fMRI‐constrained MEG source imaging and consideration of fMRI invisible sources

Abstract

Recent studies on multimodal brain source imaging have shown that the use of functional MRI (fMRI) prior information could enhance spatial resolution of magnetoencephalography (MEG), while MEG could compensate poor temporal resolution of fMRI. This article deals with a multimodal imaging method, which combines fMRI and MEG for enhancing both spatial and temporal resolutions. Recent studies on the combination of fMRI and MEG have suggested that the fMRI prior information could be very easily implemented by just giving different weighting factors to the diagonal terms of source covariance matrix in linear inverse operator. We applied the fMRI constrained imaging method to several simulation data and experimental data (Japanese language lexical judgment experiment), and found that some MEG sources may be eliminated by the introduction of the fMRI weighting and the eliminated sources may affect source estimation in fMRI activation regions. In this article, in order to check whether the eliminated sources were fMRI invisible ones or just spurious ones, we placed small numbers of regional sources (rotating dipoles) around all possible activation regions and investigated their temporal changes. By investigating the results carefully, we could evaluate whether the missed sources were real or not.

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

“…The reduction in the signal-to-noise ratio per source element due to cancellation may result in some active areas being detected with fMRI but not with EEG or MEG. This appears not to be a serious problem, however, for approaches in which fMRI is used to suggest likely locations for the EEG and MEG sources: simulation studies have indicated that the inclusion of such non-contributing regions as fMRI priors have only little effect on the EEG and MEG source estimates [Ahlfors and Simpson, 2004; Im et al, 2005; Liu et al, 1998]. …”

Simultaneous recording of EEG and BOLD responses: A historical perspective

“…In a number of studies it has been shown that not all activity visible in MEG also results in BOLD responses, and vice versa (e.g. Liu et al, 1998;Schulz et al, 2004;Im et al, 2005). This has resulted in the notion of so-called fMRI-blind EEG/MEG sources and EEG/MEG-blind fMRI sources (Ritter and Villringer, Fig.…”

“…However, it is likely that some EEG/MEG correlates of cognitive processing may not result in measurable changes of the BOLD signal, whereas other patterns of neuro-cognitive activity may be detectable with fMRI but not EEG (Liu et al, 1998;Nunez and Silberstein, 2000;Schulz et al, 2004;Im et al, 2005). Accordingly, a main goal of simultaneous EEGfMRI is to shed light on the foundations of the two measures and their interrelations.…”

Spatial resolution of EEG cortical source imaging revealed by localization of retinotopic organization in human primary visual cortex

“…Since we already identified from previous studies and our fMRI studies that the EEG sources related to the early visual process would appear around the visual cortex, we gave the source points located around occipital lobe a higher probability to be estimated in the EEG inverse. We imposed the probability to the EEG inverse solution by giving different weighting values to the diagonal terms of R. If a source belonged to the predetermined regions, 1 was multiplied by its corresponding diagonal term; otherwise, 0.1 was multiplied (Liu et al, 1998, Im et al, 2005b. The area where the functional constraints were imposed was large enough to cover the entire human visual cortex (e.g.…”