The use of functional constraints for the neuroelectromagnetic inverse problem: alternatives and caveats.

Abstract

The use of functional neuroimages as a constraint for the solution of the neuroelectromagnetic inverse problem (NIP) constitutes an appealing alternative to deal with the non uniqueness of the solution. Among the functional techniques it is probably the fMRI the most attractive one due to its high spatial resolution. A limitation to this integration is that the relationships between neuronal activity and BOLD responses are poorly defined. This paper starts discussing some alternatives to integrate functional information as constraints for the inverse solution. Concrete examples of situations where functional images substantially diverge from electrophysiological methods are presented to promote the discussion about the most reasonable alternatives to combine these image modalities. The results of an anatomically constrained inverse solution that employs a sound physical model are compared with the EEG triggered fMRI in an epileptic patient. This example serves to show that the spatial resolution attainable with inverse solutions is comparable in some situations with that of functional images. Finally, some concrete strategies to ameliorate the quality and reliability of linear inverse solutions maps in more general situations are briefly described. The main conclusion of this paper is that integration of functional modalities into the solution of the NIP should be cautiously considered until a more tight coupling between BOLD effects and electrophysiological measurements could be established.