Abstracts

Mechanisms leading to the occurrence of epileptic seizures are still poorly understood. In order to investigate intra- and interhemispheric interactions as well as processes of seizure generation we analyzed long-lasting intracranial EEG recordings using an information theoretic measure. This measure - called transfer entropy - quantifies the flow of information between signals from different brain regions and, moreover, indicates the directionality of the flow., We studied intracranial EEG recordings of up to now 6 patients suffering from unilateral medically medial temporal lobe epilepsy. All patients underwent invasive presurgical diagnostics and are postoperatively seizure free. EEG signals were recorded from bilateral intrahippocampal depth electrodes, each equipped with 10 contacts and implanted stereotactically along the longitudinal axis of the hippocampal formation. The total EEG recording time amounted to more than 46 days during which the patients had 40 seizures. The transfer entropy was calculated for all channel combinations using a moving window technique., During seizure activity, we observed the mesial temporal structures in one hemisphere to be much more active and driving homologuoes structures in the opposite hemisphere. These active structures, however, not necessarily coincided with the epileptic focus and were frequently observed in the contralateral hemisphere. Moreover, in many patients the dynamics of these structures allowed to identify long-lasting preictal states. During the interictal state a comparable active-passive relationship could be observed again indicating more active contralateral structures. Intrahemispherically, we observed a posterior-anterior gradient, i.e., from hippocampal to enthorinal structures, both druring the interictal state and during seizures., Measuring directionality in the human epileptic brain may provide relevant information about the location of the epileptic focus. In addition, our seemingly counter-intuitive findings may indicate the importance of brain outside of the ictal onset zone but within the epileptic network in ictogenesis., (Supported by Deutsche Forschungsgemeinschaft.)