Connectivity measures in the electroencephalogram of sleep and epilepsy
Authors: Σακελλαρίου Δημήτριος 
Published: September 2021

From birth, we spend at least one third of our lives asleep. After decades of research the reasons still remain to be elucidated. Greek mythology describes sleep as the brother of death while Edgar Allan Poe reads that the boundaries that divide life from death are, at best shadowy and vague.A neuropathy of the brain, closely related to sleep is epilepsy. Epilepsy is a group of neurological disease that express by epileptic seizures and has been characterised by Hippocrates as the Sacred Disease. The ause of most cases of epilepsy is unknown.Both sleep physiology and epilepsy diagnosis are studied via the Electroencephalogram (EEG). Although many studies have looked into the EEG macrostructural architecture of both, the modern technological advancements have made available the detailed investigation of microstructural EEG elements, such as the sleep spindle and the spike and wave discharge, using various computational techniques with brain connectivity being amongst them.In this study we focus on the computational investigation of the sleep spindle and the spike and wave discharge, hallmark EEG elements of sleep and epilepsy respectively. The sleep spindle has been associated with various cognitive functions and neuropsychological diseases, whereas the spike and wave discharge is the primary EEG indicator for epilepsy.For the computational study of the above we have used a range of computational techniques, such as time averaging, voltage topographies and time frequency analysis. Moreover, we developed a connectivity method specifically designed for the investigation of EEG microelements. This connectivity tool makes use of the Imaginary part of Coherence, reduces the volume conduction and reference EEG problems while it features sophisticated error-assessment statistical analysis.Secondly, in a network examination of ten healthy subjects a strongly interacting network that involves parietal-occipital and left frontal areas was identified. Furthermore, another two substantially interconnected nodes related to occipital and right frontotemporal regions were found. These findings suggest that these areas are primarily crucial in the maintenance and regional distribution of the sleep spindle and furthermore important in the understanding of various physiological and abnormal br.ain functions the sleep spindle function are involved with. In order to study the fundamental origins of epileptiform EEG manifestations, the boundaries between the survivable condition and irreversible coma conditions, GSWD were invastigated in Non Colvulsive Status Epilepticus and Anoxic Brain Injury patients. Although the morphological aspects of the GPEDs between NCSE and ABI patients were indifferent, a clear differentiation between the two was found with regards to the voltage topography of the negative GPD phase.Finally, in order to elucidate possible links between sleep spindles and spikes generators we looked into those in 3 groups of idiopathic focal epilepsies of childhood. A positive correlation between spike and spindle density as well as between the morphology of spikes and spindles was found. The latter suggest that the generation of spikes and spindles is likely to be under led by similar neurophysiological mechanisms.