Abstracts

The first objective is to compare two imaging modalities to better identify brain areas where spikes are generated:
- [italic]EEG source localization[/italic], which allows estimating a current density along the cortical surface at each time sample of the spike
- [italic]Simultaneous EEG/fMRI[/italic], which allows measuring the hemodynamic correlates of EEG activity
Those modalities highlight several aspects of spike generation, because signals have different physiological origins and complementary spatial and temporal resolutions. Moreover, EEG source localization has no unique solution and adding prior information is necessary. The second objective is to evaluate whether it is relevant to include fMRI information in EEG source localization. 9 patients with focal epilepsy underwent EEG/fMRI examination and EEG recording outside the scanner. The EEG/fMRI protocol consists of recording 19 EEG channels inside the MR scanner. After manual detection of interictal spikes, data were analysed using a linear model combining several hemodynamic responses after each spike. T maps showing brain areas with significant fMRI responses were interpolated on the cortical surface, to compare with EEG source localization. Source localization was applied to averaged spikes recorded outside the scanner (43 channels). We applied the Maximum Entropy on the Mean (MEM) approach ¹ to estimate the current density on the cortical surface at each time sample of the spike.
fMRI results were compared with MEM results obtained at the peaks of the spike. The level of concordance was assessed by measuring the minimal geodesic distance D between local extrema of the fMRI and MEM maps. We also measured an index A to assess whether fMRI could be included as a prior for EEG source localization, using a hierarchical Bayesian model ². Large positive values of A mean that fMRI data were highly relevant information regarding EEG data. 3 patients did not show any fMRI response. Among the 6 other patients, MEM and fMRI results showed good concordance in 4 (D [lt] 2.5 cm). However, qualitative analysis revealed that most exhibited concordant areas and areas unique to each modality. Our index A was in good agreement with the qualitative comparison of MEM and fMRI results. Our study highlights that EEG source localization and EEG/fMRI clearly explored different phenomena linked to spike generation. Brain areas involved could be concordant or complementary regarding both modalities. The index of relevance A seems promising to decide whether fMRI or a part of the fMRI map should be used as prior for source localization. By analysing more data, those approaches may provide a better understanding of the mechanisms involved during spike generation and propagation.
¹Amblard C [italic]et al[/italic] 2004 [italic]IEEE TBME, [/italic](3)
²Daunizeau J [italic]et al[/italic] 2005 [italic]IEEE TSPS, [/italic]in press (Supported by Canadian Institute of Health Research, Jeanne Timmins Costello Fellowship.)