Abstracts

Rationale: We consider the question whether the clinically defined epileptic focus drives or is driven by local or even remote areas in large scale brain networks. For this purpose we analyzed short- and long-ranged directed interactions in functional brain networks of patients suffering from focal epilepsies. Methods: We investigated EEG recordings from up to now 20 epilepsy patients (15 with unifocal and 5 with multifocal epilepsies; all patients are seizure-free post-operatively). EEGs were recorded from on average 56 intracranial recording sites for a period of 117 hours per patient. A total of 131 seizures occurred. Directionality of interactions was assessed in a time-resolved manner by calculating the transfer entropy (TE) as well as an index based on phase interactions (DI) for all channel combinations using a moving window approach. Results: When considering all interactions, both directionality measures indicated the epileptic focus as a driving structure in more than 50% of cases. However, in these cases the driving effect could mainly be observed for long-ranged interactions (that even extended to the contralateral hemisphere) while the focus appeared to be driven by its immediate surroundings. For the other patients the aforementioned short-ranged interactions could also be observed with TE but the effect reversed with DI. Long-ranged interactions were negligible. In all cases neither pre-ictal phenomena nor daily rhythms appeared to affect the observed driving phenomena. Conclusions: Our findings indicate that the epileptic focus appears to be driven, however, only if short-ranged interactions are taken into account. The directionality indices used here capture different aspects of directed interactions within epileptic brain networks and can help to improve our understanding of the epileptic process. This work was supported by the Deutsche Forschungsgemeinschaft (Grant No: LE 660/5-2)