Cancellation of EEG and MEG signals generated by extended and distributed sources

Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital/Harvard Medical School, Charlestown, 02129, USA.
Human Brain Mapping (Impact Factor: 5.97). 01/2009; 31(1):140-9. DOI: 10.1002/hbm.20851
Source: PubMed


Extracranial patterns of scalp potentials and magnetic fields, as measured with electro- and magnetoencephalography (EEG, MEG), are spatially widespread even when the underlying source in the brain is focal. Therefore, loss in signal magnitude due to cancellation is expected when multiple brain regions are simultaneously active. We characterized these cancellation effects in EEG and MEG using a forward model with sources constrained on an anatomically accurate reconstruction of the cortical surface. Prominent cancellation was found for both EEG and MEG in the case of multiple randomly distributed source dipoles, even when the number of simultaneous dipoles was small. Substantial cancellation occurred also for locally extended patches of simulated activity, when the patches extended to opposite walls of sulci and gyri. For large patches, a difference between EEG and MEG cancellation was seen, presumably due to selective cancellation of tangentially vs. radially oriented sources. Cancellation effects can be of importance when electrophysiological data are related to hemodynamic measures. Furthermore, the selective cancellation may be used to explain some observed differences between EEG and MEG in terms of focal vs. widespread cortical activity.

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    • "Indeed, we observed prominent activity in the ventral temporal lobe (for faces, see Figure 4A) and the ventral surface of the frontal lobe (for bodies, see Figures 3, 4 and 5), areas that are expected to yield a relatively low signal-to-noise ratio [78]. It is, however, feasible that sources in these areas can be detected by MEG [34], [40] when extended patches are activated, as the summating sources from the crown of a gyrus and bottom of a sulcus have a predominantly tangential orientation whereas the canceling sources from opposing sites of the sulcal wall are predominantly radial [79]. "
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    • "Even with current anatomical and functional constraints, the estimates are spatially distributed, which is why the present discussion has been concentrated on the overall statistical patterns. A limitation of MEG/EEG is also their limited capability to detect simultaneous synchronous activity on the opposite banks of sulci due to cancellation effects (Ahlfors et al., 2009). This limitation might, for example, explain why the present alpha lateralization effects were more prominent in the lateral than medial parieto-occipital cortices. "
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    • "This finding is consistent with previous studies showing that the interregional hemodynamic timing differences are longer than reaction times (Bellgowan et al., 2003; Henson et al., 2002). Potential explanations include low-pass filtering (Logothetis et al., 2001) and nonlinear (Devor et al., 2003) nature of neurovascular coupling, the exact way in which hemodynamic and neuronal latencies were measured, statistical power issues , possible cancelations of MEG sources with opposing directions (Ahlfors et al., 2010), and especially – because the interregional delays were much more similar between the induced MEG responses (OTI) and BOLD fMRI – the averaging process that removes out-of-phase MEG components from evoked responses. Nonetheless, the present results suggest new possibilities for what fMRI might tell us about the dynamic brain. "
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