MEG: good enough.

In this issue, Barkley (2004) and Baumgartner (2004) debate whether magnetoencephalography (MEG) is superior to electroencephalography (EEG) for detecting interictal epileptiform activity. As the authors note, MEG has now been with us for several decades. Whole-head MEG has been available for about a decade. What have we learned about and with MEG? What is its place in clinical neurophysiology? This question is in part, and perhaps inevitably, a money question: the most capable MEG machine, together with magnetic shielding and appropriate source analysis software, can be 15 times as expensive as the most capable EEG machine, together with similar source analysis software, and a 3-dimensional electrode localization device. The cost question may be particularly acute for MEG manufacturers who have moved quickly to take advances in technology and develop them into MEG machines, and who may hope to show MEG to be 15 times better, so that hospitals and doctors can justify a purchase. Money issues are also important for clinical MEGers, who need to pay for the machines, and who would like to be reimbursed for their clinical services. However, these financial considerations, although important, make it harder to understand what the contribution of MEG has been to neurophysiology, and what its contribution might be in the future. MEG is different from EEG, but the difference is strictly defined, and described by the ‘right hand rule’ of physics. Stick your right hand out, with your thumb pointed and your fingers curled, like a hitchhiker. The thumb indicates the direction of current, measurable by EEG. The curling fingers indicate the direction of the related magnetic field, measured by MEG. At the cellular level, both ultimately depend upon current flow within pyramidal cells, with this measured directly by MEG, and indirectly by EEG, which measures the extracellular currents generated by the intracellular flow. By the law of charge conservation, the two should be the ‘same.’ Taking one step back, when recording from the surface of the scalp, with EEG, or from the air just beyond it, with MEG, the former is thought to record from both radial and tangential sources located in the cortical mantel, the latter from tangential sources only, such as sulci. There are some practical problems with this. First, cortical gray matter is not always parallel with the undersurface of the skull. When it is not, it might generate activity recordable by MEG. Second, consider current sources located very close to one another, but with currents going in opposite directions. This might occur, for instance, if two gyri or microgyri were close to one another. In this case, the currents would cancel out, so both EEG and MEG might record nothing, A number of papers had reported that individual spikes may be seen with one method (either EEG or MEG), but not the other, when recording from the cortical convexity (Barkley and Baumgartner, 2003). In summary, these studies have led to the conclusion that MEG and EEG together can provide more information than can either by itself, when recording from the cortical convexity. There had been hope that MEG might be better able to record from deep sources, such as the hippocampus, however, coverage of the inferior temporal surfaces can be limited with MEG machines (Leijten et al., 2003). It is now clear that a particular discharge might be better seen on MEG or EEG, but that the overall results are relatively similar, especially when using the same, or a similar, number of recording channels (Barkley and Baumgartner, 2003; Barkley, 2004; Baumgartner, 2004). As we all know, many studies and many neurophysiologists use a very valuable computer shortcut called ‘source analysis’ to identify where spikes or other activity originate, and to determine whether this origination is deep or superficial to the cortical surface. This analysis can and has been performed with EEG (Gevins et al., 1995, 1999; Scherg and Berg, 1991; Scherg and Ebersole, 1994), and can