Normalized power variance change between pre-ictal and ictal phase of an epilepsy patient using NAT analysis: A case study

Variance of state variables shifts due to phase-instability and may serve as an early-warning signal of phase transition of complex systems such as an epileptic seizure of brain cortical activity. Neuronal Activity Topology (NAT) analysis calculates a normalized-power-variance (NPV) of electroencephalogram (EEG) data in each frequency band to obtain relative values comparable among different power states. In this study, using AT analysis, we investigated PV changes between the pre-ictal and ictal phase in a patient with mesial frontal lobe epilepsy. We also investigated interictal cortical electrical activity in each frequency band using exact low resolution brain electromagnetic tomography (eLORETA) analysis. AT analysis demonstrated a PV increase in beta frequency band at mesial frontal lobe in the pre-ictal period, and PV decrease in beta frequency band at mesial frontal lobe in ictal period. In addition, eLORETA analysis localized a beta band cortical electrical activity at mesial frontal lobe in the interictal period. These results indicate instability of cortical electrical activity at seizure onset zone in the pre-ictal phase and its stabilization by phase transition to ictal phase. Overall findings indicate that AT analysis can sensitively detect instability of cortical electrical activity and may serve as an early-warning signal of phase transition of cortical electrical activity.