Abstracts

Rationale: Previous studies have analyzed effective connectivity independently in scalp EEG, intracranial EEG and MEG in patients with intractable epilepsy using partial directed coherence (PDC). PDC is similar to granger causality, but is used to perform multivariate analysis of EEG signals in the frequency domain. Additionally, a few studies have analyzed the relationship between signals recorded from noninvasive modalities like, MEG and fMRI, with intracranial EEG. Analysis of simultaneous scalp and intracranial EEG data may provide insight into dynamic interaction of connectivity parameters in the interictal and ictal states. In the future scalp EEG alone may be sufficient for connectivity analysis if the parameters that affect its co variation with intracranial EEG are better understood. In this project, we attempt to answer the following question: What are the electrophysiological and clinical parameters (state of the patient, phase of the seizure) that affect the co variation of connectivity recorded from scalp and intracranial EEG? Is scalp EEG data enough to provide information on connectivity in patients with epilepsy?Methods: All patients who underwent simultaneous scalp and intracranial EEG studies at the University of Pittsburgh were included in the anaylsis(n=11) Three EEG epochs during wakefulness, stage 2 sleep, and the three most typical seizures semiologically and electrophysiologically were selected as visually identified by two epileptologists. (Sampling rates: scalp EEG: 256 Hz, intracranial EEG 1024 Hz ). PDC was calculated for the three epochs in the scalp and intracranial EEG, and results were compared. The analysis was performed using MATLAB (v. 2012a) and HERMES. PDC was also calculated separately for unfiltered scalp EEG and intracranial EEG data to study the correlation between the two modalities. Intracranial EEG was down sampled to 500 Hz and filtered to delta (1-4 Hz), theta (5-7 Hz), alpha (8-13 Hz), beta (15-30 Hz), gamma (30-100 Hz) and high frequency oscillations (100-250 Hz) and band specific results will be obtained.Results: Figure 1 shows a) EEG signals b) power spectrum c) PDC in the awake state at a threshold of 0.5 of the normalized PDC values. d) PDC in the sleep state at a threshold of 0.5 of the normalized PDC values. Figure 2 shows PDC with a threshold of 0.4 computed using both scalp and intracranial EEG in the first ten seconds of three typical seizures. The extent of connected nodes exhibiting suprathreshold PDC varied widely across the seizures studied, however, a subset of these connected nodes was common to all in the same patient.Conclusions: Nodes with significant input assessed by PDC co-vary in a consistent fashion in the scalp and intracranial EEG recorded in ictal state suggesting consistent activation of ictal networks in the same patient. Scalp EEG can be used to provide some information regarding the connectivity in the intracranial EEG.