Spectral Analysis of Pre- and Post-Ictal Periods using Intracranial EEG

Epilepsy is a neurological disease that results in behavioral seizures due to electrical activity abnormalities in the brain (Fisher et al. 2005). Seizures are composed of periods in which electrical activity is of mixed frequency (pre- or inter-ictal periods), of high amplitude synchronous activity across all recording electrodes (ictal periods), and of periods where brain activity is reduced in amplitude (depressed, post-ictal periods). Electroencephalography (EEG) measures electrical activity through electrodes placed on the scalp. EEGs provides a great modality for studying the electrical abnormalities associated with epilepsy. Intracranial EEG (iEEG) is an invasive procedure in which a grid of electrodes is placed directly on the brain to measure brain waves and is often used for resection surgery candidates to provide enhanced localization of the epileptic focus so it can be removed. The purpose of this study is to determine if the post-ictal ‘state’ of the brain following the post-ictal depression phase returns to that seen pre-ictally. The data used in this study are iEEG recordings that were obtained from the Epilepsy Monitoring Unit (EMU) at the University of Arkansas for Medical Sciences (UAMS). The data were processed in Brainstorm to derive the power spectral densities (PSD) of pre- and post-ictal periods. Data were band-limited according to the Welch Method into four frequency bands: delta/theta band (2-5 Hz), theta/alpha band (6-9 Hz), and two gamma bands (30-80, 80-120 Hz). These frequency bands were chosen based on previous studies indicating that they are important in epileptic patients. Power in these frequency bands is expected to be higher during ictal events and to drop as the brain returns to its normal, functional waking state. We will compare post-ictal spectral power over a 15-minute period to determine if the brain returns to the pre-seizure state rapidly or slowly