a), Laplacian (b), and MEG (c) sensitivity distributions for a fixed electrode location (yellow circle at right inset figure) and magnetic coil position (green line at right inset figure) are shown. The cortical surface was constructed from the MRI of one subject. Simulated “dipole sources” P(r,t) (dipole moment per unit volume, 100,000 in one hemisphere) were assumed normal to the local cortical surface. Scalp surface potentials, scalp Laplacians, and surface normal magnetic fields due to each dipole were calculated at the electrode and coil locations shown in the inset using Green's functions (one for each measure) based on the confocal 3-ellipsoid head model. The three Green's functions were normalized with respect to their maximum values so that the relative sensitivity of the three measured could be compared. (a) The EEG electrode is most sensitive to gyral sources under the electrode, but this electrode is also sensitive to large source regions occupying relatively remote gyral crowns and much less sensitive to sources in cortical folds. (b) The Laplacian is most sensitive to gyral sources under the electrode; sensitivity falls off rapidly at moderate and large distances. (c) The MEG is most sensitive to sources in cortical folds that tend to be tangent to MEG coils. Maximum MEG sensitivity occurs in folds that are roughly 4 cm from the coil in directions tangent to the surface. Regions in blue provide contributions to MEG of opposite sign to those of yellow/orange, reflecting dipoles on opposite sides of folds that tend to produce canceling magnetic fields at the coil.