Connectivity of Epileptic Brain Regions in Wake and Sleep

Focal epileptic brain is characterized by a region of pathological tissue seizure onset zone (SOZ) - the pathologic tissue generating seizures. During the interictal period (non-seizure) the SOZ is characterized by epileptiform activity - interictal spikes & high-frequency oscillations (HFO). The SOZ also exhibits hyper-synchrony and functional disconnection from the surrounding areas. Recent studies have described the synchrony inside the SOZ and surrounding tissue for just small sets of patients (2-4) and without any distinction in behavioral states. Wake and sleep cycles can, however, have a significant influence on SOZ activity. Here we show the results of connectivity analysis in three fundamental areas of the epileptic brain - inside SOZ, outside SOZ and bridging areas in 7 patients during wake and sleep. We observed increased synchrony inside SOZ and decreased synchrony on its edges (bridging areas) in specific frequency bands. We also detected significant differences of synchrony levels between wake and sleep periods in HFO frequencies. Our results provide additional insight into the properties of SOZ connectivity. Knowledge of these principles may prove useful for SOZ localization and understanding epileptic brain function in general.