On the Relationship of Synaptic Activity to Macroscopic Measurements: Does Co-Registration of EEG with fMRI Make Sense?

Paul L. Nunez*^ and Richard B. Silberstein*+^

Summary: A two-scale theoretical description outlines relationships between brain current sources and the resulting extracranial electric field, recorded as EEG. Finding unknown sources of EEG, the so-called "inverse problem", is discussed in general terms, with emphasis on the fundamental non-uniqueness of inverse solutions. Hemodynamic signatures, measured with fMRI, are expressed as voxel integrals to facilitate comparisons with EEG. Two generally distinct cell groups (1 and 2), generating EEG and fMRI signals respectively, are embedded within the much broader class of synaptic action fields. Cell groups 1 and 2mayormaynot overlap in specific experiments. Implications of this incomplete overlap for co-registration studies are considered. Each experimental measure of brain function is generally sensitive to a different kind of source activity and to different spatial and temporal scales. Failure to appreciate such distinctions can exacerbate conflicting views of brain function that emphasize either global integration or functional localization.
Over the past few years, brain science has exhibited an explosive growth in hemodynamic/metabolic data on brain function. In particular, Positron Emission Tomography (PET) and functional Magnetic Resonance Imaging (fMRI) provide excellent spatial resolution, but their temporal resolutions are severely limited by relatively slow responses of brain metabolism. By contrast, electroencephalography (EEG) and magnetoencephalography (MEG) are able to track modulations of current source activity at millisecond time scales, but suffer from poor spatial resolution. An apparently plausible approach for
achieving both high spatial and temporal resolution is to combine metabolic and electric/magnetic measures in some way, e.g., by co-registration.