Sensitivity of MEG and EEG to Source Orientation

An important difference between magnetoencephalography (MEG) and electroencephalography (EEG) is that MEG is insensitive to radially oriented sources. We quantified computationally the dependency of MEG and EEG on the source orientation using a forward model with realistic tissue boundaries. Similar to the simpler case of a spherical head model, in which MEG cannot see radial sources at all, for most cortical locations there was a source orientation to which MEG was insensitive. The median value for the ratio of the signal magnitude for the source orientation of the lowest and the highest sensitivity was 0.06 for MEG and 0.63 for EEG. The difference in the sensitivity to the source orientation is expected to contribute to systematic differences in the signal-to-noise ratio between MEG and EEG.National Institutes of Health (U.S.) (Grant NS057500)National Institutes of Health (U.S.) (Grant NS037462)National Institutes of Health (U.S.) (Grant HD040712)National Center for Research Resources (U.S.) (P41RR14075)Mind Research Networ

Crystal field effects in the compounds RbFeCl\u3csub\u3e3\u3c/sub\u3e·2H\u3csub\u3e2\u3c/sub\u3eO and CsFeCl\u3csub\u3e3\u3c/sub\u3e·2H\u3csub\u3e2\u3c/sub\u3eO

\u3cp\u3eMössbauer experiments on the compounds RbFeCl\u3csub\u3e3\u3c/sub\u3e·2H\u3csub\u3e2\u3c/sub\u3eO and CsFeCl\u3csub\u3e3\u3c/sub\u3e·2H\u3csub\u3e2\u3c/sub\u3eO have been performed in the temperature region of 4.2 to 300 K. The temperature dependence of the center shift and the quadrupole splitting is reported. A qualitative analysis of these data indicates an orbital-doublet ground state for the octahedrally coordinated ferrous ion. This is confirmed by the results from experiments in an applied magnetic field. A quantitative analysis of the quadrupole splitting data, using the crystal field model, results in the assignment of a singlet 685±50 cm\u3csup\u3e-1\u3c/sup\u3e above the orbital-doublet. The rhombic distortion will lift the degeneracy of the orbital ground state by a splitting of no more than 30 cm\u3csup\u3e-\u3c/sup\u3e. Single-crystal absorbers of RbFeCl\u3csub\u3e3\u3c/sub\u3e·2H\u3csub\u3e2\u3c/sub\u3eO were measured along the crystal axes, as well as in several directions within the bc plane, in order to determine the principal axes of the electric field gradient tensor (EFG). Additional experiments were performed in the paramagnetic region, by the application of an external magnetic field, and in the magnetically ordered region. Analysis of the spectra results in the assignment of the Cl\u3csub\u3eI\u3c/sub\u3eFeCl\u3csub\u3eI\u3c/sub\u3e axis as the maing axis of the EFG tensor.\u3c/p\u3