How can EEG/MEG and fMRI/PET data be combined?

In the last few years, the functional brain imaging community has witnessed numerous efforts (and perhaps even more discussion) directed at multimodality data fusion: combining high-quality localization information provided by the hemodynamic-based brain imaging methods such as PET and fMRI with highquality temporal data generated by the electromagnetic-based techniques such as EEG and MEG [Dale and Halgren, 2001]. The article in this issue by Vitacco et al. [2002] provides an interesting research effort aimed at this problem, one that forces us to confront a number of critical questions about the entire data fusion enterprise. Almost every neuroscientist, and certainly every functional neuroimager, tries in one way or another to combine data from multiple methods. Three distinct approaches are used.