Abstracts

Rationale: We consider the important and yet unsolved question whether directional interactions between the seizure generating area of the human epileptic brain and remote areas can be identified, which may improve our understanding of network processes underlying seizure generation. In particular, we analyzed intracranial EEG recordings of epileptic seizures using an information theoretic measure, called symbolic transfer entropy [1], which identifies couplings as well as the preferential direction of information transport between different brain areas. Methods: We analyzed EEG recordings including focal seizure activity obtained from patients suffering from refractory medial temporal lobe epilepsy. The EEG was recorded via intrahippocampal depth electrodes, each equipped with 10 cylindrical contacts, using an average common reference. Electrodes were implanted stereotactically in the medial temporal lobes via the longitudinal axis of the hippocampus. The recording time of each EEG sample amounted to 30 min including a 20 min preictal recording. The measure for directionality was estimated for each combination of pairs of electrode contacts in a moving window fashion. Results: Preictally, we observed almost stable interactions among different brain structures indicating active brain regions in one hemisphere as compared to activity in contralateral medial temporal structures. However, these active structures were not necessarily confined to the seizure generating area. In addition, we observed the active brain regions to vary for different preictal recordings. During the initial part of a seizure, however, the active medial temporal structures perfectly coincided with the seizure generating area as determined by the presurgical workup and the complete postoperative seizure control. Conclusions: Our findings indicate that remote and even contralateral brain areas appear to be involved in the generation of focal seizures within the epileptic network. Relevant information about the seizure generating area as well as the epileptic network may be achieved by analyzing the directionality of interactions in the human epileptic brain. [1]Staniek and Lehnertz, Symbolic Transfer Entropy, Phys. Rev. Lett. 100, 158101 (2008).