Spatial filter approach for comparison of the forward and inverse problemsof electroencephalography and magnetoencephalography

We present an analysis of the relative information content of cortical curr ent source reconstructions from electroencephalogram (EEG) and magnetoencep halogram (MEG) forward calculations by examining the spatial filters that r elate the internal sources with the externally measured electric potentials and magnetic fields. The forward spatial filters are seen to be low-pass f unctions of spatial frequency and spatial resolution degrades in external m easurements. Inverse spatial filters may be used to reconstruct cortical so urces from external data, but since they are high-pass functions of spatial frequency, they must be regularized to avoid instabilities caused by noise at higher spatial frequencies. The regularization process limits the spati al resolution of source reconstructions. EEG forward spa tial filters fall off at lower spatial frequencies than MEG filters; hence, there is less inf ormation available in higher spatial frequencies resulting in lower spatial resolution in inverse reconstructions. The tangential component of the mag netic field provides even higher spatial resolution than can be obtained us ing the radial component. An accompanying article examines the surface Lapl acian for both the EEG and the MEG. (C) 2001 Biomedical Engineering Society .