Synchrony in Neural Networks Underlying Seizure Generation in Human Partial Epilepsies

Abstract

Focal (or partial) epilepsies are characterized by recurrent seizures generated in an abnormal region of the brain, the epileptogenic zone (EZ). Approximately 30% of cases are resistant to antiepileptic drugs. In this situation, surgical resection of the EZ is the only therapeutic option able to suppress seizures or, at least, to significantly reduce their frequency. The localization and the definition of the EZ are therefore crucial issues in epileptology and are addressed through detailed analysis of anatomo-functional data acquired in epileptic patients during pre-surgical evaluation. Among investigation methods used during this evaluation, intracerebral exploration remains the only way to directly record the electrophysiological activity (depth-EEG) from brain structures and to formulate hypotheses about their potential involvement in epileptogenic processes. In this context, a large number of studies have been dedicated to the analysis of depth-EEG signals. Based on the estimation of interdependences (i.e., statistical coupling) between signals recorded from distinct sites, some reports have demonstrated that the areas involved in the generation of seizures (defining the EZ) are characterized by synchronous oscillations at seizure onset (Bartolomei et al., 2005b; Bartolomei et al., 2001; Bartolomei et al., 2004b; Bartolomei et al., 1999; Duckrow and Spencer, 1992; Gotman and Levtova, 1996; Le Van Quyen et al., 1998; Lieb et al., 1987). The “synchronization” of activities recorded from brain structures is therefore an important phenomenon that may be used for identifying epileptogenic networks (i.e., promoting the initiation of seizures). Other studies based on nonlinear associations in multivariate signals (Guye et al., 2006) have also reported that long distance functional connectivity is dramatically altered during seizures, or indicated that the topology of networks changes as ictal activity develops (Ponten et al., 2007).