Abstract
The evolution of EEG/MEG source connectivity is both, a promising, and controversial advance in the characterization of epileptic brain activity. In this narrative review we elucidate the potential of this technology to provide an intuitive view of the epileptic network at its origin, the different brain regions involved in the epilepsy, without the limitation of electrodes at the scalp level. Several studies have confirmed the added value of using source connectivity to localize the seizure onset zone and irritative zone or to quantify the propagation of epileptic activity over time. It has been shown in pilot studies that source connectivity has the potential to obtain prognostic correlates, to assist in the diagnosis of the epilepsy type even in the absence of visually noticeable epileptic activity in the EEG/MEG, and to predict treatment outcome. Nevertheless, prospective validation studies in large and heterogeneous patient cohorts are still lacking and are needed to bring these techniques into clinical use. Moreover, the methodological approach is challenging, with several poorly examined parameters that most likely impact the resulting network patterns. These fundamental challenges affect all potential applications of EEG/MEG source connectivity analysis, be it in a resting, spiking, or ictal state, and also its application to cognitive activation of the eloquent area in presurgical evaluation. However, such method can allow unique insights into physiological and pathological brain functions and have great potential in (clinical) neuroscience.
Highlights
Epilepsy is commonly considered an archetypical network disease [1], with seizures and interictal activity generated and spreading in networks involving one or both hemispheres
Elshoff et al showed that the topology changes from a star-like topography with the seizure onset zone (SOZ) as the main hub in the beginning of the seizure to a circular pattern with no hub in the middle of the seizure (Figure 2). These results suggest an important information transfer from the SOZ at seizure onset, that was reduced during the seizure and resulted in a reduction of the efficiency of information transfer [58]
Source connectivity derived from EEG or MEG opens up new perspectives on the network disease epilepsy
Summary
Epilepsy is commonly considered an archetypical network disease [1], with seizures and interictal activity generated and spreading in networks involving one or both hemispheres. Klamer et al studied seizures and auras of a patient with musicogenic epilepsy using DCM based on prior selection of regions of interest from fMRI [60] In this application, the technique was used to infer hidden neuronal states from measurements of brain activity, to localize the SOZ from simultaneous high density EEG/MEG. Coito et al studied the time-varying frequency specific directed connectivity between brain regions during spikes in temporal epilepsy using high density EEG [25]. Comparison of microstates to the time-restricted view of source space connectivity is of interest, as time scale plays an important role in determining directed networks
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